LiveFire Labs' UNIX Tip, Trick, or Shell Script of the Week

Using $$ to Create Unique Filenames in UNIX or Linux

There may be times when your Korn shell script will need to dynamically create temporary files that may be accessed intermittently while the script is running.  How would you ensure the integrity of these files if two or more users were running the script simultaneously?  One option is to use the $ shell parameter to create a unique filename for each temporary file.

The $ shell parameter is a special parameter (set by the Korn shell*) that contains the process ID, or the PID, of the running shell.  The PID is a unique integer that is assigned to each process when it is created.  The system and process' owner use this number to manage the process.

To access the value stored in $, you simply preface it with a second $.  For example, the following command will print the running shell's PID to standard output:

# print $$
31853

To uniquely name temporary shell script files, you would append the value stored in $ to the end of the temporary file's base name:

/tmp/filename.$$

This syntax may be used when you use exec to open a file for writing:

exec 3> /tmp/filename.$$

...or within simple I/O redirection statements such as:

print "Script starting at $(date +%m%d%Y_%H%M%S)" > /tmp/filename.$$

Like anything else in UNIX, there are multiple methods for generating unique filenames but using a shell's PID is one of the most frequently-used techniques.

*You cannot assign your own value to this parameter