Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
m4
exit 0
A hidden treasure, m4 is a powerful macro processing filter, [37] virtually a complete language.
Although originally written as a pre−processor for RatFor, m4 turned out to be useful as a
stand−alone utility. In fact, m4 combines some of the functionality of eval, tr, and awk, in addition to
its extensive macro expansion facilities.
The April, 2002 issue of Linux Journal has a very nice article on m4 and its uses.
Example 12−45. Using m4
doexec
#!/bin/bash
# m4.sh: Using the m4 macro processor
# Strings
string=abcdA01
echo "len($string)" | m4 # 7
echo "substr($string,4)" | m4
# A01
echo "regexp($string,[0−1][0−1],\&Z)" | m4
# 01Z
# Arithmetic
echo "incr(22)" | m4 # 23
echo "eval(99 / 3)" | m4 # 33
exit 0
The doexec command enables passing an arbitrary list of arguments to a binary executable. In
particular, passing argv[0] (which corresponds to $0 in a script) lets the executable be invoked by
various names, and it can then carry out different sets of actions, according to the name by which it
was called. What this amounts to is roundabout way of passing options to an executable.
For example, the /usr/local/bin directory might contain a binary called "aaa". Invoking doexec
/usr/local/bin/aaa list would list all those files in the current working directory beginning with an "a",
while invoking (the same executable with) doexec /usr/local/bin/aaa delete would delete those files.
The various behaviors of the executable must be defined
within the code of the executable itself, analogous to
something like the following in a shell script:
case `basename $0` in
"name1" ) do_something;;
"name2" ) do_something_else;;
"name3" ) do_yet_another_thing;;
* ) bail_out;;
esac
Chapter 12. External Filters, Programs and Commands 213