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shell script

Posted on 2011-09-29
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Last Modified: 2012-05-12
what is the meaning of below .. pls give me some examples to understand better

        if [ x$TEMP_LIFE != "x" ]
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Question by:damodar4u
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tel2 earned 500 total points
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This is not Perl, so this should be in a shell scripting zone.

This code:
    if [ x$TEMP_LIFE != "x" ]
is another way of writing:
    if [ $TEMP_LIFE != "" ]
(i.e. $TEMP_LIFE is not empty), but the latter would fail, because if $TEMP_LIFE is empty, it's like writing this:
    if [   != "" ]
which is a syntax error, because the != operator requires 1 argument on each side.
But if you prefix both with some character like "x", then if $TEMP_LIFE is empty, then it would be like writing this:
    if [ "x" != "x" ]
which works fine.

There are tidier ways to achieve the same, e.g.
    if [ "$TEMP_LIFE" != "" ]
or:
    if [ ! -z $TEMP_LIFE ]
To see what the -z switch means, do: man test
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