Bash, version 3

Advanced Bash-Scripting Guide:
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34.2. Bash, version 3

On July 27, 2004, Chet Ramey released version 3 of Bash. This update fixes quite a number of bug in Bash and adds some new features.

Some of the added features are:

A new, more generalized {a..z} brace expansion operator.

#!/bin/bash

for i in {1..10}
#  Simpler and more straightforward than
#+ for i in $(seq 10)
do
  echo -n "$i "
done

echo

# 1 2 3 4 5 6 7 8 9 10



# Or just . . .

echo {a..z}    #  a b c d e f g h i j k l m n o p q r s t u v w x y z
echo {z..a}    #  z y x w v u t s r q p o n m l k j i h g f e d c b a
               #  Works backwards, too.
echo {3..-2}   #  3 2 1 0 -1 -2
echo {X..d}    #  X Y Z [  ] ^ _ ` a b c d
               #  Shows (some of) the the ASCII characters between Z and a,
               #+ but don't rely on this type of behavior because . . .
echo {]..a}    #  {]..a}
               #  Why?

The ${!array[@]} operator, which expands to all the indices of a given array.

#!/bin/bash

Array=(element-zero element-one element-two element-three)

echo ${Array[0]}   # element-zero
                   # First element of array.

echo ${!Array[@]}  # 0 1 2 3
                   # All the indices of Array.

for i in ${!Array[@]}
do
  echo ${Array[i]} # element-zero
                   # element-one
                   # element-two
                   # element-three
                   #
                   # All the elements in Array.
done

The =~ Regular Expression matching operator within a double brackets test expression. (Perl has a similar operator.)

#!/bin/bash

variable="This is a fine mess."

echo "$variable"

if [[ "$variable" =~ "T*fin*es*" ]]
# Regex matching with =~ operator within [[ double brackets ]].
then
  echo "match found"
      # match found
fi

Or, more usefully:

#!/bin/bash

input=$1


if [[ "$input" =~ "[1-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]" ]]
# NNN-NN-NNNN
# Where each N is a digit.
# But, initial digit must not be 0.
then
  echo "Social Security number."
  # Process SSN.
else
  echo "Not a Social Security number!"
  # Or, ask for corrected input.
fi

For additional examples of using the =~ operator, see Example A-29 and Example 18-14.

The new set -o pipefail option is useful for debugging pipes. If this option is set, then the exit status of a pipe is the exit status of the last command in the pipe to fail (return a non-zero value), rather than the actual final command in the pipe.
See Example 15-39.

The update to version 3 of Bash breaks a few scripts that worked under earlier versions. Test critical legacy scripts to make sure they still work!

As it happens, a couple of the scripts in the Advanced Bash Scripting Guide had to be fixed up (see Example A-20 and Example 9-4, for instance).

34.2.1. Bash, version 3.1

The version 3.1 update of Bash introduces a number of bugfixes and a few minor changes.

The += operator is now permitted in in places where previously only the = assignment operator was recognized.

a=1
echo $a        # 1

a+=5           # Won't work under versions of Bash earlier than 3.1.
echo $a        # 15

a+=Hello
echo $a        # 15Hello

Here, += functions as a string concatenation operator. Note that its behavior in this particular context is different than within a let construct.

a=1
echo $a        # 1

let a+=5       # Integer arithmetic, rather than string concatenation.
echo $a        # 6

let a+=Hello   # Doesn't "add" anything to a.
echo $a        # 6

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