Shell scripting (BASH) : How to create temporary random file name

Sometimes you need to create a temporary file in your shell script. There are various methods exist to create a random temporary file name. It is quite useful if your application/shell scripting needs temporary unique file names.

Method # 1 Use of mktemp or tempfile utility

As the name suggest, both of the following commands create a unique temporary file or directory. Just type the mktemp at the shell prompt to create it a temp file or dir:
$ mktemp
Sample outputs

/tmp/tmp.Le4jmo6TrH

OR
$ tempfile
Sample outputs:

/tmp/file1wqzcO

Please note that the tempfile command is deprecated; you should use always use mktemp command instead. So to create a temp file:

tfile=$(mktemp /tmp/foo.XXXXXXXXX)
echo "a file: $tfile"

To create a temp dir:

tdir=$(mktemp -d /tmp/foo.XXXXXXXXX)
echo "a direcotry: $tdir"

How to make a directory

Make a unique temporary directory instead of a file using -d option. The syntax is:
$ mktemp -d

A shell script example

#!/bin/bash
s="https://server1.cyberciti.biz/?Download=ips-v4&Format=text"
f="$(mktemp /tmp/myscript.XXXXXX)"
wget -q -O $f $s
echo "IPv4 address downloaded to '$f'.."
echo "Processing..."
#
# logic to do something on $f here
#
# Delete the temp file
rm -f "$f"

Rest of the following methods are insecure and do not use them in production. They are here for historical reasons only.

Method #2: Use $RANDOM bash shell variable

At shell prompt type the command:
$ echo $RANDOM
Sample outputs:

You can use it as follows:

file="/tmp/myscript.$RANDOM"
echo "Working on temp $file ..."
echo "Deleting $file ..."
rm -f "$file"

Method # 3 Use of $$ variable

This is old and classic method. $$ shell variable returns the current running process this can be use to create unique temporary file as demonstrated in following script:
vi random2.bash

#!/bin/bash
#
TFILE="/tmp/$(basename $0).$$.tmp"
ls &gt; $TFILE
echo "See directory listing in $TFILE"

Save the script and execute as follows:
$ chmod +x random2.bash $ ./ random2.bash
Use this method if your script needs only ONE temporary file.

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.

ADVERTISEMENTS

14 comment

how can i record the log in the log file in shell script?
like during running whatever the user is seeing, that should be recorded in log file.

Use logger command

createtempfiles.bash is missing… But article is still useful 🙂

Artem,

Thanks for the heads up! The post has beeb updated.

$ tempfile -d does not create a temporary directory. Instead, it requires a directory as an argument and creates a temporary file inside that directory.

MKTEMP(1)                        User Commands                       MKTEMP(1)
...
       -d, --directory
              create a directory, not a file

So your comment seems to be incorrect.
To specify directory to create a file in use either
 --tmpdir[=DIR]
              interpret TEMPLATE relative to DIR.  If DIR is not specified, use $TMPDIR if set, else /tmp.  With this option, TEMPLATE must not be an absolute name.  Unlike with -t, TEM-
              PLATE may contain slashes, but mktemp creates only the final component.
or
-p DIR use DIR as a prefix; implies -t [deprecated]

Thank’s. Obrigado ! Very good tutorial !

useful posts.
But i need to have filenames in sequence eachtime when i run the script in which i am creating the file.

CWS says:
April 30, 2013 at 11:09 pm
This is very necro, but for others who find this rather high ranking search and want to create some “temp” files in sequence, you could do something like:
BASE=$(mktemp)
for a in {0..5}; do
cp $BASE $BASE.$a
done
That said, I really don’t understand why you’d care if they were in sequence.
Reply

This article is very useful: it helps me solve my problem. Thank you.

After reading http://www.linuxsecurity.com/content/view/115462/151/ , I think some of your examples may be vulnerable to symlink attacks.

$RANDOM (once in the filename) or $$ alone are not enough because it may be possible for an attacker to create symlinks for all filenames.

Great summary. Thank you.
It helps writing shell script!

mktemp /path/of/dorectory/filename.XXXXXX
The more X’s you add the more random characters it adds.

Suggest that you list the preferred method first rather than last.