Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
exit 0
Example 9−22. Passing an indirect reference to awk
#!/bin/bash
# Another version of the "column totaler" script
# that adds up a specified column (of numbers) in the target file.
# This uses indirect references.
ARGS=2
E_WRONGARGS=65
if [ $# −ne "$ARGS" ] # Check for proper no. of command line args.
then
echo "Usage: `basename $0` filename column−number"
exit $E_WRONGARGS
fi
filename=$1
column_number=$2
#===== Same as original script, up to this point =====#
# A multi−line awk script is invoked by awk ' ..... '
# Begin awk script.
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
awk "
{ total += \$${column_number} # indirect reference
}
END {
print total
}
" "$filename"
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# End awk script.
# Indirect variable reference avoids the hassles
# of referencing a shell variable within the embedded awk script.
# Thanks, Stephane Chazelas.
exit 0
This method of indirect referencing is a bit tricky. If the second order variable changes its value, then the
first order variable must be properly dereferenced (as in the above example). Fortunately, the
${!variable} notation introduced with version 2 of Bash (see Example 35−2) makes indirect
referencing more intuitive.
Chapter 9. Variables Revisited 95