Problem with backticks in shellscript

shell backticks
benw picture benw · Oct 13, 2010 · Viewed 24.5k times · Source

I am having a problem getting my shellscript working using backticks. Here is an example version of the script I am having an issue with:

#!/bin/sh

ECHO_TEXT="Echo this"
ECHO_CMD="echo ${ECHO_TEXT} | awk -F' ' '{print \$1}'"

result=`${ECHO_CMD}`;
echo $result;

result=`echo ${ECHO_TEXT} | awk -F' ' '{print \$1}'`;
echo $result;

The output of this script is:

sh-3.2$ ./test.sh 
Echo this | awk -F' ' '{print $1}'
Echo

Why does the first backtick using a variable for the command not actually execute the full command but only returns the output of the first command along with the second command? I am missing something in order to get the first backtick to execute the command?

Answer

codaddict picture codaddict · Oct 13, 2010

You need to use eval to get it working

result=`eval ${ECHO_CMD}`;

in place of

result=`${ECHO_CMD}`;

Without eval 

${ECHO_TEXT} | awk -F' ' '{print \$1}

which will be expanded to

Echo this | awk -F' ' '{print \$1}

will be treated as argument to echo and will be output verbatim. With eval that line will actually be run.

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