Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
return 0
}
It is even possible for a script to source itself, though this does not seem to have any practical
applications.
Example 11−19. A (useless) script that sources itself
#!/bin/bash
# self−source.sh: a script sourcing itself "recursively."
# From "Stupid Script Tricks," Volume II.
MAXPASSCNT=100
# Maximum number of execution passes.
echo −n "$pass_count "
# At first execution pass, this just echoes two blank spaces,
#+ since $pass_count still uninitialized.
let "pass_count += 1"
# Assumes the uninitialized variable $pass_count
#+ can be incremented the first time around.
# This works with Bash and pdksh, but
#+ it relies on non−portable (and possibly dangerous) behavior.
# Better would be to set $pass_count to 0 if non−initialized.
while [ "$pass_count" −le $MAXPASSCNT ]
do
. $0 # Script "sources" itself, rather than calling itself.
# ./$0 (which would be true recursion) doesn't work here.
done
# What occurs here is not actually recursion,
#+ since the script effectively "expands" itself
#+ (generates a new section of code)
#+ with each pass throught the 'while' loop',
# with each 'source' in line 20.
#
# Of course, the script interprets each newly 'sourced' "#!" line
#+ as a comment, and not as the start of a new script.
echo
exit 0 # The net effect is counting from 1 to 100.
# Very impressive.
exit
exec
# Exercise:
# −−−−−−−−
# Write a script that uses this trick to do something useful.
Unconditionally terminates a script. The exit command may optionally take an integer argument,
which is returned to the shell as the exit status of the script. It is good practice to end all but the
simplest scripts with an exit 0, indicating a successful run.
If a script terminates with an exit lacking an argument, the exit status of the script is
the exit status of the last command executed in the script, not counting the exit.
Chapter 11. Internal Commands and Builtins 141