shell-script – How to Create a Temporary File in Shell Script

I've ran into this problem when examining behavior of a particular application I didn't trust. The application would create and later delete its temp files. kenorb's solution is good, however using cat can lead to race condition( that is a file may be removed while cat is working, so partial data only is retrieved).

A lower chance of race condition can be done with creating a hard-link to the file itself. greping and creating hard link can be combined via awk. Thus what I've came up with is the following;

inotifywait -e create -m --format "%w/%f" /tmp/suspicious_dir/ 2>&1 | 
awk 'NR>2{n=split($0,a,"/");system("ln "$0" /tmp/hardlink_to_"a[n]);}'

Since we're using -e create flag , we're only interested in temporary files that are going to be immediately created, and the output format gives us the full path to created temp file. Ignoring first two lines of output via NR>2. For every reported file, there will be created a hard link in form /tmp/hardlink_to_<original filename>

To delete the temporary file as soon as possible, you can write a wrapper, like:

#!/bin/sh
the-application "$1"
rm -f "$1"

replacing the-application by the name of the real executable, and ask Firefox to use this wrapper instead of the application. Or:

#!/bin/sh
the-application "$1"
case "$1" in
  /tmp/*) rm -f "$1" ;;
esac

This form is safer in case future Firefox versions do not create a new file for "file:" URL's (you don't want the file to be removed in this case).

Note that some applications return immediately and may fail to work correctly if the file is removed before the application is quit. In such a case, there isn't much you can do (at least in a reliable way).

Concerning the alternative (to replace an existing file with the same name), this can yield clashes with other applications that use /tmp, with possible security implications. This is not a good idea in general.