Two simple tricks for better shell script error handling

Here are two simple tricks that are easy to use and will make your scripts much more robust.

1. Turn on -e mode (do you feel lucky - punk?)

   In this mode any command your script runs which returns a non-zero exitcode - an error in the world of shell - will cause your script to itself terminate immediately with an error.
   You can do that in your shebang line:

   #!/bin/sh -e
   

   Or using set:

   set -e
   

   Yes, this is what you want. A neat predictable failure is infinitely better than a noisy unreliable failure.
   If you REALLY want to ignore an error, be explicit about it:

   # I don't care if evil-broken-command fails
   evil-broken-command || true
   

   Oh and as long as you're messing with shell modes, -e goes well with -x (which I like to think of as shell X-ray).
   Like this:

   #!/bin/sh -ex
   

   Or like this:

   # turn -x on if DEBUG is set to a non-empty string
   [ -n "$DEBUG" ] && set -x
   

   That way you can actually see what your script was doing right before it failed.

2. Use trap for robust clean-ups

   A trap is a snippet of code that the shell executes when it exits or receives a signal. For example, pressing CTRL-C in the terminal where the script is running generates the INT signal. killing the process by default generates a TERM (I.e., terminate) signal.
   I find traps most useful for making sure my scripts clean-up after themselves whatever happens (e.g., a non-zero error code in -e mode).
   For example:

   #!/bin/sh -e
   TMPFILE=$(tempfile)
   trap 'echo "removing $TMPFILE"; rm -f $TMPFILE' INT TERM EXIT
   echo TMPFILE=$TMPFILE
   echo hello world > $TMPFILE
   cat $TMPFILE
   # gives user a chance to press CTRL-C
   sleep 3
   # false always returns an error
   false
   echo "NEVER REACHED"
   

   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