GROUPING COMMAND IN LINUX

Monday, 1 April 2013

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GROUPING COMMAND IN LINUX

GROUPING COMMAND

Two ways to group commands:

Compound:date ; who WC -I

Command run back-to-back

Sub shell: (date ; who WC -I)>>/tmp/trace

All output is sent to a single STDOUT and STDERR

Aggregating Output Redirection:

Suppose you want to maintain a count of the number of users logged on,along with a time/date stamp,in a log file.This could be accomplished with two commands:

[student@tationX ~] $ date >> log-file
[student@tationX ~] $ who / WC -1 >> log-file

This command sequence required that you enter two lines of command,append to log-file twice,and in general,type much more than is necessary.When writing to the terminal,this task can be simplified by combining the command on one line separated by semicolons:

[student@tationX ~] $date; who /WC -1

But if your intent is to redirect standard output,this will not work as expected:

[student@tationX ~] $ (date; who /WC -1) >> log-file

Both commands will run,but only the second one will redirect its output to log-file.

subs-hells

Commands inside parentheses are run in their own instance of bash,called a subshell. The output of all commands run inside a subshell are sent to the subshell's STDOUT and STDERR,making it possible to send multiple programs through the same pipe:

[student@tationX ~]$ (date; who /WC -1) >> log-file

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