This trap on ERR halts the script. Other examples show that a trap can be used to continue program execution, so why does this halt?
#!/bin/sh -e
trap 'echo error' ERR
echo begin
false
echo end
Returns
$ ./test.sh
begin
$
ERR traps are still affected by the shell's errexit option (set -e). The problem here is that the shebang sets errexit. This will work as expected:
#!/bin/sh
trap 'echo error' ERR
echo begin
false
echo end
returning
$ ./test.sh
begin
error
end
$
errexit is very aggressive. It will instantly kill the script if any statement is non-true.
This will push you as a developer to write very defensively. For example, you cannot ever use $? with errexit because the script itself will not continue if the exit code is non-zero. This "push the developer to get it perfect" approach is useful for some scenarios, e.g. commonly errexit is used to run very early boot scripts when automatically provisioning a system.
If you find yourself in an errexit scenario but needing to handle expected errors, here is the common approach:
#!/bin/sh -e
trap 'echo error' ERR
echo begin
false || { echo "It was false"; }
echo end
Which will output:
begin
It was false
end
Note that the trap is still not fired, because no error happened. As far as I can tell, the "error" condition referred to by ERRtrap and errexit is functionally identical
Related
I have a Bash shell script that invokes a number of commands.
I would like to have the shell script automatically exit with a return value of 1 if any of the commands return a non-zero value.
Is this possible without explicitly checking the result of each command?
For example,
dosomething1
if [[ $? -ne 0 ]]; then
exit 1
fi
dosomething2
if [[ $? -ne 0 ]]; then
exit 1
fi
Add this to the beginning of the script:
set -e
This will cause the shell to exit immediately if a simple command exits with a nonzero exit value. A simple command is any command not part of an if, while, or until test, or part of an && or || list.
See the bash manual on the "set" internal command for more details.
It's really annoying to have a script stubbornly continue when something fails in the middle and breaks assumptions for the rest of the script. I personally start almost all portable shell scripts with set -e.
If I'm working with bash specifically, I'll start with
set -Eeuo pipefail
This covers more error handling in a similar fashion. I consider these as sane defaults for new bash programs. Refer to the bash manual for more information on what these options do.
To add to the accepted answer:
Bear in mind that set -e sometimes is not enough, specially if you have pipes.
For example, suppose you have this script
#!/bin/bash
set -e
./configure > configure.log
make
... which works as expected: an error in configure aborts the execution.
Tomorrow you make a seemingly trivial change:
#!/bin/bash
set -e
./configure | tee configure.log
make
... and now it does not work. This is explained here, and a workaround (Bash only) is provided:
#!/bin/bash
set -e
set -o pipefail
./configure | tee configure.log
make
The if statements in your example are unnecessary. Just do it like this:
dosomething1 || exit 1
If you take Ville Laurikari's advice and use set -e then for some commands you may need to use this:
dosomething || true
The || true will make the command pipeline have a true return value even if the command fails so the the -e option will not kill the script.
If you have cleanup you need to do on exit, you can also use 'trap' with the pseudo-signal ERR. This works the same way as trapping INT or any other signal; bash throws ERR if any command exits with a nonzero value:
# Create the trap with
# trap COMMAND SIGNAME [SIGNAME2 SIGNAME3...]
trap "rm -f /tmp/$MYTMPFILE; exit 1" ERR INT TERM
command1
command2
command3
# Partially turn off the trap.
trap - ERR
# Now a control-C will still cause cleanup, but
# a nonzero exit code won't:
ps aux | grep blahblahblah
Or, especially if you're using "set -e", you could trap EXIT; your trap will then be executed when the script exits for any reason, including a normal end, interrupts, an exit caused by the -e option, etc.
The $? variable is rarely needed. The pseudo-idiom command; if [ $? -eq 0 ]; then X; fi should always be written as if command; then X; fi.
The cases where $? is required is when it needs to be checked against multiple values:
command
case $? in
(0) X;;
(1) Y;;
(2) Z;;
esac
or when $? needs to be reused or otherwise manipulated:
if command; then
echo "command successful" >&2
else
ret=$?
echo "command failed with exit code $ret" >&2
exit $ret
fi
Run it with -e or set -e at the top.
Also look at set -u.
On error, the below script will print a RED error message and exit.
Put this at the top of your bash script:
# BASH error handling:
# exit on command failure
set -e
# keep track of the last executed command
trap 'LAST_COMMAND=$CURRENT_COMMAND; CURRENT_COMMAND=$BASH_COMMAND' DEBUG
# on error: print the failed command
trap 'ERROR_CODE=$?; FAILED_COMMAND=$LAST_COMMAND; tput setaf 1; echo "ERROR: command \"$FAILED_COMMAND\" failed with exit code $ERROR_CODE"; put sgr0;' ERR INT TERM
An expression like
dosomething1 && dosomething2 && dosomething3
will stop processing when one of the commands returns with a non-zero value. For example, the following command will never print "done":
cat nosuchfile && echo "done"
echo $?
1
#!/bin/bash -e
should suffice.
I am just throwing in another one for reference since there was an additional question to Mark Edgars input and here is an additional example and touches on the topic overall:
[[ `cmd` ]] && echo success_else_silence
Which is the same as cmd || exit errcode as someone showed.
For example, I want to make sure a partition is unmounted if mounted:
[[ `mount | grep /dev/sda1` ]] && umount /dev/sda1
I want to display a error log line for one particular command when its return value is nonzero .
I am using ' set -e ' for terminating if any command returns nonzero value along with ' trap ' for this
#!/bin/bash
set -e
log_report() {
echo "Error on line $1"
}
trap 'log_report $LINENO' ERR
echo "starting ..."
first_badcommand
echo "running"
second_badcommd
OUTPUT:
starting ...
/tmp/test1.sh: line 10: first_badcommand: command not found
Error on line 10
since i use set -e the script exit and showing my error log for first_badcommand.. itself.
I want to exit with error log for only a particular command giving non zero return code
and for rest of commands giving non zero return code, exit without error log
After clarification, it appears that the requirement is to exit the script if any error happens, but that the commands which are described as "badcommand" in the question, might or might not fail.
In this answer, I am naming the commands simply first_command etc, to reflect the fact they might or might not fail.
The set -e command, as suggested in the question, will indeed terminate the script if an error occurs, and the trap ... ERR installs a handler which will run after an error (and before the script exits where set -e has been used).
In this case, you should:
wait until the trap is required before installing it (it does not need to be done at/near the start of the script)
disable the trap again when it is no longer required, using trap - ERR
so that commands to enable and disable the trap surround the command for which the trap is required.
For example:
#!/bin/bash
set -e
log_report() {
echo "Error on line $1"
}
echo "starting ..."
first_command
trap 'log_report $LINENO' ERR
echo "running"
second_command
trap - ERR
echo "still running"
third_command
This will exit if any command fails (because of the set -e at the top), but the trap will only be run if second_command fails.
(Note also that set -e similarly does not need to be applied at the start of the script. It can be enabled at any point, and disabled again using set +e. But in this example, it appears that the exit-on-error behaviour is required throughout.)
set -e does not stop you from checking the status of a command line you'd otherwise do:
if some_command
then
echo "It succeeded"
else
echo "It failed. Some message here."
exit 1
fi
I have a Bash shell script that invokes a number of commands.
I would like to have the shell script automatically exit with a return value of 1 if any of the commands return a non-zero value.
Is this possible without explicitly checking the result of each command?
For example,
dosomething1
if [[ $? -ne 0 ]]; then
exit 1
fi
dosomething2
if [[ $? -ne 0 ]]; then
exit 1
fi
Add this to the beginning of the script:
set -e
This will cause the shell to exit immediately if a simple command exits with a nonzero exit value. A simple command is any command not part of an if, while, or until test, or part of an && or || list.
See the bash manual on the "set" internal command for more details.
It's really annoying to have a script stubbornly continue when something fails in the middle and breaks assumptions for the rest of the script. I personally start almost all portable shell scripts with set -e.
If I'm working with bash specifically, I'll start with
set -Eeuo pipefail
This covers more error handling in a similar fashion. I consider these as sane defaults for new bash programs. Refer to the bash manual for more information on what these options do.
To add to the accepted answer:
Bear in mind that set -e sometimes is not enough, specially if you have pipes.
For example, suppose you have this script
#!/bin/bash
set -e
./configure > configure.log
make
... which works as expected: an error in configure aborts the execution.
Tomorrow you make a seemingly trivial change:
#!/bin/bash
set -e
./configure | tee configure.log
make
... and now it does not work. This is explained here, and a workaround (Bash only) is provided:
#!/bin/bash
set -e
set -o pipefail
./configure | tee configure.log
make
The if statements in your example are unnecessary. Just do it like this:
dosomething1 || exit 1
If you take Ville Laurikari's advice and use set -e then for some commands you may need to use this:
dosomething || true
The || true will make the command pipeline have a true return value even if the command fails so the the -e option will not kill the script.
If you have cleanup you need to do on exit, you can also use 'trap' with the pseudo-signal ERR. This works the same way as trapping INT or any other signal; bash throws ERR if any command exits with a nonzero value:
# Create the trap with
# trap COMMAND SIGNAME [SIGNAME2 SIGNAME3...]
trap "rm -f /tmp/$MYTMPFILE; exit 1" ERR INT TERM
command1
command2
command3
# Partially turn off the trap.
trap - ERR
# Now a control-C will still cause cleanup, but
# a nonzero exit code won't:
ps aux | grep blahblahblah
Or, especially if you're using "set -e", you could trap EXIT; your trap will then be executed when the script exits for any reason, including a normal end, interrupts, an exit caused by the -e option, etc.
The $? variable is rarely needed. The pseudo-idiom command; if [ $? -eq 0 ]; then X; fi should always be written as if command; then X; fi.
The cases where $? is required is when it needs to be checked against multiple values:
command
case $? in
(0) X;;
(1) Y;;
(2) Z;;
esac
or when $? needs to be reused or otherwise manipulated:
if command; then
echo "command successful" >&2
else
ret=$?
echo "command failed with exit code $ret" >&2
exit $ret
fi
Run it with -e or set -e at the top.
Also look at set -u.
On error, the below script will print a RED error message and exit.
Put this at the top of your bash script:
# BASH error handling:
# exit on command failure
set -e
# keep track of the last executed command
trap 'LAST_COMMAND=$CURRENT_COMMAND; CURRENT_COMMAND=$BASH_COMMAND' DEBUG
# on error: print the failed command
trap 'ERROR_CODE=$?; FAILED_COMMAND=$LAST_COMMAND; tput setaf 1; echo "ERROR: command \"$FAILED_COMMAND\" failed with exit code $ERROR_CODE"; put sgr0;' ERR INT TERM
An expression like
dosomething1 && dosomething2 && dosomething3
will stop processing when one of the commands returns with a non-zero value. For example, the following command will never print "done":
cat nosuchfile && echo "done"
echo $?
1
#!/bin/bash -e
should suffice.
I am just throwing in another one for reference since there was an additional question to Mark Edgars input and here is an additional example and touches on the topic overall:
[[ `cmd` ]] && echo success_else_silence
Which is the same as cmd || exit errcode as someone showed.
For example, I want to make sure a partition is unmounted if mounted:
[[ `mount | grep /dev/sda1` ]] && umount /dev/sda1
I have a simple script :
#!/bin/bash
set -e
trap "echo BOO!" ERR
function func(){
ls /root/
}
func
I would like to trap ERR if my script fails (as it will here b/c I do not have the permissions to look into /root). However, when using set -e it is not trapped. Without set -e ERR is trapped.
According to the bash man page, for set -e :
... A trap on ERR, if set, is executed before the shell exits. ...
Why isn't my trap executed? From the man page it seems like it should.
chepner's answer is the best solution: If you want to combine set -e (same as: set -o errexit) with an ERR trap, also use set -o errtrace (same as: set -E).
In short: use set -eE in lieu of just set -e:
#!/bin/bash
set -eE # same as: `set -o errexit -o errtrace`
trap 'echo BOO!' ERR
function func(){
ls /root/
}
# Thanks to -E / -o errtrace, this still triggers the trap,
# even though the failure occurs *inside the function*.
func
A more sophisticated example trap example that prints the message in red and also prints the exit code:
trap 'printf "\e[31m%s: %s\e[m\n" "BOO!" $?' ERR
man bash says about set -o errtrace / set -E:
If set, any trap on ERR is inherited by shell functions, command substitutions, and commands executed in a subshell environment. The ERR trap is normally not inherited in such cases.
What I believe is happening:
Without -e: The ls command fails inside your function, and, due to being the last command in the function, the function reports ls's nonzero exit code to the caller, your top-level script scope. In that scope, the ERR trap is in effect, and it is invoked (but note that execution will continue, unless you explicitly call exit from the trap).
With -e (but without -E): The ls command fails inside your function, and because set -e is in effect, Bash instantly exits, directly from the function scope - and since there is no ERR trap in effect there (because it wasn't inherited from the parent scope), your trap is not called.
While the man page is not incorrect, I agree that this behavior is not exactly obvious - you have to infer it.
You need to use set -o errtrace for the function to inherit the trap.
We have these options for debugging:
-e Exit immediately on failure
-E If set, any trap on ERR is inherited by shell functions
-u Exit when there is an unbound variable
-o Give a option-name to set
pipefail The return values of last (rightmost) command (exit code)
-v Print all shell input lines as they are read
-x Print trace of commands
For handling the errors we can catch directory with trap
trap 'echo >&2 "Error - exited with status $? at line $LINENO' ERR
Or a better version ref :
trap 'echo >&2 "Error - exited with status $? at line $LINENO:";
pr -tn $0 | tail -n+$((LINENO - 3)) | head -n7' ERR
Or a function:
function __error_handing__(){
local last_status_code=$1;
local error_line_number=$2;
echo 1>&2 "Error - exited with status $last_status_code at line $error_line_number";
perl -slne 'if($.+5 >= $ln && $.-4 <= $ln){ $_="$. $_"; s/$ln/">" x length($ln)/eg; s/^\D+.*?$/\e[1;31m$&\e[0m/g; print}' -- -ln=$error_line_number $0
}
and call it this way:
trap '__error_handing__ $? $LINENO' ERR
Replace ERR with EXIT and it will work.
The syntax of the trap command is: trap [COMMANDS] [SIGNALS]
For more info, please read http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_12_02.html
I'm trying to remove some bashisms from a shell script.
I can't figure out how I would replicate 'trap ERR': ERR is not a signal, and is not in the standard, but is a common bashism (see e.g. the LDP guide).
How do I replicate trap ERR in a standards-compliant /bin/sh script?
You can use a combination of set -e and trap ... EXIT.
#!/bin/sh
set -e
err_handler () {
[ $? -eq 0 ] && exit
# Code for non-zero exit status here
}
trap err_handler EXIT
set -e will cause the script to exit whenever an unguarded command has an non-zero exit status. The error handler will be called unconditionally when the script exits, but inside the handler you can simply exit if the current exit status is 0, i.e., we reached it without any errors occurring.
By "unguarded" command I mean a command that isn't run in a context where a non-zero exit status is reasonably expected to occur, such as in the condition for an if statement.
ERR is not really a signal. See: http://en.wikibooks.org/wiki/Bourne_Shell_Scripting/Debugging_and_signal_handling#Err..._ERR.3F at the bottom.