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

Subshells & Subshell Grouping - Part I

Invoking a shell script by entering only the script's name on the command line creates a subshell in which the script runs.  It is important to understand that the subshell has a separate environment from the login/parent shell.  Alternatively, prefacing the script name with a "." (dot) and a space causes the script to run within the context of the login session, as if you had typed each line of the script in at the command prompt.  A simple shell script named display_info will be used to demonstrate this concept:

[livefire@hawk] # cat display_info
print
ps -f
print
print "The value of x is: $x"
print
[livefire@hawk] #

Prior to running display_info as a subshell, a value will be assigned to the variable x, and the current session's process information will be displayed.

[livefire@hawk] # x=50
[livefire@hawk] # print $x
50
[livefire@hawk] # ps -f
UID PID PPID C STIME TTY TIME CMD
livefire 455 453 0 13:27:20 pts/2 0:00 -ksh
[livefire@hawk] #
[livefire@hawk] # display_info

UID PID PPID C STIME TTY TIME CMD
livefire 540 455 0 15:11:09 pts/2 0:00 -ksh
livefire 455 453 0 13:27:20 pts/2 0:00 -ksh

The value of x is:

[livefire@hawk] #

Notice that a child process running another instance of ksh (the Korn shell program) is created, and the value assigned to the local variable x in the parent shell is not visible in the subshell.  This time the same script will be executed without creating a subshell:

[livefire@hawk] # . display_info

UID PID PPID C STIME TTY TIME CMD
livefire 455 453 0 13:27:20 pts/2 0:00 -ksh

The value of x is: 50

[livefire@hawk] #

As you can see from the output, a second process is not created and the previously defined value for x is displayed.

Next week we'll discuss when the Korn shell creates and does not create subshells, and why or why not you may want to use subshells.

Read the NEXT article in this series - Subshells & Subshell Grouping - Part II