Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
# then "$*" and "$@" do not echo positional params as expected.
mecho () # Echo positional parameters.
{
echo "$1,$2,$3";
}
IFS=""
set a b c
mecho "$*"
mecho $*
mecho $@
mecho "$@"
# Set, but empty.
# Positional parameters.
# abc,,
# a,b,c
# a,b,c
# a,b,c
# The behavior of $* and $@ when $IFS is empty depends
# on whatever Bash or sh version being run.
# It is therefore inadvisable to depend on this "feature" in a script.
# Thanks, S.C.
exit 0
Other Special Parameters
$−
$!
Flags passed to script (using set). See Example 11−13.
This was originally a ksh construct adopted into Bash, and unfortunately it does not
seem to work reliably in Bash scripts. One possible use for it is to have a script
self−test whether it is interactive.
PID (process id) of last job run in background
LOG=$0.log
COMMAND1="sleep 100"
echo "Logging PIDs background commands for script: $0" >> "$LOG"
# So they can be monitored, and killed as necessary.
echo >> "$LOG"
# Logging commands.
echo −n "PID of \"$COMMAND1\": " >> "$LOG"
${COMMAND1} &
echo $! >> "$LOG"
# PID of "sleep 100": 1506
$_
# Thank you, Jacques Lederer, for suggesting this.
Special variable set to last argument of previous command executed.
Example 9−9. underscore variable
Chapter 9. Variables Revisited 78