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What is your favorite method to handle errors in Bash?
The best example of handling errors I have found on the web was written by William Shotts, Jr at http://www.linuxcommand.org.
He suggests using the following function for error handling in Bash:
#!/bin/bash
# A slicker error handling routine
# I put a variable in my scripts named PROGNAME which
# holds the name of the program being run. You can get this
# value from the first item on the command line ($0).
# Reference: This was copied from <http://www.linuxcommand.org/wss0150.php>
PROGNAME=$(basename $0)
function error_exit
{
# ----------------------------------------------------------------
# Function for exit due to fatal program error
# Accepts 1 argument:
# string containing descriptive error message
# ----------------------------------------------------------------
echo "${PROGNAME}: ${1:-"Unknown Error"}" 1>&2
exit 1
}
# Example call of the error_exit function. Note the inclusion
# of the LINENO environment variable. It contains the current
# line number.
echo "Example of error with line number and message"
error_exit "$LINENO: An error has occurred."
Do you have a better error handling routine that you use in Bash scripts?
Use a trap!
tempfiles=( )
cleanup() {
rm -f "${tempfiles[#]}"
}
trap cleanup 0
error() {
local parent_lineno="$1"
local message="$2"
local code="${3:-1}"
if [[ -n "$message" ]] ; then
echo "Error on or near line ${parent_lineno}: ${message}; exiting with status ${code}"
else
echo "Error on or near line ${parent_lineno}; exiting with status ${code}"
fi
exit "${code}"
}
trap 'error ${LINENO}' ERR
...then, whenever you create a temporary file:
temp_foo="$(mktemp -t foobar.XXXXXX)"
tempfiles+=( "$temp_foo" )
and $temp_foo will be deleted on exit, and the current line number will be printed. (set -e will likewise give you exit-on-error behavior, though it comes with serious caveats and weakens code's predictability and portability).
You can either let the trap call error for you (in which case it uses the default exit code of 1 and no message) or call it yourself and provide explicit values; for instance:
error ${LINENO} "the foobar failed" 2
will exit with status 2, and give an explicit message.
Alternatively shopt -s extdebug and give the first lines of the trap a little modification to trap all non-zero exit codes across the board (mind set -e non-error non-zero exit codes):
error() {
local last_exit_status="$?"
local parent_lineno="$1"
local message="${2:-(no message ($last_exit_status))}"
local code="${3:-$last_exit_status}"
# ... continue as above
}
trap 'error ${LINENO}' ERR
shopt -s extdebug
This then is also "compatible" with set -eu.
That's a fine solution. I just wanted to add
set -e
as a rudimentary error mechanism. It will immediately stop your script if a simple command fails. I think this should have been the default behavior: since such errors almost always signify something unexpected, it is not really 'sane' to keep executing the following commands.
Reading all the answers on this page inspired me a lot.
So, here's my hint:
file content: lib.trap.sh
lib_name='trap'
lib_version=20121026
stderr_log="/dev/shm/stderr.log"
#
# TO BE SOURCED ONLY ONCE:
#
###~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~##
if test "${g_libs[$lib_name]+_}"; then
return 0
else
if test ${#g_libs[#]} == 0; then
declare -A g_libs
fi
g_libs[$lib_name]=$lib_version
fi
#
# MAIN CODE:
#
###~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~##
set -o pipefail # trace ERR through pipes
set -o errtrace # trace ERR through 'time command' and other functions
set -o nounset ## set -u : exit the script if you try to use an uninitialised variable
set -o errexit ## set -e : exit the script if any statement returns a non-true return value
exec 2>"$stderr_log"
###~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~##
#
# FUNCTION: EXIT_HANDLER
#
###~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~##
function exit_handler ()
{
local error_code="$?"
test $error_code == 0 && return;
#
# LOCAL VARIABLES:
# ------------------------------------------------------------------
#
local i=0
local regex=''
local mem=''
local error_file=''
local error_lineno=''
local error_message='unknown'
local lineno=''
#
# PRINT THE HEADER:
# ------------------------------------------------------------------
#
# Color the output if it's an interactive terminal
test -t 1 && tput bold; tput setf 4 ## red bold
echo -e "\n(!) EXIT HANDLER:\n"
#
# GETTING LAST ERROR OCCURRED:
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #
#
# Read last file from the error log
# ------------------------------------------------------------------
#
if test -f "$stderr_log"
then
stderr=$( tail -n 1 "$stderr_log" )
rm "$stderr_log"
fi
#
# Managing the line to extract information:
# ------------------------------------------------------------------
#
if test -n "$stderr"
then
# Exploding stderr on :
mem="$IFS"
local shrunk_stderr=$( echo "$stderr" | sed 's/\: /\:/g' )
IFS=':'
local stderr_parts=( $shrunk_stderr )
IFS="$mem"
# Storing information on the error
error_file="${stderr_parts[0]}"
error_lineno="${stderr_parts[1]}"
error_message=""
for (( i = 3; i <= ${#stderr_parts[#]}; i++ ))
do
error_message="$error_message "${stderr_parts[$i-1]}": "
done
# Removing last ':' (colon character)
error_message="${error_message%:*}"
# Trim
error_message="$( echo "$error_message" | sed -e 's/^[ \t]*//' | sed -e 's/[ \t]*$//' )"
fi
#
# GETTING BACKTRACE:
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #
_backtrace=$( backtrace 2 )
#
# MANAGING THE OUTPUT:
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #
local lineno=""
regex='^([a-z]{1,}) ([0-9]{1,})$'
if [[ $error_lineno =~ $regex ]]
# The error line was found on the log
# (e.g. type 'ff' without quotes wherever)
# --------------------------------------------------------------
then
local row="${BASH_REMATCH[1]}"
lineno="${BASH_REMATCH[2]}"
echo -e "FILE:\t\t${error_file}"
echo -e "${row^^}:\t\t${lineno}\n"
echo -e "ERROR CODE:\t${error_code}"
test -t 1 && tput setf 6 ## white yellow
echo -e "ERROR MESSAGE:\n$error_message"
else
regex="^${error_file}\$|^${error_file}\s+|\s+${error_file}\s+|\s+${error_file}\$"
if [[ "$_backtrace" =~ $regex ]]
# The file was found on the log but not the error line
# (could not reproduce this case so far)
# ------------------------------------------------------
then
echo -e "FILE:\t\t$error_file"
echo -e "ROW:\t\tunknown\n"
echo -e "ERROR CODE:\t${error_code}"
test -t 1 && tput setf 6 ## white yellow
echo -e "ERROR MESSAGE:\n${stderr}"
# Neither the error line nor the error file was found on the log
# (e.g. type 'cp ffd fdf' without quotes wherever)
# ------------------------------------------------------
else
#
# The error file is the first on backtrace list:
# Exploding backtrace on newlines
mem=$IFS
IFS='
'
#
# Substring: I keep only the carriage return
# (others needed only for tabbing purpose)
IFS=${IFS:0:1}
local lines=( $_backtrace )
IFS=$mem
error_file=""
if test -n "${lines[1]}"
then
array=( ${lines[1]} )
for (( i=2; i<${#array[#]}; i++ ))
do
error_file="$error_file ${array[$i]}"
done
# Trim
error_file="$( echo "$error_file" | sed -e 's/^[ \t]*//' | sed -e 's/[ \t]*$//' )"
fi
echo -e "FILE:\t\t$error_file"
echo -e "ROW:\t\tunknown\n"
echo -e "ERROR CODE:\t${error_code}"
test -t 1 && tput setf 6 ## white yellow
if test -n "${stderr}"
then
echo -e "ERROR MESSAGE:\n${stderr}"
else
echo -e "ERROR MESSAGE:\n${error_message}"
fi
fi
fi
#
# PRINTING THE BACKTRACE:
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #
test -t 1 && tput setf 7 ## white bold
echo -e "\n$_backtrace\n"
#
# EXITING:
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #
test -t 1 && tput setf 4 ## red bold
echo "Exiting!"
test -t 1 && tput sgr0 # Reset terminal
exit "$error_code"
}
trap exit_handler EXIT # ! ! ! TRAP EXIT ! ! !
trap exit ERR # ! ! ! TRAP ERR ! ! !
###~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~##
#
# FUNCTION: BACKTRACE
#
###~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~##
function backtrace
{
local _start_from_=0
local params=( "$#" )
if (( "${#params[#]}" >= "1" ))
then
_start_from_="$1"
fi
local i=0
local first=false
while caller $i > /dev/null
do
if test -n "$_start_from_" && (( "$i" + 1 >= "$_start_from_" ))
then
if test "$first" == false
then
echo "BACKTRACE IS:"
first=true
fi
caller $i
fi
let "i=i+1"
done
}
return 0
Example of usage:
file content: trap-test.sh
#!/bin/bash
source 'lib.trap.sh'
echo "doing something wrong now .."
echo "$foo"
exit 0
Running:
bash trap-test.sh
Output:
doing something wrong now ..
(!) EXIT HANDLER:
FILE: trap-test.sh
LINE: 6
ERROR CODE: 1
ERROR MESSAGE:
foo: unassigned variable
BACKTRACE IS:
1 main trap-test.sh
Exiting!
As you can see from the screenshot below, the output is colored and the error message comes in the used language.
An equivalent alternative to "set -e" is
set -o errexit
It makes the meaning of the flag somewhat clearer than just "-e".
Random addition: to temporarily disable the flag, and return to the default (of continuing execution regardless of exit codes), just use
set +e
echo "commands run here returning non-zero exit codes will not cause the entire script to fail"
echo "false returns 1 as an exit code"
false
set -e
This precludes proper error handling mentioned in other responses, but is quick & effective (just like bash).
Inspired by the ideas presented here, I have developed a readable and convenient way to handle errors in bash scripts in my bash boilerplate project.
By simply sourcing the library, you get the following out of the box (i.e. it will halt execution on any error, as if using set -e thanks to a trap on ERR and some bash-fu):
There are some extra features that help handle errors, such as try and catch, or the throw keyword, that allows you to break execution at a point to see the backtrace. Plus, if the terminal supports it, it spits out powerline emojis, colors parts of the output for great readability, and underlines the method that caused the exception in the context of the line of code.
The downside is - it's not portable - the code works in bash, probably >= 4 only (but I'd imagine it could be ported with some effort to bash 3).
The code is separated into multiple files for better handling, but I was inspired by the backtrace idea from the answer above by Luca Borrione.
To read more or take a look at the source, see GitHub:
https://github.com/niieani/bash-oo-framework#error-handling-with-exceptions-and-throw
I prefer something really easy to call. So I use something that looks a little complicated, but is easy to use. I usually just copy-and-paste the code below into my scripts. An explanation follows the code.
#This function is used to cleanly exit any script. It does this displaying a
# given error message, and exiting with an error code.
function error_exit {
echo
echo "$#"
exit 1
}
#Trap the killer signals so that we can exit with a good message.
trap "error_exit 'Received signal SIGHUP'" SIGHUP
trap "error_exit 'Received signal SIGINT'" SIGINT
trap "error_exit 'Received signal SIGTERM'" SIGTERM
#Alias the function so that it will print a message with the following format:
#prog-name(#line#): message
#We have to explicitly allow aliases, we do this because they make calling the
#function much easier (see example).
shopt -s expand_aliases
alias die='error_exit "Error ${0}(#`echo $(( $LINENO - 1 ))`):"'
I usually put a call to the cleanup function in side the error_exit function, but this varies from script to script so I left it out. The traps catch the common terminating signals and make sure everything gets cleaned up. The alias is what does the real magic. I like to check everything for failure. So in general I call programs in an "if !" type statement. By subtracting 1 from the line number the alias will tell me where the failure occurred. It is also dead simple to call, and pretty much idiot proof. Below is an example (just replace /bin/false with whatever you are going to call).
#This is an example useage, it will print out
#Error prog-name (#1): Who knew false is false.
if ! /bin/false ; then
die "Who knew false is false."
fi
Another consideration is the exit code to return. Just "1" is pretty standard, although there are a handful of reserved exit codes that bash itself uses, and that same page argues that user-defined codes should be in the range 64-113 to conform to C/C++ standards.
You might also consider the bit vector approach that mount uses for its exit codes:
0 success
1 incorrect invocation or permissions
2 system error (out of memory, cannot fork, no more loop devices)
4 internal mount bug or missing nfs support in mount
8 user interrupt
16 problems writing or locking /etc/mtab
32 mount failure
64 some mount succeeded
OR-ing the codes together allows your script to signal multiple simultaneous errors.
I use the following trap code, it also allows errors to be traced through pipes and 'time' commands
#!/bin/bash
set -o pipefail # trace ERR through pipes
set -o errtrace # trace ERR through 'time command' and other functions
function error() {
JOB="$0" # job name
LASTLINE="$1" # line of error occurrence
LASTERR="$2" # error code
echo "ERROR in ${JOB} : line ${LASTLINE} with exit code ${LASTERR}"
exit 1
}
trap 'error ${LINENO} ${?}' ERR
I've used
die() {
echo $1
kill $$
}
before; i think because 'exit' was failing for me for some reason. The above defaults seem like a good idea, though.
This has served me well for a while now. It prints error or warning messages in red, one line per parameter, and allows an optional exit code.
# Custom errors
EX_UNKNOWN=1
warning()
{
# Output warning messages
# Color the output red if it's an interactive terminal
# #param $1...: Messages
test -t 1 && tput setf 4
printf '%s\n' "$#" >&2
test -t 1 && tput sgr0 # Reset terminal
true
}
error()
{
# Output error messages with optional exit code
# #param $1...: Messages
# #param $N: Exit code (optional)
messages=( "$#" )
# If the last parameter is a number, it's not part of the messages
last_parameter="${messages[#]: -1}"
if [[ "$last_parameter" =~ ^[0-9]*$ ]]
then
exit_code=$last_parameter
unset messages[$((${#messages[#]} - 1))]
fi
warning "${messages[#]}"
exit ${exit_code:-$EX_UNKNOWN}
}
Not sure if this will be helpful to you, but I modified some of the suggested functions here in order to include the check for the error (exit code from prior command) within it.
On each "check" I also pass as a parameter the "message" of what the error is for logging purposes.
#!/bin/bash
error_exit()
{
if [ "$?" != "0" ]; then
log.sh "$1"
exit 1
fi
}
Now to call it within the same script (or in another one if I use export -f error_exit) I simply write the name of the function and pass a message as parameter, like this:
#!/bin/bash
cd /home/myuser/afolder
error_exit "Unable to switch to folder"
rm *
error_exit "Unable to delete all files"
Using this I was able to create a really robust bash file for some automated process and it will stop in case of errors and notify me (log.sh will do that)
This trick is useful for missing commands or functions. The name of the missing function (or executable) will be passed in $_
function handle_error {
status=$?
last_call=$1
# 127 is 'command not found'
(( status != 127 )) && return
echo "you tried to call $last_call"
return
}
# Trap errors.
trap 'handle_error "$_"' ERR
This function has been serving me rather well recently:
action () {
# Test if the first parameter is non-zero
# and return straight away if so
if test $1 -ne 0
then
return $1
fi
# Discard the control parameter
# and execute the rest
shift 1
"$#"
local status=$?
# Test the exit status of the command run
# and display an error message on failure
if test ${status} -ne 0
then
echo Command \""$#"\" failed >&2
fi
return ${status}
}
You call it by appending 0 or the last return value to the name of the command to run, so you can chain commands without having to check for error values. With this, this statement block:
command1 param1 param2 param3...
command2 param1 param2 param3...
command3 param1 param2 param3...
command4 param1 param2 param3...
command5 param1 param2 param3...
command6 param1 param2 param3...
Becomes this:
action 0 command1 param1 param2 param3...
action $? command2 param1 param2 param3...
action $? command3 param1 param2 param3...
action $? command4 param1 param2 param3...
action $? command5 param1 param2 param3...
action $? command6 param1 param2 param3...
<<<Error-handling code here>>>
If any of the commands fail, the error code is simply passed to the end of the block. I find it useful when you don't want subsequent commands to execute if an earlier one failed, but you also don't want the script to exit straight away (for example, inside a loop).
Sometimes set -e , trap ERR ,set -o ,set -o pipefail and set -o errtrace not work properly because they attempt to add automatic error detection to the shell. This does not work well in practice.
In my opinion, instead of using set -e and other stuffs, you should write your own error checking code. If you wise to use set -e, be aware of potential gotchas.
To avoid Error while running the code you can use exec 1>/dev/null or exec 2>/dev/null
/dev/null in Linux is a null device file. This will discard anything written to it and will return EOF on reading. you can use this at end of the command
For try/catch you can use && or || to achieve Similar behaviour
use can use && like this
{ # try
command &&
# your command
} || {
# catch exception
}
or you can use if else :
if [[ Condition ]]; then
# if true
else
# if false
fi
$? show output of the last command ,it return 1 or 0
Using trap is not always an option. For example, if you're writing some kind of re-usable function that needs error handling and that can be called from any script (after sourcing the file with helper functions), that function cannot assume anything about exit time of the outer script, which makes using traps very difficult. Another disadvantage of using traps is bad composability, as you risk overwriting previous trap that might be set earlier up in the caller chain.
There is a little trick that can be used to do proper error handling without traps. As you may already know from other answers, set -e doesn't work inside commands if you use || operator after them, even if you run them in a subshell; e.g., this wouldn't work:
#!/bin/sh
# prints:
#
# --> outer
# --> inner
# ./so_1.sh: line 16: some_failed_command: command not found
# <-- inner
# <-- outer
set -e
outer() {
echo '--> outer'
(inner) || {
exit_code=$?
echo '--> cleanup'
return $exit_code
}
echo '<-- outer'
}
inner() {
set -e
echo '--> inner'
some_failed_command
echo '<-- inner'
}
outer
But || operator is needed to prevent returning from the outer function before cleanup. The trick is to run the inner command in background, and then immediately wait for it. The wait builtin will return the exit code of the inner command, and now you're using || after wait, not the inner function, so set -e works properly inside the latter:
#!/bin/sh
# prints:
#
# --> outer
# --> inner
# ./so_2.sh: line 27: some_failed_command: command not found
# --> cleanup
set -e
outer() {
echo '--> outer'
inner &
wait $! || {
exit_code=$?
echo '--> cleanup'
return $exit_code
}
echo '<-- outer'
}
inner() {
set -e
echo '--> inner'
some_failed_command
echo '<-- inner'
}
outer
Here is the generic function that builds upon this idea. It should work in all POSIX-compatible shells if you remove local keywords, i.e. replace all local x=y with just x=y:
# [CLEANUP=cleanup_cmd] run cmd [args...]
#
# `cmd` and `args...` A command to run and its arguments.
#
# `cleanup_cmd` A command that is called after cmd has exited,
# and gets passed the same arguments as cmd. Additionally, the
# following environment variables are available to that command:
#
# - `RUN_CMD` contains the `cmd` that was passed to `run`;
# - `RUN_EXIT_CODE` contains the exit code of the command.
#
# If `cleanup_cmd` is set, `run` will return the exit code of that
# command. Otherwise, it will return the exit code of `cmd`.
#
run() {
local cmd="$1"; shift
local exit_code=0
local e_was_set=1; if ! is_shell_attribute_set e; then
set -e
e_was_set=0
fi
"$cmd" "$#" &
wait $! || {
exit_code=$?
}
if [ "$e_was_set" = 0 ] && is_shell_attribute_set e; then
set +e
fi
if [ -n "$CLEANUP" ]; then
RUN_CMD="$cmd" RUN_EXIT_CODE="$exit_code" "$CLEANUP" "$#"
return $?
fi
return $exit_code
}
is_shell_attribute_set() { # attribute, like "x"
case "$-" in
*"$1"*) return 0 ;;
*) return 1 ;;
esac
}
Example of usage:
#!/bin/sh
set -e
# Source the file with the definition of `run` (previous code snippet).
# Alternatively, you may paste that code directly here and comment the next line.
. ./utils.sh
main() {
echo "--> main: $#"
CLEANUP=cleanup run inner "$#"
echo "<-- main"
}
inner() {
echo "--> inner: $#"
sleep 0.5; if [ "$1" = 'fail' ]; then
oh_my_god_look_at_this
fi
echo "<-- inner"
}
cleanup() {
echo "--> cleanup: $#"
echo " RUN_CMD = '$RUN_CMD'"
echo " RUN_EXIT_CODE = $RUN_EXIT_CODE"
sleep 0.3
echo '<-- cleanup'
return $RUN_EXIT_CODE
}
main "$#"
Running the example:
$ ./so_3 fail; echo "exit code: $?"
--> main: fail
--> inner: fail
./so_3: line 15: oh_my_god_look_at_this: command not found
--> cleanup: fail
RUN_CMD = 'inner'
RUN_EXIT_CODE = 127
<-- cleanup
exit code: 127
$ ./so_3 pass; echo "exit code: $?"
--> main: pass
--> inner: pass
<-- inner
--> cleanup: pass
RUN_CMD = 'inner'
RUN_EXIT_CODE = 0
<-- cleanup
<-- main
exit code: 0
The only thing that you need to be aware of when using this method is that all modifications of Shell variables done from the command you pass to run will not propagate to the calling function, because the command runs in a subshell.
I have a function that runs a set of scripts that set variables, functions, and aliases in the current shell.
reloadVariablesFromScript() {
for script in "${scripts[#]}"; do
. "$script"
done
}
If one of the scripts has an error, I want to exit the script and then exit the function, but not to kill the shell.
reloadVariablesFromScript() {
for script in "${scripts[#]}"; do
{(
set -e
. "$script"
)}
if [[ $? -ne 0 ]]; then
>&2 echo $script failed. Skipping remaining scripts.
return 1
fi
done
}
This would do what I want except it doesn't set the variables in the script whether the script succeeds or fails.
Without the subshell, set -e causes the whole shell to exit, which is undesirable.
Is there a way I can either prevent the called script from continuing on an error without killing the shell or else set/export variables, aliases, and functions from within a subshell?
The following script simulates my problem:
test() {
{(
set -e
export foo=bar
false
echo Should not have gotten here!
export bar=baz
)}
local errorCode=$?
echo foo="'$foo'". It should equal 'bar'.
echo bar="'$bar'". It should not be set.
if [[ $errorCode -ne 0 ]]; then
echo Script failed correctly. Exiting function.
return 1
fi
echo Should not have gotten here!
}
test
If worst comes to worse, since these scripts don't actually edit the filesystem, I can run each script in a subshell, check the exit code, and if it succeeds, run it outside of a subshell.
Note that set -e has a number of surprising behaviors -- relying on it is not universally considered a good idea. That caveat being give, though: We can shuffle environment variables, aliases, and shell functions out as text:
envTest() {
local errorCode newVars
newVars=$(
set -e
{
export foo=bar
false
echo Should not have gotten here!
export bar=baz
} >&2
# print generate code which, when eval'd, recreates our functions and variables
declare -p | egrep -v '^declare -[^[:space:]]*r'
declare -f
alias -p
); errorCode=$?
if (( errorCode == 0 )); then
eval "$newVars"
fi
printf 'foo=%q. It should equal %q\n' "$foo" "bar"
printf 'bar=%q. It should not be set.\n' "$bar"
if [[ $errorCode -ne 0 ]]; then
echo 'Script failed correctly. Exiting function.'
return 1
fi
echo 'Should not have gotten here!'
}
envTest
Note that this code only evaluates either export should the entire script segment succeed; the question text and comments appear to indicate that this is acceptable if not desired.
The following script calls another program reading its output in a while loop (see Bash - How to pipe input to while loop and preserve variables after loop ends):
while read -r col0 col1; do
# [...]
done < <(other_program [args ...])
How can I check for the exit code of other_program to see if the loop was executed properly?
Note: ls -d / /nosuch is used as an example command below, because it fails (exit code 1) while still producing stdout output (/) (in addition to stderr output).
Bash v4.2+ solution:
ccarton's helpful answer works well in principle, but by default the while loop runs in a subshell, which means that any variables created or modified in the loop will not be visible to the current shell.
In Bash v4.2+, you can change this by turning the lastpipe option on, which makes the last segment of a pipeline run in the current shell;
as in ccarton's answer, the pipefail option must be set to have $? reflect the exit code of the first failing command in the pipeline:
shopt -s lastpipe # run the last segment of a pipeline in the current shell
shopt -so pipefail # reflect a pipeline's first failing command's exit code in $?
ls -d / /nosuch | while read -r line; do
result=$line
done
echo "result: [$result]; exit code: $?"
The above yields (stderr output omitted):
result: [/]; exit code: 1
As you can see, the $result variable, set in the while loop, is available, and the ls command's (nonzero) exit code is reflected in $?.
Bash v3+ solution:
ikkachu's helpful answer works well and shows advanced techniques, but it is a bit cumbersome.
Here is a simpler alternative:
while read -r line || { ec=$line && break; }; do # Note the `|| { ...; }` part.
result=$line
done < <(ls -d / /nosuch; printf $?) # Note the `; printf $?` part.
echo "result: [$result]; exit code: $ec"
By appending the value of $?, the ls command's exit code, to the output without a trailing \n (printf $?), read reads it in the last loop operation, but indicates failure (exit code 1), which would normally exit the loop.
We can detect this case with ||, and assign the exit code (that was still read into $line) to variable $ec and exit the loop then.
On the off chance that the command's output doesn't have a trailing \n, more work is needed:
while read -r line ||
{ [[ $line =~ ^(.*)/([0-9]+)$ ]] && ec=${BASH_REMATCH[2]} && line=${BASH_REMATCH[1]};
[[ -n $line ]]; }
do
result=$line
done < <(printf 'no trailing newline'; ls /nosuch; printf "/$?")
echo "result: [$result]; exit code: $ec"
The above yields (stderr output omitted):
result: [no trailing newline]; exit code: 1
At least one way would be to redirect the output of the background process through a named pipe. This would allow to pick up its PID and then get the exit status through waiting on the PID.
#!/bin/bash
mkfifo pipe || exit 1
(echo foo ; exit 19) > pipe &
pid=$!
while read x ; do echo "read: $x" ; done < pipe
wait $pid
echo "exit status of bg process: $?"
rm pipe
If you can use a direct pipe (i.e. don't mind the loop being run in a subshell), you could use Bash's PIPESTATUS, which contains the exit codes of all commands in the pipeline:
(echo foo ; exit 19) | while read x ; do
echo "read: $x" ; done;
echo "status: ${PIPESTATUS[0]}"
A simple way is to use the bash pipefail option to propagate the first error code from a pipeline.
set -o pipefail
other_program | while read x; do
echo "Read: $x"
done || echo "Error: $?"
Another way is to use coproc (requires 4.0+).
coproc other_program [args ...]
while read -r -u ${COPROC[0]} col0 col1; do
# [...]
done
wait $COPROC_PID || echo "Error exit status: $?"
coproc frees you from having to setup asynchronicity and stdin/stdout redirection that you'd otherwise need to do in an equivalent mkfifo.
When I use the trap command in Bash, the previous trap for the given signal is replaced.
Is there a way of making more than one trap fire for the same signal?
Technically you can't set multiple traps for the same signal, but you can add to an existing trap:
Fetch the existing trap code using trap -p
Add your command, separated by a semicolon or newline
Set the trap to the result of #2
Here is a bash function that does the above:
# note: printf is used instead of echo to avoid backslash
# processing and to properly handle values that begin with a '-'.
log() { printf '%s\n' "$*"; }
error() { log "ERROR: $*" >&2; }
fatal() { error "$#"; exit 1; }
# appends a command to a trap
#
# - 1st arg: code to add
# - remaining args: names of traps to modify
#
trap_add() {
trap_add_cmd=$1; shift || fatal "${FUNCNAME} usage error"
for trap_add_name in "$#"; do
trap -- "$(
# helper fn to get existing trap command from output
# of trap -p
extract_trap_cmd() { printf '%s\n' "$3"; }
# print existing trap command with newline
eval "extract_trap_cmd $(trap -p "${trap_add_name}")"
# print the new trap command
printf '%s\n' "${trap_add_cmd}"
)" "${trap_add_name}" \
|| fatal "unable to add to trap ${trap_add_name}"
done
}
# set the trace attribute for the above function. this is
# required to modify DEBUG or RETURN traps because functions don't
# inherit them unless the trace attribute is set
declare -f -t trap_add
Example usage:
trap_add 'echo "in trap DEBUG"' DEBUG
Edit:
It appears that I misread the question. The answer is simple:
handler1 () { do_something; }
handler2 () { do_something_else; }
handler3 () { handler1; handler2; }
trap handler3 SIGNAL1 SIGNAL2 ...
Original:
Just list multiple signals at the end of the command:
trap function-name SIGNAL1 SIGNAL2 SIGNAL3 ...
You can find the function associated with a particular signal using trap -p:
trap -p SIGINT
Note that it lists each signal separately even if they're handled by the same function.
You can add an additional signal given a known one by doing this:
eval "$(trap -p SIGUSR1) SIGUSR2"
This works even if there are other additional signals being processed by the same function. In other words, let's say a function was already handling three signals - you could add two more just by referring to one existing one and appending two more (where only one is shown above just inside the closing quotes).
If you're using Bash >= 3.2, you can do something like this to extract the function given a signal. Note that it's not completely robust because other single quotes could appear.
[[ $(trap -p SIGUSR1) =~ trap\ --\ \'([^\047]*)\'.* ]]
function_name=${BASH_REMATCH[1]}
Then you could rebuild your trap command from scratch if you needed to using the function name, etc.
No
About the best you could do is run multiple commands from a single trap for a given signal, but you cannot have multiple concurrent traps for a single signal. For example:
$ trap "rm -f /tmp/xyz; exit 1" 2
$ trap
trap -- 'rm -f /tmp/xyz; exit 1' INT
$ trap 2
$ trap
$
The first line sets a trap on signal 2 (SIGINT). The second line prints the current traps — you would have to capture the standard output from this and parse it for the signal you want.
Then, you can add your code to what was already there — noting that the prior code will most probably include an 'exit' operation. The third invocation of trap clears the trap on 2/INT. The last one shows that there are no traps outstanding.
You can also use trap -p INT or trap -p 2 to print the trap for a specific signal.
I didn't like having to play with these string manipulations which are confusing at the best of times, so I came up with something like this:
(obviously you can modify it for other signals)
exit_trap_command=""
function cleanup {
eval "$exit_trap_command"
}
trap cleanup EXIT
function add_exit_trap {
local to_add=$1
if [[ -z "$exit_trap_command" ]]
then
exit_trap_command="$to_add"
else
exit_trap_command="$exit_trap_command; $to_add"
fi
}
Here's another option:
on_exit_acc () {
local next="$1"
eval "on_exit () {
local oldcmd='$(echo "$next" | sed -e s/\'/\'\\\\\'\'/g)'
local newcmd=\"\$oldcmd; \$1\"
trap -- \"\$newcmd\" 0
on_exit_acc \"\$newcmd\"
}"
}
on_exit_acc true
Usage:
$ on_exit date
$ on_exit 'echo "Goodbye from '\''`uname`'\''!"'
$ exit
exit
Sat Jan 18 18:31:49 PST 2014
Goodbye from 'FreeBSD'!
tap#
I liked Richard Hansen's answer, but I don't care for embedded functions so an alternate is:
#===================================================================
# FUNCTION trap_add ()
#
# Purpose: appends a command to a trap
#
# - 1st arg: code to add
# - remaining args: names of traps to modify
#
# Example: trap_add 'echo "in trap DEBUG"' DEBUG
#
# See: http://stackoverflow.com/questions/3338030/multiple-bash-traps-for-the-same-signal
#===================================================================
trap_add() {
trap_add_cmd=$1; shift || fatal "${FUNCNAME} usage error"
new_cmd=
for trap_add_name in "$#"; do
# Grab the currently defined trap commands for this trap
existing_cmd=`trap -p "${trap_add_name}" | awk -F"'" '{print $2}'`
# Define default command
[ -z "${existing_cmd}" ] && existing_cmd="echo exiting # `date`"
# Generate the new command
new_cmd="${existing_cmd};${trap_add_cmd}"
# Assign the test
trap "${new_cmd}" "${trap_add_name}" || \
fatal "unable to add to trap ${trap_add_name}"
done
}
I have been wrote a set of functions for myself to a bit resolve this task in a convenient way.
Update: The implementation here is obsoleted and left here as a demonstration. The new implementation is more complex, having dependencies, supports a wider range of cases and quite big to be placed here.
New implementation: https://sf.net/p/tacklelib/tacklelib/HEAD/tree/trunk/bash/tacklelib/traplib.sh
Here is the list of features of the new implementation:
Pros:
Automatically restores the previous trap handler in nested functions.
Originally the RETURN trap restores ONLY if ALL functions in the stack did set it.
The RETURN signal trap can support other signal traps to achieve the RAII pattern as in other languages.
For example, to temporary disable interruption handling and auto restore it at the end of a function
while an initialization code is executing.
Protection from call not from a function context in case of the RETURN signal trap.
The not RETURN signal handlers in the whole stack invokes together in a bash process from the
bottom to the top and executes them in order reversed to the tkl_push_trap function calls
The RETURN signal trap handlers invokes only for a single function from the bottom to the top in
reverse order to the tkl_push_trap function calls.
Because the EXIT signal does not trigger the RETURN signal trap handler, then the EXIT signal trap
handler does setup automatically at least once per bash process when the RETURN signal trap handler
makes setup at first time in a bash process.
That includes all bash processes, for example, represented as (...) or $(...) operators.
So the EXIT signal trap handlers automatically handles all the RETURN trap handlers before to run itself.
The RETURN signal trap handler still can call to tkl_push_trap and tkl_pop_trap functions to process
the not RETURN signal traps
The RETURN signal trap handler can call to tkl_set_trap_postponed_exit function from both the EXIT and
RETURN signal trap handlers.
If is called from the RETURN signal trap handler, then the EXIT trap handler will be called after all the
RETURN signal trap handlers in the bash process.
If is called from the EXIT signal trap handler, then the EXIT trap handler will change the exit code after
the last EXIT signal trap handler is invoked.
Faster access to trap stack as a global variable instead of usage the (...) or $(...) operators
which invokes an external bash process.
The source command ignores by the RETURN signal trap handler, so all calls to the source command will not
invoke the RETURN signal trap user code (marked in the Pros, because RETURN signal trap handler has to be
called only after return from a function in the first place and not from a script inclusion).
Cons:
You must not use builtin trap command in the handler passed to the tkl_push_trap function as tkl_*_trap functions
does use it internally.
You must not use builtin exit command in the EXIT signal handlers while the EXIT signal trap
handler is running. Otherwise that will leave the rest of the RETURN and EXIT signal trap handlers not executed.
To change the exit code from the EXIT handler you can use tkl_set_trap_postponed_exit function for that.
You must not use builtin return command in the RETURN signal trap handler while the RETURN signal trap handler
is running. Otherwise that will leave the rest of the RETURN and EXIT signal trap handlers not executed.
All calls to the tkl_push_trap and tkl_pop_trap functions has no effect if has been called from a trap
handler for a signal the trap handler is handling (recursive call through the signal).
You have to replace all builtin trap commands in nested or 3dparty scripts by tkl_*_trap functions if
already using the library.
The source command ignores by the RETURN signal trap handler, so all calls to the source command will not
invoke the RETURN signal trap user code (marked in the Cons, because of losing the back compatability here).
Old implementation:
traplib.sh
#!/bin/bash
# Script can be ONLY included by "source" command.
if [[ -n "$BASH" && (-z "$BASH_LINENO" || ${BASH_LINENO[0]} -gt 0) ]] && (( ! ${#SOURCE_TRAPLIB_SH} )); then
SOURCE_TRAPLIB_SH=1 # including guard
function GetTrapCmdLine()
{
local IFS=$' \t\r\n'
GetTrapCmdLineImpl RETURN_VALUES "$#"
}
function GetTrapCmdLineImpl()
{
local out_var="$1"
shift
# drop return values
eval "$out_var=()"
local IFS
local trap_sig
local stack_var
local stack_arr
local trap_cmdline
local trap_prev_cmdline
local i
i=0
IFS=$' \t\r\n'; for trap_sig in "$#"; do
stack_var="_traplib_stack_${trap_sig}_cmdline"
declare -a "stack_arr=(\"\${$stack_var[#]}\")"
if (( ${#stack_arr[#]} )); then
for trap_cmdline in "${stack_arr[#]}"; do
declare -a "trap_prev_cmdline=(\"\${$out_var[i]}\")"
if [[ -n "$trap_prev_cmdline" ]]; then
eval "$out_var[i]=\"\$trap_cmdline; \$trap_prev_cmdline\"" # the last srored is the first executed
else
eval "$out_var[i]=\"\$trap_cmdline\""
fi
done
else
# use the signal current trap command line
declare -a "trap_cmdline=(`trap -p "$trap_sig"`)"
eval "$out_var[i]=\"\${trap_cmdline[2]}\""
fi
(( i++ ))
done
}
function PushTrap()
{
# drop return values
EXIT_CODES=()
RETURN_VALUES=()
local cmdline="$1"
[[ -z "$cmdline" ]] && return 0 # nothing to push
shift
local IFS
local trap_sig
local stack_var
local stack_arr
local trap_cmdline_size
local prev_cmdline
IFS=$' \t\r\n'; for trap_sig in "$#"; do
stack_var="_traplib_stack_${trap_sig}_cmdline"
declare -a "stack_arr=(\"\${$stack_var[#]}\")"
trap_cmdline_size=${#stack_arr[#]}
if (( trap_cmdline_size )); then
# append to the end is equal to push trap onto stack
eval "$stack_var[trap_cmdline_size]=\"\$cmdline\""
else
# first stack element is always the trap current command line if not empty
declare -a "prev_cmdline=(`trap -p $trap_sig`)"
if (( ${#prev_cmdline[2]} )); then
eval "$stack_var=(\"\${prev_cmdline[2]}\" \"\$cmdline\")"
else
eval "$stack_var=(\"\$cmdline\")"
fi
fi
# update the signal trap command line
GetTrapCmdLine "$trap_sig"
trap "${RETURN_VALUES[0]}" "$trap_sig"
EXIT_CODES[i++]=$?
done
}
function PopTrap()
{
# drop return values
EXIT_CODES=()
RETURN_VALUES=()
local IFS
local trap_sig
local stack_var
local stack_arr
local trap_cmdline_size
local trap_cmd_line
local i
i=0
IFS=$' \t\r\n'; for trap_sig in "$#"; do
stack_var="_traplib_stack_${trap_sig}_cmdline"
declare -a "stack_arr=(\"\${$stack_var[#]}\")"
trap_cmdline_size=${#stack_arr[#]}
if (( trap_cmdline_size )); then
(( trap_cmdline_size-- ))
RETURN_VALUES[i]="${stack_arr[trap_cmdline_size]}"
# unset the end
unset $stack_var[trap_cmdline_size]
(( !trap_cmdline_size )) && unset $stack_var
# update the signal trap command line
if (( trap_cmdline_size )); then
GetTrapCmdLineImpl trap_cmd_line "$trap_sig"
trap "${trap_cmd_line[0]}" "$trap_sig"
else
trap "" "$trap_sig" # just clear the trap
fi
EXIT_CODES[i]=$?
else
# nothing to pop
RETURN_VALUES[i]=""
fi
(( i++ ))
done
}
function PopExecTrap()
{
# drop exit codes
EXIT_CODES=()
local IFS=$' \t\r\n'
PopTrap "$#"
local cmdline
local i
i=0
IFS=$' \t\r\n'; for cmdline in "${RETURN_VALUES[#]}"; do
# execute as function and store exit code
eval "function _traplib_immediate_handler() { $cmdline; }"
_traplib_immediate_handler
EXIT_CODES[i++]=$?
unset _traplib_immediate_handler
done
}
fi
test.sh
#/bin/bash
source ./traplib.sh
function Exit()
{
echo exitting...
exit $#
}
pushd ".." && {
PushTrap "echo popd; popd" EXIT
echo 111 || Exit
PopExecTrap EXIT
}
GetTrapCmdLine EXIT
echo -${RETURN_VALUES[#]}-
pushd ".." && {
PushTrap "echo popd; popd" EXIT
echo 222 && Exit
PopExecTrap EXIT
}
Usage
cd ~/test
./test.sh
Output
~ ~/test
111
popd
~/test
--
~ ~/test
222
exitting...
popd
~/test
In this answer I implemented a simple solution. Here I implement another solution that is based on extracting of previous trap commands from trap -p output. But I don't know how much it is portable because I'm not sure that trap -p output is regulated. Maybe its format can be changed in future (but I doubt that).
trap_add()
{
local new="$1"
local sig="$2"
# Get raw trap output.
local old="$(trap -p "$sig")"
# Extract previous commands from raw trap output.
old="${old#*\'}" # Remove first ' and everything before it.
old="${old%\'*}" # Remove last ' and everything after it.
old="${old//\'\\\'\'/\'}" # Replace every '\'' (escaped ') to just '.
# Combine new and old commands. Separate them by newline.
trap -- "$new
$old" "$sig"
}
trap_add 'echo AAA' EXIT
trap_add '{ echo BBB; }' EXIT
But this solution doesn't work well with subshells. Unfortunately trap -p prints commands of outer shell. And we execute them in subshell after extracting.
trap_add 'echo AAA' EXIT
( trap_add 'echo BBB' EXIT; )
In the above example echo AAA is executed twice: first time in subshell and second time in outer shell.
We have to check whether we are in new subshell and if we are then we must not take commands from trap -p.
trap_add()
{
local new="$1"
# Avoid inheriting trap commands from outer shell.
if [[ "${trap_subshell:-}" != "$BASH_SUBSHELL" ]]; then
# We are in new subshell, don't take commands from outer shell.
trap_subshell="$BASH_SUBSHELL"
local old=
else
# Get raw trap output.
local old="$(trap -p EXIT)"
# Extract previous commands from trap output.
old="${old#*\'}" # Remove first ' and everything before it.
old="${old%\'*}" # Remove last ' and everything after it.
old="${old//\'\\\'\'/\'}" # Replace every '\'' (escaped ') to just '.
fi
# Combine new and old commands. Separate them by newline.
trap -- "$new
$old" EXIT
}
Note that to avoid security issue you have to reset the trap_subshell variable at script startup.
trap_subshell=
Unfortunately the solution above works only with EXIT signal now. A generic solution that works with any signal is below.
# Check if argument is number.
is_num()
{
[ -n "$1" ] && [ "$1" -eq "$1" ] 2>/dev/null
}
# Convert signal name to signal number.
to_sig_num()
{
if is_num "$1"; then
# Signal is already number.
kill -l "$1" >/dev/null # Check that signal number is valid.
echo "$1" # Return result.
else
# Convert to signal number.
kill -l "$1"
fi
}
trap_add()
{
local new="$1"
local sig="$2"
local sig_num
sig_num=$(to_sig_num "$sig")
# Avoid inheriting trap commands from outer shell.
if [[ "${trap_subshell[$sig_num]:-}" != "$BASH_SUBSHELL" ]]; then
# We are in new subshell, don't take commands from outer shell.
trap_subshell[$sig_num]="$BASH_SUBSHELL"
local old=
else
# Get raw trap output.
local old="$(trap -p "$sig")"
# Extract previous commands from trap output.
old="${old#*\'}" # Remove first ' and everything before it.
old="${old%\'*}" # Remove last ' and everything after it.
old="${old//\'\\\'\'/\'}" # Replace every '\'' (escaped ') to just '.
fi
# Combine new and old commands. Separate them by newline.
trap -- "$new
$old" "$sig"
}
trap_subshell=
trap_add 'echo AAA' EXIT
trap_add '{ echo BBB; }' 0 # The same as EXIT.
There's no way to have multiple handlers for the same trap, but the same handler can do multiple things.
The one thing I don't like in the various other answers doing the same thing is the use of string manipulation to get at the current trap function. There are two easy ways of doing this: arrays and arguments. Arguments is the most reliable one, but I'll show arrays first.
Arrays
When using arrays, you rely on the fact that trap -p SIGNAL returns trap -- ??? SIGNAL, so whatever is the value of ???, there are three more words in the array.
Therefore you can do this:
declare -a trapDecl
trapDecl=($(trap -p SIGNAL))
currentHandler="${trapDecl[#]:2:${#trapDecl[#]} - 3}"
eval "trap -- 'your handler;'${currentHandler} SIGNAL"
So let's explain this. First, variable trapDecl is declared as an array. If you do this inside a function, it will also be local, which is convenient.
Next we assign the output of trap -p SIGNAL to the array. To give an example, let's say you are running this after having sourced osht (unit testing for shell), and that the signal is EXIT. The output of trap -p EXIT will be trap -- '_osht_cleanup' EXIT, so the trapDecl assignment will be substituted like this:
trapDecl=(trap -- '_osht_cleanup' EXIT)
The parenthesis there are normal array assignment, so trapDecl becomes an array with four elements: trap, --, '_osht_cleanup' and EXIT.
Next we extract the current handler -- that could be inlined in the next line, but for explanation's sake I assigned it to a variable first. Simplifying that line, I'm doing this: currentHandler="${array[#]:offset:length}", which is the syntax used by Bash to say pick length elements starting at element offset. Since it starts counting from 0, number 2 will be '_osht_cleanup'. Next, ${#trapDecl[#]} is the number of elements inside trapDecl, which will be 4 in the example. You subtract 3 because there are three elements you don't want: trap, -- and EXIT. I don't need to use $(...) around that expression because arithmetic expansion is already performed on the offset and length arguments.
The final line performs an eval, which is used so that the shell will interpret the quoting from the output of trap. If we do parameter substitution on that line, it expands to the following in the example:
eval "trap -- 'your handler;''_osht_cleanup' EXIT"
Do not be confused by the double quote in the middle (''). Bash simply concatenates two quotes strings if they are next to each other. For example, '1'"2"'3''4' is expanded to 1234 by Bash. Or, to give a more interesting example, 1" "2 is the same thing as "1 2". So eval takes that string and evaluates it, which is equivalent to executing this:
trap -- 'your handler;''_osht_cleanup' EXIT
And that will handle the quoting correctly, turning everything between -- and EXIT into a single parameter.
To give a more complex example, I'm prepending a directory clean up to the osht handler, so my EXIT signal now has this:
trap -- 'rm -fr '\''/var/folders/7d/qthcbjz950775d6vn927lxwh0000gn/T/tmp.CmOubiwq'\'';_osht_cleanup' EXIT
If you assign that to trapDecl, it will have size 6 because of the spaces on the handler. That is, 'rm is one element, and so is -fr, instead of 'rm -fr ...' being a single element.
But currentHandler will get all three elements (6 - 3 = 3), and the quoting will work out when eval is run.
Arguments
Arguments just skips all the array handling part and uses eval up front to get the quoting right. The downside is that you replace the positional arguments on bash, so this is best done from a function. This is the code, though:
eval "set -- $(trap -p SIGNAL)"
trap -- "your handler${3:+;}${3}" SIGNAL
The first line will set the positional arguments to the output of trap -p SIGNAL. Using the example from the Arrays section, $1 will be trap, $2 will be --, $3 will be _osht_cleanup (no quotes!), and $4 will be EXIT.
The next line is pretty straightforward, except for ${3:+;}. The ${X:+Y} syntax means "output Y if the variable X is unset or null". So it expands to ; if $3 is set, or nothing otherwise (if there was no previous handler for SIGNAL).
Simple ways to do it
If all the signal handling functions are known at the same time, then the following is sufficient (has said by Jonathan):
trap 'handler1;handler2;handler3' EXIT
Else, if there is an existing handler(s) that should stay, then new handlers can easily be added like this:
trap "$( trap -p EXIT | cut -f2 -d \' );newHandler" EXIT
If you don't know if there are existing handlers but want to keep them in this case, do the following:
handlers="$( trap -p EXIT | cut -f2 -d \' )"
trap "${handlers}${handlers:+;}newHandler" EXIT
It can be factorized in a function like that:
trap-add() {
local sig="${2:?Signal required}"
hdls="$( trap -p ${sig} | cut -f2 -d \' )";
trap "${hdls}${hdls:+;}${1:?Handler required}" "${sig}"
}
export -f trap-add
Usage:
trap-add 'echo "Bye bye"' EXIT
trap-add 'echo "See you next time"' EXIT
Remark : This works only as long as the handlers are function names, or simple instructions that did not contain any simple code (simple code conflicts with cut -f2 -d \').
I add a slightly more robust version of Laurent Simon's trap-add script:
Allows using arbitrary commands as trap, including such with ' characters
Works only in bash; It could be rewritten with sed instead of bash pattern substitution, but that would make it significantly slower.
Still suffers from causing unwanted inheritance of the traps in subshells.
trap-add () {
local handler=$(trap -p "$2")
handler=${handler/trap -- \'/} # /- Strip `trap '...' SIGNAL` -> ...
handler=${handler%\'*} # \-
handler=${handler//\'\\\'\'/\'} # <- Unquote quoted quotes ('\'')
trap "${handler} $1;" "$2"
}
I would like to propose my solution of multiple trap functions for simple scripts
# Executes cleanup functions on exit
function on_exit {
for FUNCTION in $(declare -F); do
if [[ ${FUNCTION} == *"_on_exit" ]]; then
>&2 echo ${FUNCTION}
eval ${FUNCTION}
fi
done
}
trap on_exit EXIT
function remove_fifo_on_exit {
>&2 echo Removing FIFO...
}
function stop_daemon_on_exit {
>&2 echo Stopping daemon...
}
Just like to add my simple version as an example.
trap -- 'echo "Version 1";' EXIT;
function add_to_trap {
local -a TRAP;
# this will put parts of trap command into TRAP array
eval "TRAP=($(trap -p EXIT))";
# 3rd field is trap command. Concat strings.
trap -- 'echo "Version 2"; '"${TRAP[2]}" EXIT;
}
add_to_trap;
If this code is run, will print:
Version 2
Version 1
Here's how I usually do it. It's not much different from what other people have suggested here but my version seems dramatically simpler and so far it always worked as desired for me.
Somewhere in the code, set a trap:
trap "echo Hello" EXIT
and later on update it:
oldTrap=$(trap -p EXIT)
oldTrap=${oldTrap#"trap -- '"}
oldTrap=${oldTrap%"' EXIT"};
trap "$oldTrap; echo World" EXIT
finally, on exit
Hello
World
This is a simple and compact solution to run multiple trap's by executing all functions that start with the name trap_:
trap 'eval $(declare -F | grep -oP "trap_[^ ]+" | tr "\n" ";")' EXIT
Now simply add as many trap functions as you like::
# write stdout and stderr to a log file
exec &> >(tee -a "/var/log/scripts/${0//\//_}.log")
trap_shrink_logs() { echo "$(tail -n 1000 "/var/log/scripts/${0//\//_}.log")" > "/var/log/scripts/${0//\//_}.log" }
# make script race condition safe
[[ -d "/tmp/${0//\//_}" ]] || ! mkdir "/tmp/${0//\//_}" && echo "Already running!" && exit 1
trap_remove_lock() { rmdir "/tmp/${0//\//_}"; }
A special case of Richard Hansen's answer (great idea). I usually need it for EXIT traps. In such case:
extract_trap_cmd() { printf '%s\n' "${3-}"; }
get_exit_trap_cmd() {
eval "extract_trap_cmd $(trap -p EXIT)"
}
...
trap "echo '1 2'; $(get_exit_trap_cmd)" EXIT
...
trap "echo '3 4'; $(get_exit_trap_cmd)" EXIT
Or this way, if you will:
add_exit_trap_handler() {
trap "$1; $(get_exit_trap_cmd)" EXIT
}
...
add_exit_trap_handler "echo '5 6'"
...
add_exit_trap_handler "echo '7 8'"
Always assuming I remember to pass multiple code snippets in semi-colon delimited fashion (as per bash(1)s' requirement for multiple commands on a single line, it's rare that the following (or something similar to it) fails to fulfil my meagre requirements...
extend-trap() {
local sig=${1:?'Der, you forgot the sig!!!!'}
shift
local code s ; while IFS=\' read code s ; do
code="$code ; $*"
done < <(trap -p $sig)
trap "$code" $sig
}
I'd like something simpler... :)
My humble contrib:
#!/bin/bash
# global vars
TRAPS=()
# functions
function add_exit_trap() { TRAPS+=("$#") }
function execute_exit_traps() {
local I
local POSITIONS=${!TRAPS[#]} # retorna os índices válidos do array
for I in $POSITIONS
do
echo "executing TRAPS[$I]=${TRAPS[I]}"
eval ${TRAPS[I]}
done
}
# M A I N
LOCKFILE="/tmp/lock.me.1234567890"
touch $LOCKFILE
trap execute_exit_traps EXIT
add_exit_trap "rm -f $LOCKFILE && echo lock removed."
add_exit_trap "ls -l $LOCKFILE"
add_exit_trap "echo END"
echo "showing lockfile..."
ls -l $LOCKFILE
add_exit_trap() keeps adding strings (commands) to a bash global array while
execute_exit_traps() just loops thru that array and eval the commands
Executed script...
showing lockfile...
-rw-r--r--. 1 root root 0 Mar 24 10:08 /tmp/lock.me.1234567890
executing TRAPS[0]=rm -f /tmp/lock.me.1234567890 && echo lock removed.
lock removed.
executing TRAPS[1]=ls -l /tmp/lock.me.1234567890
ls: cannot access /tmp/lock.me.1234567890: No such file or directory
executing TRAPS[2]=echo END
END
A simple solution is to save commands for trap to variable and, when adding new trap, to restore them from that variable.
trap_add()
{
# Combine new and old commands. Separate them by newline.
trap_cmds="$1
$trap_cmds"
trap -- "$trap_cmds" EXIT
}
trap_add 'echo AAA'
trap_add '{ echo BBB; }'
Unfortunately this solution does not work well with subshells, because subshell inherits outer shell variables and thus outer shell trap commands are executed in subshell.
trap_add 'echo AAA'
( trap_add 'echo BBB'; )
In the above example echo AAA is executed twice: first time in subshell and second time in outer shell.
We have to check whether we are in new subshell and if we are then we must not take commands from the trap_cmds variable.
trap_add()
{
# Avoid inheriting trap commands from outer shell.
if [[ "${trap_subshell:-}" != "$BASH_SUBSHELL" ]]; then
# We are in new subshell, don't take commands from outer shell.
trap_subshell="$BASH_SUBSHELL"
trap_cmds=
fi
# Combine new and old commands. Separate them by newline.
trap_cmds="$1
$trap_cmds"
trap -- "$trap_cmds" EXIT
}
Note that to avoid security issue you have to reset the used variables at script startup.
trap_subshell=
trap_cmds=
Otherwise someone who runs your script can inject their malicious commands via environment variables.
export trap_subshell=0
export trap_cmds='echo "I hacked you"'
./your_script
Generic version that works with arbitrary signals is below.
# Check if argument is number.
is_num()
{
[ -n "$1" ] && [ "$1" -eq "$1" ] 2>/dev/null
}
# Convert signal name to signal number.
to_sig_num()
{
if is_num "$1"; then
# Signal is already number.
kill -l "$1" >/dev/null # Check that signal number is valid.
echo "$1" # Return result.
else
# Convert to signal number.
kill -l "$1"
fi
}
trap_add()
{
local cmd sig sig_num
cmd="$1"
sig="$2"
sig_num=$(to_sig_num "$sig")
# Avoid inheriting trap commands from outer shell.
if [[ "${trap_subshell[$sig_num]:-}" != "$BASH_SUBSHELL" ]]; then
# We are in new subshell, don't take commands from outer shell.
trap_subshell[$sig_num]="$BASH_SUBSHELL"
trap_cmds[$sig_num]=
fi
# Combine new and old commands. Separate them by newline.
trap_cmds[$sig_num]="$cmd
${trap_cmds[$sig_num]}"
trap -- "${trap_cmds[$sig_num]}" $sig
}
trap_subshell=
trap_cmds=
trap_add 'echo AAA' EXIT
trap_add '{ echo BBB; }' 0 # The same as EXIT.
PS In this answer I implemented another solution that gets previous commands from trap -p output.
There's two actually correct answers.
Answer #1: Create a subshell.
## some code that may contain other traps
(
trap 'whatever' EXIT HUP INT ABRT TERM
somecommand
) # 'whatever' executed when leaving the subshell.
# Previous trap(s) restored.
Do note the documentation about traps inheritance:
Signals that were ignored on entry to a non-interactive shell cannot be trapped or reset, although no error need be reported when attempting to do so.
Answer #2: Save and restore traps.
When the trap command is invoked without arguments, it will print entire list of traps formatted for shell re-evaluation. Thus, if subshell is not an option, something like this could be done:
save_traps=$(trap)
trap 'something' HUP INT ABRT TERM
somecommand
eval "$save_traps"
But the root cause of the OP is most likely an XY problem. Suppose you have various commands called in different modules of your script and producing residue temp files across filesystem. Being a good citizen, you want to cleanup after yourself. And you employ trap to call cleanup tack. Instead of trying to stack traps, stack your cleanup list:
_cleanup_list=
_cleanup() {
case "$1" in
add)
shift
for file; do _cleanup_list="${_cleanup_list:+$_cleanup_list }'$file'"; done
;;
clear)
for file in $_cleanup_list; do eval "[ -f $file ] && rm $file"; done
;;
esac
}
trap '_cleanup clear' EXIT
Richard Hansen's answer is definitely the best, but in case you don't want to use an embedded function, here's an alternative:
function foo
{
trap "echo outer" EXIT
echo "in foo: $(trap -p EXIT)"
bar
echo "post bar: $(trap -p EXIT)"
}
function bar
{
# this line is the money shot:
trap "echo inner ; $(eval "( set -- $(trap -p EXIT) ; echo \$3 )")" EXIT
echo "in bar: $(trap -p EXIT)"
}
( foo )
which produces this output:
in foo: trap -- 'echo outer' EXIT
in bar: trap -- 'echo inner ; echo outer' EXIT
post bar: trap -- 'echo inner ; echo outer' EXIT
inner
outer
Generalization to a reusable function left as an exercise for the reader.
Reasons you might not want to use an embedded function:
I suppose it's slightly inefficient, as the function is defined every time its containing function executes. I suspect, however, that this inefficiency is so negligible as to be irrelevant.
More importantly, embedded functions are not localized to the containing function. That means that you're polluting your caller's namespace, and maybe even overwriting one of their functions. That seems like a Bad Idea™.
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What is your favorite method to handle errors in Bash?
The best example of handling errors I have found on the web was written by William Shotts, Jr at http://www.linuxcommand.org.
He suggests using the following function for error handling in Bash:
#!/bin/bash
# A slicker error handling routine
# I put a variable in my scripts named PROGNAME which
# holds the name of the program being run. You can get this
# value from the first item on the command line ($0).
# Reference: This was copied from <http://www.linuxcommand.org/wss0150.php>
PROGNAME=$(basename $0)
function error_exit
{
# ----------------------------------------------------------------
# Function for exit due to fatal program error
# Accepts 1 argument:
# string containing descriptive error message
# ----------------------------------------------------------------
echo "${PROGNAME}: ${1:-"Unknown Error"}" 1>&2
exit 1
}
# Example call of the error_exit function. Note the inclusion
# of the LINENO environment variable. It contains the current
# line number.
echo "Example of error with line number and message"
error_exit "$LINENO: An error has occurred."
Do you have a better error handling routine that you use in Bash scripts?
Use a trap!
tempfiles=( )
cleanup() {
rm -f "${tempfiles[#]}"
}
trap cleanup 0
error() {
local parent_lineno="$1"
local message="$2"
local code="${3:-1}"
if [[ -n "$message" ]] ; then
echo "Error on or near line ${parent_lineno}: ${message}; exiting with status ${code}"
else
echo "Error on or near line ${parent_lineno}; exiting with status ${code}"
fi
exit "${code}"
}
trap 'error ${LINENO}' ERR
...then, whenever you create a temporary file:
temp_foo="$(mktemp -t foobar.XXXXXX)"
tempfiles+=( "$temp_foo" )
and $temp_foo will be deleted on exit, and the current line number will be printed. (set -e will likewise give you exit-on-error behavior, though it comes with serious caveats and weakens code's predictability and portability).
You can either let the trap call error for you (in which case it uses the default exit code of 1 and no message) or call it yourself and provide explicit values; for instance:
error ${LINENO} "the foobar failed" 2
will exit with status 2, and give an explicit message.
Alternatively shopt -s extdebug and give the first lines of the trap a little modification to trap all non-zero exit codes across the board (mind set -e non-error non-zero exit codes):
error() {
local last_exit_status="$?"
local parent_lineno="$1"
local message="${2:-(no message ($last_exit_status))}"
local code="${3:-$last_exit_status}"
# ... continue as above
}
trap 'error ${LINENO}' ERR
shopt -s extdebug
This then is also "compatible" with set -eu.
That's a fine solution. I just wanted to add
set -e
as a rudimentary error mechanism. It will immediately stop your script if a simple command fails. I think this should have been the default behavior: since such errors almost always signify something unexpected, it is not really 'sane' to keep executing the following commands.
Reading all the answers on this page inspired me a lot.
So, here's my hint:
file content: lib.trap.sh
lib_name='trap'
lib_version=20121026
stderr_log="/dev/shm/stderr.log"
#
# TO BE SOURCED ONLY ONCE:
#
###~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~##
if test "${g_libs[$lib_name]+_}"; then
return 0
else
if test ${#g_libs[#]} == 0; then
declare -A g_libs
fi
g_libs[$lib_name]=$lib_version
fi
#
# MAIN CODE:
#
###~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~##
set -o pipefail # trace ERR through pipes
set -o errtrace # trace ERR through 'time command' and other functions
set -o nounset ## set -u : exit the script if you try to use an uninitialised variable
set -o errexit ## set -e : exit the script if any statement returns a non-true return value
exec 2>"$stderr_log"
###~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~##
#
# FUNCTION: EXIT_HANDLER
#
###~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~##
function exit_handler ()
{
local error_code="$?"
test $error_code == 0 && return;
#
# LOCAL VARIABLES:
# ------------------------------------------------------------------
#
local i=0
local regex=''
local mem=''
local error_file=''
local error_lineno=''
local error_message='unknown'
local lineno=''
#
# PRINT THE HEADER:
# ------------------------------------------------------------------
#
# Color the output if it's an interactive terminal
test -t 1 && tput bold; tput setf 4 ## red bold
echo -e "\n(!) EXIT HANDLER:\n"
#
# GETTING LAST ERROR OCCURRED:
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #
#
# Read last file from the error log
# ------------------------------------------------------------------
#
if test -f "$stderr_log"
then
stderr=$( tail -n 1 "$stderr_log" )
rm "$stderr_log"
fi
#
# Managing the line to extract information:
# ------------------------------------------------------------------
#
if test -n "$stderr"
then
# Exploding stderr on :
mem="$IFS"
local shrunk_stderr=$( echo "$stderr" | sed 's/\: /\:/g' )
IFS=':'
local stderr_parts=( $shrunk_stderr )
IFS="$mem"
# Storing information on the error
error_file="${stderr_parts[0]}"
error_lineno="${stderr_parts[1]}"
error_message=""
for (( i = 3; i <= ${#stderr_parts[#]}; i++ ))
do
error_message="$error_message "${stderr_parts[$i-1]}": "
done
# Removing last ':' (colon character)
error_message="${error_message%:*}"
# Trim
error_message="$( echo "$error_message" | sed -e 's/^[ \t]*//' | sed -e 's/[ \t]*$//' )"
fi
#
# GETTING BACKTRACE:
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #
_backtrace=$( backtrace 2 )
#
# MANAGING THE OUTPUT:
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #
local lineno=""
regex='^([a-z]{1,}) ([0-9]{1,})$'
if [[ $error_lineno =~ $regex ]]
# The error line was found on the log
# (e.g. type 'ff' without quotes wherever)
# --------------------------------------------------------------
then
local row="${BASH_REMATCH[1]}"
lineno="${BASH_REMATCH[2]}"
echo -e "FILE:\t\t${error_file}"
echo -e "${row^^}:\t\t${lineno}\n"
echo -e "ERROR CODE:\t${error_code}"
test -t 1 && tput setf 6 ## white yellow
echo -e "ERROR MESSAGE:\n$error_message"
else
regex="^${error_file}\$|^${error_file}\s+|\s+${error_file}\s+|\s+${error_file}\$"
if [[ "$_backtrace" =~ $regex ]]
# The file was found on the log but not the error line
# (could not reproduce this case so far)
# ------------------------------------------------------
then
echo -e "FILE:\t\t$error_file"
echo -e "ROW:\t\tunknown\n"
echo -e "ERROR CODE:\t${error_code}"
test -t 1 && tput setf 6 ## white yellow
echo -e "ERROR MESSAGE:\n${stderr}"
# Neither the error line nor the error file was found on the log
# (e.g. type 'cp ffd fdf' without quotes wherever)
# ------------------------------------------------------
else
#
# The error file is the first on backtrace list:
# Exploding backtrace on newlines
mem=$IFS
IFS='
'
#
# Substring: I keep only the carriage return
# (others needed only for tabbing purpose)
IFS=${IFS:0:1}
local lines=( $_backtrace )
IFS=$mem
error_file=""
if test -n "${lines[1]}"
then
array=( ${lines[1]} )
for (( i=2; i<${#array[#]}; i++ ))
do
error_file="$error_file ${array[$i]}"
done
# Trim
error_file="$( echo "$error_file" | sed -e 's/^[ \t]*//' | sed -e 's/[ \t]*$//' )"
fi
echo -e "FILE:\t\t$error_file"
echo -e "ROW:\t\tunknown\n"
echo -e "ERROR CODE:\t${error_code}"
test -t 1 && tput setf 6 ## white yellow
if test -n "${stderr}"
then
echo -e "ERROR MESSAGE:\n${stderr}"
else
echo -e "ERROR MESSAGE:\n${error_message}"
fi
fi
fi
#
# PRINTING THE BACKTRACE:
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #
test -t 1 && tput setf 7 ## white bold
echo -e "\n$_backtrace\n"
#
# EXITING:
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #
test -t 1 && tput setf 4 ## red bold
echo "Exiting!"
test -t 1 && tput sgr0 # Reset terminal
exit "$error_code"
}
trap exit_handler EXIT # ! ! ! TRAP EXIT ! ! !
trap exit ERR # ! ! ! TRAP ERR ! ! !
###~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~##
#
# FUNCTION: BACKTRACE
#
###~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~##
function backtrace
{
local _start_from_=0
local params=( "$#" )
if (( "${#params[#]}" >= "1" ))
then
_start_from_="$1"
fi
local i=0
local first=false
while caller $i > /dev/null
do
if test -n "$_start_from_" && (( "$i" + 1 >= "$_start_from_" ))
then
if test "$first" == false
then
echo "BACKTRACE IS:"
first=true
fi
caller $i
fi
let "i=i+1"
done
}
return 0
Example of usage:
file content: trap-test.sh
#!/bin/bash
source 'lib.trap.sh'
echo "doing something wrong now .."
echo "$foo"
exit 0
Running:
bash trap-test.sh
Output:
doing something wrong now ..
(!) EXIT HANDLER:
FILE: trap-test.sh
LINE: 6
ERROR CODE: 1
ERROR MESSAGE:
foo: unassigned variable
BACKTRACE IS:
1 main trap-test.sh
Exiting!
As you can see from the screenshot below, the output is colored and the error message comes in the used language.
An equivalent alternative to "set -e" is
set -o errexit
It makes the meaning of the flag somewhat clearer than just "-e".
Random addition: to temporarily disable the flag, and return to the default (of continuing execution regardless of exit codes), just use
set +e
echo "commands run here returning non-zero exit codes will not cause the entire script to fail"
echo "false returns 1 as an exit code"
false
set -e
This precludes proper error handling mentioned in other responses, but is quick & effective (just like bash).
Inspired by the ideas presented here, I have developed a readable and convenient way to handle errors in bash scripts in my bash boilerplate project.
By simply sourcing the library, you get the following out of the box (i.e. it will halt execution on any error, as if using set -e thanks to a trap on ERR and some bash-fu):
There are some extra features that help handle errors, such as try and catch, or the throw keyword, that allows you to break execution at a point to see the backtrace. Plus, if the terminal supports it, it spits out powerline emojis, colors parts of the output for great readability, and underlines the method that caused the exception in the context of the line of code.
The downside is - it's not portable - the code works in bash, probably >= 4 only (but I'd imagine it could be ported with some effort to bash 3).
The code is separated into multiple files for better handling, but I was inspired by the backtrace idea from the answer above by Luca Borrione.
To read more or take a look at the source, see GitHub:
https://github.com/niieani/bash-oo-framework#error-handling-with-exceptions-and-throw
I prefer something really easy to call. So I use something that looks a little complicated, but is easy to use. I usually just copy-and-paste the code below into my scripts. An explanation follows the code.
#This function is used to cleanly exit any script. It does this displaying a
# given error message, and exiting with an error code.
function error_exit {
echo
echo "$#"
exit 1
}
#Trap the killer signals so that we can exit with a good message.
trap "error_exit 'Received signal SIGHUP'" SIGHUP
trap "error_exit 'Received signal SIGINT'" SIGINT
trap "error_exit 'Received signal SIGTERM'" SIGTERM
#Alias the function so that it will print a message with the following format:
#prog-name(#line#): message
#We have to explicitly allow aliases, we do this because they make calling the
#function much easier (see example).
shopt -s expand_aliases
alias die='error_exit "Error ${0}(#`echo $(( $LINENO - 1 ))`):"'
I usually put a call to the cleanup function in side the error_exit function, but this varies from script to script so I left it out. The traps catch the common terminating signals and make sure everything gets cleaned up. The alias is what does the real magic. I like to check everything for failure. So in general I call programs in an "if !" type statement. By subtracting 1 from the line number the alias will tell me where the failure occurred. It is also dead simple to call, and pretty much idiot proof. Below is an example (just replace /bin/false with whatever you are going to call).
#This is an example useage, it will print out
#Error prog-name (#1): Who knew false is false.
if ! /bin/false ; then
die "Who knew false is false."
fi
Another consideration is the exit code to return. Just "1" is pretty standard, although there are a handful of reserved exit codes that bash itself uses, and that same page argues that user-defined codes should be in the range 64-113 to conform to C/C++ standards.
You might also consider the bit vector approach that mount uses for its exit codes:
0 success
1 incorrect invocation or permissions
2 system error (out of memory, cannot fork, no more loop devices)
4 internal mount bug or missing nfs support in mount
8 user interrupt
16 problems writing or locking /etc/mtab
32 mount failure
64 some mount succeeded
OR-ing the codes together allows your script to signal multiple simultaneous errors.
I use the following trap code, it also allows errors to be traced through pipes and 'time' commands
#!/bin/bash
set -o pipefail # trace ERR through pipes
set -o errtrace # trace ERR through 'time command' and other functions
function error() {
JOB="$0" # job name
LASTLINE="$1" # line of error occurrence
LASTERR="$2" # error code
echo "ERROR in ${JOB} : line ${LASTLINE} with exit code ${LASTERR}"
exit 1
}
trap 'error ${LINENO} ${?}' ERR
I've used
die() {
echo $1
kill $$
}
before; i think because 'exit' was failing for me for some reason. The above defaults seem like a good idea, though.
This has served me well for a while now. It prints error or warning messages in red, one line per parameter, and allows an optional exit code.
# Custom errors
EX_UNKNOWN=1
warning()
{
# Output warning messages
# Color the output red if it's an interactive terminal
# #param $1...: Messages
test -t 1 && tput setf 4
printf '%s\n' "$#" >&2
test -t 1 && tput sgr0 # Reset terminal
true
}
error()
{
# Output error messages with optional exit code
# #param $1...: Messages
# #param $N: Exit code (optional)
messages=( "$#" )
# If the last parameter is a number, it's not part of the messages
last_parameter="${messages[#]: -1}"
if [[ "$last_parameter" =~ ^[0-9]*$ ]]
then
exit_code=$last_parameter
unset messages[$((${#messages[#]} - 1))]
fi
warning "${messages[#]}"
exit ${exit_code:-$EX_UNKNOWN}
}
Not sure if this will be helpful to you, but I modified some of the suggested functions here in order to include the check for the error (exit code from prior command) within it.
On each "check" I also pass as a parameter the "message" of what the error is for logging purposes.
#!/bin/bash
error_exit()
{
if [ "$?" != "0" ]; then
log.sh "$1"
exit 1
fi
}
Now to call it within the same script (or in another one if I use export -f error_exit) I simply write the name of the function and pass a message as parameter, like this:
#!/bin/bash
cd /home/myuser/afolder
error_exit "Unable to switch to folder"
rm *
error_exit "Unable to delete all files"
Using this I was able to create a really robust bash file for some automated process and it will stop in case of errors and notify me (log.sh will do that)
This trick is useful for missing commands or functions. The name of the missing function (or executable) will be passed in $_
function handle_error {
status=$?
last_call=$1
# 127 is 'command not found'
(( status != 127 )) && return
echo "you tried to call $last_call"
return
}
# Trap errors.
trap 'handle_error "$_"' ERR
This function has been serving me rather well recently:
action () {
# Test if the first parameter is non-zero
# and return straight away if so
if test $1 -ne 0
then
return $1
fi
# Discard the control parameter
# and execute the rest
shift 1
"$#"
local status=$?
# Test the exit status of the command run
# and display an error message on failure
if test ${status} -ne 0
then
echo Command \""$#"\" failed >&2
fi
return ${status}
}
You call it by appending 0 or the last return value to the name of the command to run, so you can chain commands without having to check for error values. With this, this statement block:
command1 param1 param2 param3...
command2 param1 param2 param3...
command3 param1 param2 param3...
command4 param1 param2 param3...
command5 param1 param2 param3...
command6 param1 param2 param3...
Becomes this:
action 0 command1 param1 param2 param3...
action $? command2 param1 param2 param3...
action $? command3 param1 param2 param3...
action $? command4 param1 param2 param3...
action $? command5 param1 param2 param3...
action $? command6 param1 param2 param3...
<<<Error-handling code here>>>
If any of the commands fail, the error code is simply passed to the end of the block. I find it useful when you don't want subsequent commands to execute if an earlier one failed, but you also don't want the script to exit straight away (for example, inside a loop).
Sometimes set -e , trap ERR ,set -o ,set -o pipefail and set -o errtrace not work properly because they attempt to add automatic error detection to the shell. This does not work well in practice.
In my opinion, instead of using set -e and other stuffs, you should write your own error checking code. If you wise to use set -e, be aware of potential gotchas.
To avoid Error while running the code you can use exec 1>/dev/null or exec 2>/dev/null
/dev/null in Linux is a null device file. This will discard anything written to it and will return EOF on reading. you can use this at end of the command
For try/catch you can use && or || to achieve Similar behaviour
use can use && like this
{ # try
command &&
# your command
} || {
# catch exception
}
or you can use if else :
if [[ Condition ]]; then
# if true
else
# if false
fi
$? show output of the last command ,it return 1 or 0
Using trap is not always an option. For example, if you're writing some kind of re-usable function that needs error handling and that can be called from any script (after sourcing the file with helper functions), that function cannot assume anything about exit time of the outer script, which makes using traps very difficult. Another disadvantage of using traps is bad composability, as you risk overwriting previous trap that might be set earlier up in the caller chain.
There is a little trick that can be used to do proper error handling without traps. As you may already know from other answers, set -e doesn't work inside commands if you use || operator after them, even if you run them in a subshell; e.g., this wouldn't work:
#!/bin/sh
# prints:
#
# --> outer
# --> inner
# ./so_1.sh: line 16: some_failed_command: command not found
# <-- inner
# <-- outer
set -e
outer() {
echo '--> outer'
(inner) || {
exit_code=$?
echo '--> cleanup'
return $exit_code
}
echo '<-- outer'
}
inner() {
set -e
echo '--> inner'
some_failed_command
echo '<-- inner'
}
outer
But || operator is needed to prevent returning from the outer function before cleanup. The trick is to run the inner command in background, and then immediately wait for it. The wait builtin will return the exit code of the inner command, and now you're using || after wait, not the inner function, so set -e works properly inside the latter:
#!/bin/sh
# prints:
#
# --> outer
# --> inner
# ./so_2.sh: line 27: some_failed_command: command not found
# --> cleanup
set -e
outer() {
echo '--> outer'
inner &
wait $! || {
exit_code=$?
echo '--> cleanup'
return $exit_code
}
echo '<-- outer'
}
inner() {
set -e
echo '--> inner'
some_failed_command
echo '<-- inner'
}
outer
Here is the generic function that builds upon this idea. It should work in all POSIX-compatible shells if you remove local keywords, i.e. replace all local x=y with just x=y:
# [CLEANUP=cleanup_cmd] run cmd [args...]
#
# `cmd` and `args...` A command to run and its arguments.
#
# `cleanup_cmd` A command that is called after cmd has exited,
# and gets passed the same arguments as cmd. Additionally, the
# following environment variables are available to that command:
#
# - `RUN_CMD` contains the `cmd` that was passed to `run`;
# - `RUN_EXIT_CODE` contains the exit code of the command.
#
# If `cleanup_cmd` is set, `run` will return the exit code of that
# command. Otherwise, it will return the exit code of `cmd`.
#
run() {
local cmd="$1"; shift
local exit_code=0
local e_was_set=1; if ! is_shell_attribute_set e; then
set -e
e_was_set=0
fi
"$cmd" "$#" &
wait $! || {
exit_code=$?
}
if [ "$e_was_set" = 0 ] && is_shell_attribute_set e; then
set +e
fi
if [ -n "$CLEANUP" ]; then
RUN_CMD="$cmd" RUN_EXIT_CODE="$exit_code" "$CLEANUP" "$#"
return $?
fi
return $exit_code
}
is_shell_attribute_set() { # attribute, like "x"
case "$-" in
*"$1"*) return 0 ;;
*) return 1 ;;
esac
}
Example of usage:
#!/bin/sh
set -e
# Source the file with the definition of `run` (previous code snippet).
# Alternatively, you may paste that code directly here and comment the next line.
. ./utils.sh
main() {
echo "--> main: $#"
CLEANUP=cleanup run inner "$#"
echo "<-- main"
}
inner() {
echo "--> inner: $#"
sleep 0.5; if [ "$1" = 'fail' ]; then
oh_my_god_look_at_this
fi
echo "<-- inner"
}
cleanup() {
echo "--> cleanup: $#"
echo " RUN_CMD = '$RUN_CMD'"
echo " RUN_EXIT_CODE = $RUN_EXIT_CODE"
sleep 0.3
echo '<-- cleanup'
return $RUN_EXIT_CODE
}
main "$#"
Running the example:
$ ./so_3 fail; echo "exit code: $?"
--> main: fail
--> inner: fail
./so_3: line 15: oh_my_god_look_at_this: command not found
--> cleanup: fail
RUN_CMD = 'inner'
RUN_EXIT_CODE = 127
<-- cleanup
exit code: 127
$ ./so_3 pass; echo "exit code: $?"
--> main: pass
--> inner: pass
<-- inner
--> cleanup: pass
RUN_CMD = 'inner'
RUN_EXIT_CODE = 0
<-- cleanup
<-- main
exit code: 0
The only thing that you need to be aware of when using this method is that all modifications of Shell variables done from the command you pass to run will not propagate to the calling function, because the command runs in a subshell.