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p. 307

11.8 Temporary File Management

It’s sometimes necessary to create a temporary file to collect output for use by a later command. When creating such a file, make sure that the filename is distinct enough that no other programs will accidentally write to it. Sometimes using something as simple as the shell’s PID ($$) in a filename works, but when you need to be certain that there will be no conflicts, a utility such as mktemp is often a better option.

Here’s how to use the mktemp command to create temporary filenames. This script shows you the device interrupts that have occurred in the last two seconds:

#!/bin/sh
TMPFILE1=$(mktemp /tmp/im1.XXXXXX)
TMPFILE2=$(mktemp /tmp/im2.XXXXXX)

cat /proc/interrupts > $TMPFILE1
sleep 2
cat /proc/interrupts > $TMPFILE2
diff $TMPFILE1 $TMPFILE2
rm -f $TMPFILE1 $TMPFILE2

The argument to mktemp is a template. The mktemp command converts the XXXXXX to a unique set of characters and creates an empty file with that name. Notice that this script uses variable names to store the filenames so that you only have to change one line if you want to change a filename.

note

Not all Unix flavors come with mktemp. If you’re having portability problems, it’s best to install the GNU coreutils package for your operating system.

A common problem with scripts that employ temporary files is that if the script is aborted, the temporary files could be left behind. In the preceding example, pressing CTRL-C before the second cat command leaves a temporary file in /tmp. Avoid this if possible. Instead, use the trap command to create a signal handler to catch the signal that CTRL-C generates and remove the temporary files, as in this handler:

#!/bin/sh
TMPFILE1=$(mktemp /tmp/im1.XXXXXX)
TMPFILE2=$(mktemp /tmp/im2.XXXXXX)
trap "rm -f $TMPFILE1 $TMPFILE2; exit 1" INT
 --snip--

You must use exit in the handler to explicitly end script execution, or the shell will continue running as usual after running the signal handler.

note

You don’t need to supply an argument to mktemp; if you don’t, the template will begin with a /tmp/tmp. prefix.