Raise error in a Bash script
There are a couple more ways with which you can approach this problem. Assuming one of your requirement is to run a shell script/function containing a few shell commands and check if the script ran successfully and throw errors in case of failures.
The shell commands in generally rely on exit-codes returned to let the shell know if it was successful or failed due to some unexpected events.
So what you want to do falls upon these two categories
- exit on error
- exit and clean-up on error
Depending on which one you want to do, there are shell options available to use. For the first case, the shell provides an option with set -e
and for the second you could do a trap
on EXIT
Should I use exit
in my script/function?
Using exit
generally enhances readability In certain routines, once you know the answer, you want to exit to the calling routine immediately. If the routine is defined in such a way that it doesn’t require any further cleanup once it detects an error, not exiting immediately means that you have to write more code.
So in cases if you need to do clean-up actions on script to make the termination of the script clean, it is preferred to not to use exit
.
Should I use set -e
for error on exit?
No!
set -e
was an attempt to add "automatic error detection" to the shell. Its goal was to cause the shell to abort any time an error occurred, but it comes with a lot of potential pitfalls for example,
The commands that are part of an if test are immune. In the example, if you expect it to break on the
test
check on the non-existing directory, it wouldn't, it goes through to the else conditionset -e f() { test -d nosuchdir && echo no dir; } f echo survived
Commands in a pipeline other than the last one, are immune. In the example below, because the most recently executed (rightmost) command's exit code is considered (
cat
) and it was successful. This could be avoided by setting by theset -o pipefail
option but its still a caveat.set -e somecommand that fails | cat - echo survived
Recommended for use - trap
on exit
The verdict is if you want to be able to handle an error instead of blindly exiting, instead of using set -e
, use a trap
on the ERR
pseudo signal.
The ERR
trap is not to run code when the shell itself exits with a non-zero error code, but when any command run by that shell that is not part of a condition (like in if cmd
, or cmd ||
) exits with a non-zero exit status.
The general practice is we define an trap handler to provide additional debug information on which line and what cause the exit. Remember the exit code of the last command that caused the ERR
signal would still be available at this point.
cleanup() {
exitcode=$?
printf 'error condition hit\n' 1>&2
printf 'exit code returned: %s\n' "$exitcode"
printf 'the command executing at the time of the error was: %s\n' "$BASH_COMMAND"
printf 'command present on line: %d' "${BASH_LINENO[0]}"
# Some more clean up code can be added here before exiting
exit $exitcode
}
and we just use this handler as below on top of the script that is failing
trap cleanup ERR
Putting this together on a simple script that contained false
on line 15, the information you would be getting as
error condition hit
exit code returned: 1
the command executing at the time of the error was: false
command present on line: 15
The trap
also provides options irrespective of the error to just run the cleanup on shell completion (e.g. your shell script exits), on signal EXIT
. You could also trap on multiple signals at the same time. The list of supported signals to trap on can be found on the trap.1p - Linux manual page
Another thing to notice would be to understand that none of the provided methods work if you are dealing with sub-shells are involved in which case, you might need to add your own error handling.
On a sub-shell with
set -e
wouldn't work. Thefalse
is restricted to the sub-shell and never gets propagated to the parent shell. To do the error handling here, add your own logic to do(false) || false
set -e (false) echo survived
The same happens with
trap
also. The logic below wouldn't work for the reasons mentioned above.trap 'echo error' ERR (false)
Basic error handling
If your test case runner returns a non-zero code for failed tests, you can simply write:
test_handler test_case_x; test_result=$?
if ((test_result != 0)); then
printf '%s\n' "Test case x failed" >&2 # write error message to stderr
exit 1 # or exit $test_result
fi
Or even shorter:
if ! test_handler test_case_x; then
printf '%s\n' "Test case x failed" >&2
exit 1
fi
Or the shortest:
test_handler test_case_x || { printf '%s\n' "Test case x failed" >&2; exit 1; }
To exit with test_handler's exit code:
test_handler test_case_x || { ec=$?; printf '%s\n' "Test case x failed" >&2; exit $ec; }
Advanced error handling
If you want to take a more comprehensive approach, you can have an error handler:
exit_if_error() {
local exit_code=$1
shift
[[ $exit_code ]] && # do nothing if no error code passed
((exit_code != 0)) && { # do nothing if error code is 0
printf 'ERROR: %s\n' "$@" >&2 # we can use better logging here
exit "$exit_code" # we could also check to make sure
# error code is numeric when passed
}
}
then invoke it after running your test case:
run_test_case test_case_x
exit_if_error $? "Test case x failed"
or
run_test_case test_case_x || exit_if_error $? "Test case x failed"
The advantages of having an error handler like exit_if_error
are:
- we can standardize all the error handling logic such as logging, printing a stack trace, notification, doing cleanup etc., in one place
- by making the error handler get the error code as an argument, we can spare the caller from the clutter of
if
blocks that test exit codes for errors - if we have a signal handler (using trap), we can invoke the error handler from there
Error handling and logging library
Here is a complete implementation of error handling and logging:
https://github.com/codeforester/base/blob/master/lib/stdlib.sh
Related posts
- Error handling in Bash
- The 'caller' builtin command on Bash Hackers Wiki
- Are there any standard exit status codes in Linux?
- BashFAQ/105 - Why doesn't set -e (or set -o errexit, or trap ERR) do what I expected?
- Equivalent of
__FILE__
,__LINE__
in Bash - Is there a TRY CATCH command in Bash
- To add a stack trace to the error handler, you may want to look at this post: Trace of executed programs called by a Bash script
- Ignoring specific errors in a shell script
- Catching error codes in a shell pipe
- How do I manage log verbosity inside a shell script?
- How to log function name and line number in Bash?
- Is double square brackets [[ ]] preferable over single square brackets [ ] in Bash?
This depends on where you want the error message be stored.
You can do the following:
echo "Error!" > logfile.log
exit 125
Or the following:
echo "Error!" 1>&2
exit 64
When you raise an exception you stop the program's execution.
You can also use something like exit xxx
where xxx
is the error code you may want to return to the operating system (from 0 to 255). Here 125
and 64
are just random codes you can exit with. When you need to indicate to the OS that the program stopped abnormally (eg. an error occurred), you need to pass a non-zero exit code to exit
.
As @chepner pointed out, you can do exit 1
, which will mean an unspecified error.