35.2. 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

• 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