There is a simple, useful idiom to make your bash scripts more
robust – ensuring they always perform necessary cleanup
operations, even when something unexpected goes wrong. The secret
sauce is a pseudo-signal provided by bash, called EXIT, that you
can trap;
commands or functions trapped on it will execute when the script
exits for any reason. Let’s see how this works.

The basic code structure is like this:

#!/bin/bash
function finish {
# Your cleanup code here
}
trap finish EXIT

You place any code that you want to be certain to run in this
“finish” function. A good common example: creating a temporary
scratch directory, then deleting it after.

#!/bin/bash
scratch=$(mktemp -d -t tmp.XXXXXXXXXX)
function finish {
  rm -rf "$scratch"
}
trap finish EXIT

You can then download, generate, slice and dice intermediate or
temporary files to the $scratch directory to your heart’s
content. [1]

# Download every linux kernel ever.... FOR SCIENCE!
for major in {1..4}; do
  for minor in {0..99}; do
    for patchlevel in {0..99}; do
      tarball="linux-${major}-${minor}-${patchlevel}.tar.bz2"
      curl -q "http://kernel.org/path/to/$tarball" -o "$scratch/$tarball" || true
      if [ -f "$scratch/$tarball" ]; then
        tar jxf "$scratch/$tarball"
fi
    done
  done
done
# magically merge them into some frankenstein kernel ...
# That done, copy it to a destination
cp "$scratch/frankenstein-linux.tar.bz2" "$1"
# Here at script end, the scratch directory is erased automatically

Compare this to how you’d remove the scratch directory without
the trap:

#!/bin/bash
# DON'T DO THIS!
scratch=$(mktemp -d -t tmp.XXXXXXXXXX)
# Insert dozens or hundreds of lines of code here...
# All done, now remove the directory before we exit
rm -rf "$scratch"

What’s wrong with this? Plenty:

• If some error causes the script to exit prematurely, the
  scratch directory and its contents don’t get deleted. This is
  a resource leak, and may have security
  implications too.
• If the script is designed to exit before the end, you must
  manually copy ‘n paste the rm command at each exit point.
• There are maintainability problems as well. If you later
  add a new in-script exit, it’s easy to forget to include the
  removal – potentially creating mysterious heisenleaks.

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