Tuesday, March 21, 2017

Pipe inside a bash script


Today I got stuck trying to use pipe inside a command variable in a bash script:

$CMD="echo foo | cat"
EXEC=`$CMD`                 # expecting foo
echo $EXEC                  # got foo | cat

I could solve that if I run the command directly, without variables:

EXEC=`echo foo | cat`       # expecting foo
echo $EXEC                  # got foo

But that was not an option, because $CMD was complex and carefully mounted before that snippet.

Googling around I found that the solution is to use eval (although I used it slightly different from the source):

$CMD="echo foo | cat"
EXEC=`eval $CMD`            # expecting foo
echo $EXEC                  # got foo

The reason is that eval can expand the pipe command inside a variable, while backticks can't.


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