The trap defines and activates handlers to be run when the shell receives signals or other special conditions.

trap [-lp] [[ARG] SIGNAL_SPEC...]

  • ARG is a command to be read and executed when the shell receives the signal(s)

  • SIGNAL_SPEC. If ARG is absent (and a single SIGNAL_SPEC is supplied) or ARG is a dash (“-“), each specified signal is reset to its original value. If ARG is the null string, each SIGNAL_SPEC is ignored by the shell and by the commands it invokes.

  • If a SIGNAL_SPEC is EXIT (0), ARG is executed upon exit from the shell.

  • If a SIGNAL_SPEC is DEBUG, ARG is executed before every simple command.

  • If a SIGNAL_SPEC is RETURN, ARG is executed each time a shell function or a script run by the “.” or source built-in commands finishes executing.

A SIGNAL_SPEC of ERR means to execute ARG each time a command’s failure would cause the shell to exit when the -e option is enabled. If no arguments are supplied, trap prints the list of commands associated with each signal.

In bash $$ is the process ID, which is fine if you need a single file, otherwise take a look at the mktemp command.

Example extracted from .git_template/hooks/ctags of my dotfiles:

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set -e
PATH="/usr/local/bin:$PATH"
dir="`git rev-parse --git-dir`"
trap 'rm -f "$dir/$$.tags"' EXIT
git ls-files | \
  ctags --tag-relative -L - -f"$dir/$$.tags" --languages=-javascript,sql
mv "$dir/$$.tags" "$dir/tags"

Explanation for git command: git rev-parse --git-dir

Show $GIT_DIR if defined. Otherwise show the path to the .git directory. The path shown, when relative, is relative to the current working directory.

If $GIT_DIR is not defined and the current directory is not detected to lie in a Git repository or work tree print a message to stderr and exit with nonzero status.