Shell script print line number when it errors out - bash

I have been looking for a way to print the line number inside the shell script when it errors out.
I came across '-x' option, which prints the line when running the shell script, but this is not exactly what I want. Maybe I could do $LINENO before every exit code? Is there a cleaner way to do it?
I just wanted the line number so I could open the shell script and directly go to the place where the interpreter realized the error.

Using
PS4=':$LINENO+'
will add line number to the output of set -x.
If you only want to print that on errors, there's some risk of running into bugs in recent interpreters. However, you can try the following (first given in this previous answer):
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
Again, this will not work on some recent builds of bash, which don't always have LINENO set correctly inside traps.
Another approach (which will only work on recent shells; the below uses some bash 4.0 and 4.1 features) is to use PS4 to emit the exit status and line number of each command to a dedicated file descriptor, and use tail to print only the last line given to that FD before the shell exits:
exec {BASH_XTRACEFD}> >(tail -n 1) # send set -x output to tail -n 1
PS4=':At line $LINENO; prior command exit status $?+'
set -x

Related

ERR Trap and command substitution

Running test.sh:
#!/bin/bash
set -eE
trap "echo 'in trap'" ERR
var=$(bad_command)
Output is:
./test.sh: line 7: bad_command: command not found
in trap
Running test2.sh:
#!/bin/bash
set -eE
trap "echo 'in trap'" ERR
echo $(bad_command)
Output is:
./test2.sh: line 8: bad_command: command not found
# line with whitespace printed
Why is the trap not triggered in test2.sh?
The difference only matters which of the commands var=$(badcommand) and echo $(badcommand) set the incorrect exit code to the shell. As it is clearly evident the second case, echo does not set the incorrect exit code as it is a separate process running which is capable of setting its own code depending upon its success/failure. Since echo which seemed to have exited correctly though it did not have any output to print it printed a code 0 to shell thereby masking the command not found error from the command substitution.
Just try for yourself running in shell without the set -e and the traps defined
var=$(badcommand); echo $?
bash: badcommand: command not found...
127
echo $(badcommand); echo $?
bash: badcommand: command not found...
0
The first one fires the trap and the next one doesn't.

How to handle errors in bash scripts correctly?

I use set -e options on top of my bash scripts to stop executing on any errors. But also I can to use -e on echo command like the following:
echo -e "Some text".
I have two questions:
How to handle correctly handle errors in bash scripts?
What -e options means in echo command?
The "correct" way to handle bash errors depends on the situation and what you want to accomplish.
In some cases, the if statement approach that barmar describes is the best way to handle a problem.
The vagaries of set -e
set -e will silently stop a script as soon as there is an uncaught error. It will print no message. So, if you want to know why or what line caused the script to fail, you will be frustrated.
Further, as documented on Greg's FAQ, the behavior of set -e varies from one bash version to the next and can be quite surprising.
In sum, set -e has only limited uses.
A die function
In other cases, when a command fails, you want the script to exit immediately with a message. In perl, the die function provides a handy way to do this. This feature can be emulated in shell with a function:
die () {
echo "ERROR: $*. Aborting." >&2
exit 1
}
A call to die can then be easily attached to commands which have to succeed or else the script must be stopped. For example:
cp file1 dir/ || die "Failed to cp file1 to dir."
Here, due to the use of bash's OR control operator, ||, the die command is executed only if the command which precedes it fails.
If you want to handle an error instead of stopping the script when it happens, use if:
if ! some_command
then
# Do whatever you want here, for instance...
echo some_command got an error
fi
echo -e is unrelated. This -e option tells the echo command to process escape sequences in its arguments. See man echo for the list of escape sequences.
One way of handling error is to use -e in your shebang at start of your script and using a trap handler for ERR like this:
#!/bin/bash -e
errHandler () {
d=$(date '+%D %T :: ')
echo "$d Error, Exiting..." >&2
# can do more things like print to a log file etc or some cleanup
exit 1
}
trap errHandler ERR
Now this function errHandler will be called only when an error occurs in your script.

How to display last command that failed when using bash set -e?

I am using set -e to stop execution of a script on first error.
The problem is that this does not tell me what went wrong.
How can update a bash script so it will display me the last command that failed?
Instead of set -e, use an ERR trap; you can pass $BASH_LINENO in to get the specific line number on which the error occurred. I provide a script taking advantage of this in my answer at https://stackoverflow.com/a/185900/14122
To summarize:
error() {
local sourcefile=$1
local lineno=$2
# ...logic for reporting an error at line $lineno
# of file $sourcefile goes here...
}
trap 'error "${BASH_SOURCE}" "${LINENO}"' ERR
make err.sh
set -e
trap 'echo "ERROR: $BASH_SOURCE:$LINENO $BASH_COMMAND" >&2' ERR
include it (. err.sh) in all your scripts.
replace any
... | while read X ; do ... ; done
with
while read X ; do ... ; done < <( ... )
in your scripts for the trap to give the correct line number/command in the error message
Have you tried with --verbose?
bash --verbose script.sh
You can't use set -e by itself because processing will immediately stop after any error. Take a look at the Set Builtin section of the Bash Reference Manual for more information about the -x and -v options, which you can use for debugging.
Something like:
set -e
set -v
will exit on any error, while showing you each input line as it is read. It will not, however, show you just the line with the error. For that, you will need to do your own explicit error checking.
For example:
set +e
if false; then
real_exit_status=$?
echo 'Some useful error message.' >&2
exit $real_exit_status
fi
set -ex will show (all) lines as they are executed and stop at the first command returning nonzero (not as part of if/while/until constructs).

Echoing the last command run in Bash?

I am trying to echo the last command run inside a bash script. I found a way to do it with some history,tail,head,sed which works fine when commands represent a specific line in my script from a parser standpoint. However under some circumstances I don't get the expected output, for instance when the command is inserted inside a case statement:
The script:
#!/bin/bash
set -o history
date
last=$(echo `history |tail -n2 |head -n1` | sed 's/[0-9]* //')
echo "last command is [$last]"
case "1" in
"1")
date
last=$(echo `history |tail -n2 |head -n1` | sed 's/[0-9]* //')
echo "last command is [$last]"
;;
esac
The output:
Tue May 24 12:36:04 CEST 2011
last command is [date]
Tue May 24 12:36:04 CEST 2011
last command is [echo "last command is [$last]"]
[Q] Can someone help me find a way to echo the last run command regardless of how/where this command is called within the bash script?
My answer
Despite the much appreciated contributions from my fellow SO'ers, I opted for writing a run function - which runs all its parameters as a single command and display the command and its error code when it fails - with the following benefits:
-I only need to prepend the commands I want to check with run which keeps them on one line and doesn't affect the conciseness of my script
-Whenever the script fails on one of these commands, the last output line of my script is a message that clearly displays which command fails along with its exit code, which makes debugging easier
Example script:
#!/bin/bash
die() { echo >&2 -e "\nERROR: $#\n"; exit 1; }
run() { "$#"; code=$?; [ $code -ne 0 ] && die "command [$*] failed with error code $code"; }
case "1" in
"1")
run ls /opt
run ls /wrong-dir
;;
esac
The output:
$ ./test.sh
apacheds google iptables
ls: cannot access /wrong-dir: No such file or directory
ERROR: command [ls /wrong-dir] failed with error code 2
I tested various commands with multiple arguments, bash variables as arguments, quoted arguments... and the run function didn't break them. The only issue I found so far is to run an echo which breaks but I do not plan to check my echos anyway.
Bash has built in features to access the last command executed. But that's the last whole command (e.g. the whole case command), not individual simple commands like you originally requested.
!:0 = the name of command executed.
!:1 = the first parameter of the previous command
!:4 = the fourth parameter of the previous command
!:* = all of the parameters of the previous command
!^ = the first parameter of the previous command (same as !:1)
!$ = the final parameter of the previous command
!:-3 = all parameters in range 0-3 (inclusive)
!:2-5 = all parameters in range 2-5 (inclusive)
!! = the previous command line
etc.
So, the simplest answer to the question is, in fact:
echo !!
...alternatively:
echo "Last command run was ["!:0"] with arguments ["!:*"]"
Try it yourself!
echo this is a test
echo !!
In a script, history expansion is turned off by default, you need to enable it with
set -o history -o histexpand
The command history is an interactive feature. Only complete commands are entered in the history. For example, the case construct is entered as a whole, when the shell has finished parsing it. Neither looking up the history with the history built-in (nor printing it through shell expansion (!:p)) does what you seem to want, which is to print invocations of simple commands.
The DEBUG trap lets you execute a command right before any simple command execution. A string version of the command to execute (with words separated by spaces) is available in the BASH_COMMAND variable.
trap 'previous_command=$this_command; this_command=$BASH_COMMAND' DEBUG
…
echo "last command is $previous_command"
Note that previous_command will change every time you run a command, so save it to a variable in order to use it. If you want to know the previous command's return status as well, save both in a single command.
cmd=$previous_command ret=$?
if [ $ret -ne 0 ]; then echo "$cmd failed with error code $ret"; fi
Furthermore, if you only want to abort on a failed commands, use set -e to make your script exit on the first failed command. You can display the last command from the EXIT trap.
set -e
trap 'echo "exit $? due to $previous_command"' EXIT
Note that if you're trying to trace your script to see what it's doing, forget all this and use set -x.
After reading the answer from Gilles, I decided to see if the $BASH_COMMAND var was also available (and the desired value) in an EXIT trap - and it is!
So, the following bash script works as expected:
#!/bin/bash
exit_trap () {
local lc="$BASH_COMMAND" rc=$?
echo "Command [$lc] exited with code [$rc]"
}
trap exit_trap EXIT
set -e
echo "foo"
false 12345
echo "bar"
The output is
foo
Command [false 12345] exited with code [1]
bar is never printed because set -e causes bash to exit the script when a command fails and the false command always fails (by definition). The 12345 passed to false is just there to show that the arguments to the failed command are captured as well (the false command ignores any arguments passed to it)
I was able to achieve this by using set -x in the main script (which makes the script print out every command that is executed) and writing a wrapper script which just shows the last line of output generated by set -x.
This is the main script:
#!/bin/bash
set -x
echo some command here
echo last command
And this is the wrapper script:
#!/bin/sh
./test.sh 2>&1 | grep '^\+' | tail -n 1 | sed -e 's/^\+ //'
Running the wrapper script produces this as output:
echo last command
history | tail -2 | head -1 | cut -c8-
tail -2 returns the last two command lines from history
head -1 returns just first line
cut -c8- returns just command line, removing PID and spaces.
There is a racecondition between the last command ($_) and last error ( $?) variables. If you try to store one of them in an own variable, both encountered new values already because of the set command. Actually, last command hasn't got any value at all in this case.
Here is what i did to store (nearly) both informations in own variables, so my bash script can determine if there was any error AND setting the title with the last run command:
# This construct is needed, because of a racecondition when trying to obtain
# both of last command and error. With this the information of last error is
# implied by the corresponding case while command is retrieved.
if [[ "${?}" == 0 && "${_}" != "" ]] ; then
# Last command MUST be retrieved first.
LASTCOMMAND="${_}" ;
RETURNSTATUS='✓' ;
elif [[ "${?}" == 0 && "${_}" == "" ]] ; then
LASTCOMMAND='unknown' ;
RETURNSTATUS='✓' ;
elif [[ "${?}" != 0 && "${_}" != "" ]] ; then
# Last command MUST be retrieved first.
LASTCOMMAND="${_}" ;
RETURNSTATUS='✗' ;
# Fixme: "$?" not changing state until command executed.
elif [[ "${?}" != 0 && "${_}" == "" ]] ; then
LASTCOMMAND='unknown' ;
RETURNSTATUS='✗' ;
# Fixme: "$?" not changing state until command executed.
fi
This script will retain the information, if an error occured and will obtain the last run command. Because of the racecondition i can not store the actual value. Besides, most commands actually don't even care for error noumbers, they just return something different from '0'. You'll notice that, if you use the errono extention of bash.
It should be possible with something like a "intern" script for bash, like in bash extention, but i'm not familiar with something like that and it wouldn't be compatible as well.
CORRECTION
I didn't think, that it was possible to retrieve both variables at the same time. Although i like the style of the code, i assumed it would be interpreted as two commands. This was wrong, so my answer devides down to:
# Because of a racecondition, both MUST be retrieved at the same time.
declare RETURNSTATUS="${?}" LASTCOMMAND="${_}" ;
if [[ "${RETURNSTATUS}" == 0 ]] ; then
declare RETURNSYMBOL='✓' ;
else
declare RETURNSYMBOL='✗' ;
fi
Although my post might not get any positive rating, i solved my problem myself, finally.
And this seems appropriate regarding the intial post. :)

In a Bash script, how can I exit the entire script if a certain condition occurs?

I'm writing a script in Bash to test some code. However, it seems silly to run the tests if compiling the code fails in the first place, in which case I'll just abort the tests.
Is there a way I can do this without wrapping the entire script inside of a while loop and using breaks? Something like a dun dun dun goto?
Try this statement:
exit 1
Replace 1 with appropriate error codes. See also Exit Codes With Special Meanings.
Use set -e
#!/bin/bash
set -e
/bin/command-that-fails
/bin/command-that-fails2
The script will terminate after the first line that fails (returns nonzero exit code). In this case, command-that-fails2 will not run.
If you were to check the return status of every single command, your script would look like this:
#!/bin/bash
# I'm assuming you're using make
cd /project-dir
make
if [[ $? -ne 0 ]] ; then
exit 1
fi
cd /project-dir2
make
if [[ $? -ne 0 ]] ; then
exit 1
fi
With set -e it would look like:
#!/bin/bash
set -e
cd /project-dir
make
cd /project-dir2
make
Any command that fails will cause the entire script to fail and return an exit status you can check with $?. If your script is very long or you're building a lot of stuff it's going to get pretty ugly if you add return status checks everywhere.
A SysOps guy once taught me the Three-Fingered Claw technique:
yell() { echo "$0: $*" >&2; }
die() { yell "$*"; exit 111; }
try() { "$#" || die "cannot $*"; }
These functions are *NIX OS and shell flavor-robust. Put them at the beginning of your script (bash or otherwise), try() your statement and code on.
Explanation
(based on flying sheep comment).
yell: print the script name and all arguments to stderr:
$0 is the path to the script ;
$* are all arguments.
>&2 means > redirect stdout to & pipe 2. pipe 1 would be stdout itself.
die does the same as yell, but exits with a non-0 exit status, which means “fail”.
try uses the || (boolean OR), which only evaluates the right side if the left one failed.
$# is all arguments again, but different.
If you will invoke the script with source, you can use return <x> where <x> will be the script exit status (use a non-zero value for error or false). But if you invoke an executable script (i.e., directly with its filename), the return statement will result in a complain (error message "return: can only `return' from a function or sourced script").
If exit <x> is used instead, when the script is invoked with source, it will result in exiting the shell that started the script, but an executable script will just terminate, as expected.
To handle either case in the same script, you can use
return <x> 2> /dev/null || exit <x>
This will handle whichever invocation may be suitable. That is assuming you will use this statement at the script's top level. I would advise against directly exiting the script from within a function.
Note: <x> is supposed to be just a number.
I often include a function called run() to handle errors. Every call I want to make is passed to this function so the entire script exits when a failure is hit. The advantage of this over the set -e solution is that the script doesn't exit silently when a line fails, and can tell you what the problem is. In the following example, the 3rd line is not executed because the script exits at the call to false.
function run() {
cmd_output=$(eval $1)
return_value=$?
if [ $return_value != 0 ]; then
echo "Command $1 failed"
exit -1
else
echo "output: $cmd_output"
echo "Command succeeded."
fi
return $return_value
}
run "date"
run "false"
run "date"
Instead of if construct, you can leverage the short-circuit evaluation:
#!/usr/bin/env bash
echo $[1+1]
echo $[2/0] # division by 0 but execution of script proceeds
echo $[3+1]
(echo $[4/0]) || exit $? # script halted with code 1 returned from `echo`
echo $[5+1]
Note the pair of parentheses which is necessary because of priority of alternation operator. $? is a special variable set to exit code of most recently called command.
I have the same question but cannot ask it because it would be a duplicate.
The accepted answer, using exit, does not work when the script is a bit more complicated. If you use a background process to check for the condition, exit only exits that process, as it runs in a sub-shell. To kill the script, you have to explicitly kill it (at least that is the only way I know).
Here is a little script on how to do it:
#!/bin/bash
boom() {
while true; do sleep 1.2; echo boom; done
}
f() {
echo Hello
N=0
while
((N++ <10))
do
sleep 1
echo $N
# ((N > 5)) && exit 4 # does not work
((N > 5)) && { kill -9 $$; exit 5; } # works
done
}
boom &
f &
while true; do sleep 0.5; echo beep; done
This is a better answer but still incomplete a I really don't know how to get rid of the boom part.
You can close your program by program name on follow way:
for soft exit do
pkill -9 -x programname # Replace "programmname" by your programme
for hard exit do
pkill -15 -x programname # Replace "programmname" by your programme
If you like to know how to evaluate condition for closing a program, you need to customize your question.
#!/bin/bash -x
# exit and report the failure if any command fails
exit_trap () { # ---- (1)
local lc="$BASH_COMMAND" rc=$?
echo "Command [$lc] exited with code [$rc]"
}
trap exit_trap EXIT # ---- (2)
set -e # ---- (3)
Explanation:
This question is also about how to write clean code. Let's divide the above script into multiple parts:
Part - 1:
exit_trap is a function that gets called when any step failed and captures the last executed step using $BASH_COMMAND and captures the return code of that step. This is the function that can be used for any clean-up, similar to shutdownhooks
The command currently being executed or about to be executed, unless the shell is executing a command as the result of a trap, in which case it is the command executing at the time of the trap.
Doc.
Part - 2:
trap [action] [signal]
Register the trap action (here exit_trap function) in case of EXIT signal.
Part - 3:
Exit immediately if a sequence of one or more commands returns a non-zero status. The shell does not exit if the command that fails is part of the command list immediately following a while or until keyword, part of the test in an if statement, part of any command executed in a && or || list except the command following the final && or ||, any command in a pipeline but the last, or if the command’s return status is being inverted with !. If a compound command other than a subshell returns a non-zero status because a command failed while -e was being ignored, the shell does not exit. A trap on ERR, if set, is executed before the shell exits.
Doc.
Part - 4:
You can create a common.sh file and source it in all of your scripts.
source common.sh

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