Advanced Bash−Scripting Guide

Chapter 9. Variables Revisited
Used properly, variables can add power and flexibility to scripts. This requires learning their subtleties and
nuances.
9.1. Internal Variables
Builtin variables
variables affecting bash script behavior
$BASH
the path to the Bash binary itself
bash$ echo $BASH
/bin/bash
$BASH_ENV
an environmental variable pointing to a Bash startup file to be read when a script is invoked
$BASH_VERSINFO[n]
a 6−element array containing version information about the installed release of Bash. This is similar
to $BASH_VERSION, below, but a bit more detailed.
# Bash version info:
for n in 0 1 2 3 4 5
do
echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}"
done
# BASH_VERSINFO[0] = 2 # Major version no.
# BASH_VERSINFO[1] = 05 # Minor version no.
# BASH_VERSINFO[2] = 8 # Patch level.
# BASH_VERSINFO[3] = 1 # Build version.
# BASH_VERSINFO[4] = release # Release status.
# BASH_VERSINFO[5] = i386−redhat−linux−gnu # Architecture
# (same as $MACHTYPE).
$BASH_VERSION
the version of Bash installed on the system
bash$ echo $BASH_VERSION
2.04.12(1)−release
tcsh% echo $BASH_VERSION
BASH_VERSION: Undefined variable.
Checking $BASH_VERSION is a good method of determining which shell is running. $SHELL does
not necessarily give the correct answer.
$DIRSTACK
the top value in the directory stack (affected by pushd and popd)
This builtin variable corresponds to the dirs command, however dirs shows the entire contents of the
directory stack.
Chapter 9. Variables Revisited 64