How to: create a temporary file securely
Q. How do I create a temporary file securely under UNIX Bash shell?
A. There are many ways:
[a] mktemp command - make temporary unique filename
[b] $RANDOM - Use $RANDOM variable
mktemp command
From the man page:
The mktemp utility takes the given filename template and overwrites a portion of it to create a unique filename. The template may be any filename with some number of ‘Xs’ appended to it, for example /tmp/tfile.XXXXXXXXXX.
For example, create a temporary file:
$ mktemp /tmp/output.XXXXXXXXXX
Output:
/tmp/output.qBYDtF7199
Send ls command output to /tmp/output.qBYDtF7199:
$ ls > /tmp/output.qBYDtF7199
However, you may need to store temporary file name /tmp/output.qBYDtF7199 to a shell variable:
TMPFILE=$(mktemp /tmp/output.XXXXXXXXXX)
ls > $TMPFILE
Create a temporary directory
The -d option makes a directory instead of a file.
TMPDIR=$(mktemp -d /tmp/output.XXXXXXXXXX)
cd $TMPDIR
# do something
Another example:
TMPFILE=‘mktemp -t /tmp/out.myapp.XXXXXXXXXX‘ && {
# Safe to use $TMPFILE in this block
echo data > $TMPFILE
...
# do something
# clean up
rm -f $TMPFILE
}
Using $RANDOM variable
Bash also provide $RANDOM variable with random value, you can use the same to create a file or directory:
TEMFILE=/tmp/$RANDOM > $TEMFILE # create directory TEMDIR=/tmp/$RANDOM.$RANDOM mkdir $TEMDIR # do something...
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Tags: $RANDOM, /tmp/output.XXXXXXXXXX file, BASH Shell, mktemp_command, random variable, shell variable, temporary file, tmp, UNIX
November 30th, 2007 at 1:27 pm
do not ever use $RANDOM for creating files or directories…! mktemp can create also a directory if you use -d option. the “mkdir somethin” (where something can be $RANDOM or whatever else) suffers from race conditions. what would you do if directory was already there and not owned by you? with mktemp -d you have no problems.
November 30th, 2007 at 2:41 pm
matej,
Actually, I talked about -d option, but forgot to type -d.
I appreciate your post.