Question

I have a shell script that constructs an awk program as a string then pass that string to awk. This is because I want to use values of shell variables in the awk program.

My code looks like this:

awk_prog="'{if (\$4~/$shell_var/) print \$1,\$2}'"
echo $awk_prog
awk $awk_prog $FILENAME

However, when I pass the string to awk, I always get the error:

'{if ($4~/regex/) print $1,$2}'
awk: '{if
awk: ^ invalid char ''' in expression

What does that error message mean? I tried the -F: switch but it does not help. How can I settle this issue?

Thank you.

Answer 1

This is caused by shell quoting. The following will work:

awk_prog="{ if (\$4 ~ /$shell_var/) print \$1, \$2 }"
echo "$awk_prog"
awk "$awk_prog" $FILENAME

When you run awk '{ print }' foo from the command line, the shell interprets and removes the quotes around the program so awk receives two arguments – the first is the program text and the second is the filename foo. Your example was sending awk the program text '{if ...}' which is invalid syntax as far as awk is concerned. The outer quotes should not be present.

In the snippet that I gave above, the shell uses the quotes in the awk_prog= line to group the contents of the string into a single value and then assigns it to the variable awk_prog. When it executes the awk "$awk_prog"... line, you have to quote the expansion of $awk_prog so awk receives the program text as a single argument.