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Advanced Bash-Scripting Guide:
PrevChapter 34. Bash, versions 2 and 3Next

34.2. Bash, version 3

On July 27, 2004, Chet Ramey released version 3 of Bash. This update fixed quite a number of bugs and added new features.

Some of the more important added features:

  • 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 ASCII characters between Z and a,
               #+ but don't rely on this type of behavior because . . .
echo {]..a}    #  {]..a}
               #  Why?



# Unfortunately, this does not lend itself to parameterization.
var1=1
var2=5
echo {$var1..$var2}   # {1..5}

  • 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"

# Regex matching with =~ operator within [[ double brackets ]].
if [[ "$variable" =~ "T.........fin*es*" ]]
#                    ^                 ^
# NOTE: Quoting not necessary, as of version 3.2 of Bash.
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]" ]]
#                 ^ NOTE: Quoting not necessary, as of version 3.2 of Bash.
# NNN-NN-NNNN (where each N is a digit). 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-31, Example 18-14, Example A-37, and Example A-26.

  • 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 15-43.

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-21 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

34.2.2. Bash, version 3.2

This is pretty much a bugfix update.

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