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command substitutions

summary

    This subchapter looks at command substitutions in Unix (and Linux).

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command substitutions

    This subchapter looks at command substitutions in Unix (and Linux).

command substitution

    Command substitution is used to assign the output of a command to a variable.

    Place the command in sideways ticks (`) around the command. Do not confuse these with regular single quotation marks (').

    You can place a simple command, a pipeline, or a command list inside the tick marks.

    An example of a simple command:

    $ DATE=`date`

    An example of a pipeline:

    $ CONSOLEUSER=`who | grep console`

    An example of a command list:

    $ FILEUSAGE=`date ; df`

    You can use command substitution to create parameters for other commands.

    In the following example, the user name is used for grep word search of the file named names:

    $ grep `id -un` names

    You can use $( ) as a replacement for ` `. If you nest backticks inside of each other, you need to escape the internal backticks. You do not need to escape nested $( ).

    $ DATEVAR=$(date)
    $ echo $DATEVAR
    $Mon Aug 26 19:35:47 PDT 2013
    $

    If there is only one level of nesting, some people use the convention of using $() for the outer expression and backicks (``) fr the inner expression.

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