# Errors in shell scripts

June 27, 2016
(shell)

Writing shell scripts might be pretty tedious because of all those failing commands leading to a corrupted state. And it gets even worse when you overpopulate your script with if statements which distract you from solving your initial problem. And fortunately, there are several neat tricks for handling errors in shell scripts that everyone should be familiar with.

Before we start investigating these tricks, let’s define two helper functions - success and failure. We will use them in our examples to represent abstract commands that always succeeds and always fails respectively.

# just like true, but also echoes
function success {
echo "success"
return 0
}

# just like false, but also echoes
function failure {
echo "failure"
return 1
}

Now let’s implement our first script.

success
failure
success

# produces following output:
# success
# failure
# success
# result is 0 (success)

All commands will be executed, even the last success despite the fact that failure, well, failed. In most cases, this is not what we want. The usual solution is to use && operator that conditionally executes the second command.

success &&
failure &&
success

# produces following output:
# success
# failure
# result is 1 (failure)

This was quite easy, wasn’t it? Such behaviour looks just like imperative code in pretty much any language. Whenever one piece of execution chain fails - the whole chain fails unless we handle it somehow. And here is a fundamental problem about the previous script - you must be explicit about behaviour that should be on by default.

And you’ll be pleased to know that it’s possible to enable such behaviour. Sh has a special exit-on-error mode, which when enabled forces the shell to exit when the simple command returns non-zero status code. In order to enable this mode run sh with -e option or just call set -e.

set -e
success
failure
success

# produces following output:
# success
# failure
# result is 1 (failure)

This looks much better. Just for reference, I would like to quote manual for the set -e.

Exit immediately if a pipeline (see Pipelines), which may consist of a single simple command (see Simple Commands), a list (see Lists), or a compound command (see Compound 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.

This option applies to the shell environment and each subshell environment separately (see Command Execution Environment), and may cause subshells to exit before executing all the commands in the subshell.

If a compound command or shell function executes in a context where -e is being ignored, none of the commands executed within the compound command or function body will be affected by the -e setting, even if -e is set and a command returns a failure status. If a compound command or shell function sets -e while executing in a context where -e is ignored, that setting will not have any effect until the compound command or the command containing the function call completes.

Just as a side note, there are several other useful options, like -u (to fail when any undefined variable is referenced) and -x (to print each command before it’s executed).

Using exit-on-error mode helps to mitigate lots of problems caused by executing commands in a corrupted state. If you ever need to disable it after enabling it - just call set +e (note + instead of -).

There is also another problem that must be solved. Sometimes we need to handle specific signal (like pressing C-c to stop script execution). Let’s investigate pretty classical example.

set -e

TMPFILE="path/to/tmp_file"

function cleanup () {
echo "removing $TMPFILE" rm -f$TMPFILE
}

echo "hello world" > $TMPFILE cat$TMPFILE

# gives user a chance to press CTRL-C
sleep 3

# here we are doing something that might fail
failure
# oh, this always fails

cleanup

As a result of running this script - there will be a temporary file that wasn’t removed due to the failure during execution. The good thing is that it’s possible to handle even such problem using trap command which allows performing an action when a specific signal is received. You can check the list of supported signals by calling trap -l.

set -e

TMPFILE="tmp_file"

function cleanup () {
echo "removing $TMPFILE" rm -f$TMPFILE
}

# execute cleanup function whenever INT, TERM or EXIT singal is received
trap cleanup INT TERM EXIT

echo "Hello World!" > $TMPFILE cat$TMPFILE

# gives user a chance to press CTRL-C
sleep 3

# here we are doing something that might fail
failure
# oh, this always fails

cleanup

If you press C-c, the program immediately stops the execution, but the cleanup function is still called. It is also called when failure is executed. And this is really nice.

Note that you can only set one trap per signal. If you set a new trap you’re implicitly disabling the old one. You can also disable a trap by specifying - as the argument, like this:

trap - INT TERM EXIT

# Conclusion

As you can see, the shell provides several good tricks for dealing with errors and corrupted state. In simple situations, it’s good enough to just use && and || operators, but with more complicated problems exit-on-error mode combined with trap command is a very powerful tool.