Advanced Bash−Scripting Guide

Chapter 35. Bash, version 2
The current version of Bash, the one you have running on your machine, is actually version 2.XX.Y.
bash$ echo $BASH_VERSION
2.05.8(1)−release
This update of the classic Bash scripting language added array variables, [66] string and parameter expansion,
and a better method of indirect variable references, among other features.
Example 35−1. String expansion
#!/bin/bash
# String expansion.
# Introduced with version 2 of Bash.
# Strings of the form $'xxx'
# have the standard escaped characters interpreted.
echo $'Ringing bell 3 times \a \a \a'
echo $'Three form feeds \f \f \f'
echo $'10 newlines \n\n\n\n\n\n\n\n\n\n'
exit 0
Example 35−2. Indirect variable references − the new way
#!/bin/bash
# Indirect variable referencing.
# This has a few of the attributes of references in C++.
a=letter_of_alphabet
letter_of_alphabet=z
echo "a = $a"
# Direct reference.
echo "Now a = ${!a}" # Indirect reference.
# The ${!variable} notation is greatly superior to the old "eval var1=\$$var2"
echo
t=table_cell_3
table_cell_3=24
echo "t = ${!t}" # t = 24
table_cell_3=387
echo "Value of t changed to ${!t}" # 387
# This is useful for referencing members of an array or table,
# or for simulating a multi−dimensional array.
# An indexing option would have been nice (sigh).
Chapter 35. Bash, version 2 355