Parameter Substitution

Advanced Bash-Scripting Guide:
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# 10.2. Parameter Substitution

Manipulating and/or expanding variables

`${parameter}`

  Same as `$parameter`, i.e., value of the variable `parameter`. In certain contexts, only the less ambiguous `${parameter}` form works.

  May be used for concatenating variables with strings.

      your_id=${USER}-on-${HOSTNAME}
      echo "$your_id"
      #
      echo "Old \$PATH = $PATH"
      PATH=${PATH}:/opt/bin  # Add /opt/bin to $PATH for duration of script.
      echo "New \$PATH = $PATH"

`${parameter-default}`, `${parameter:-default}`

  If parameter not set, use default.

      var1=1
      var2=2
      # var3 is unset.

      echo ${var1-$var2}   # 1
      echo ${var3-$var2}   # 2
      #           ^          Note the $ prefix.

      echo ${username-`whoami`}
      # Echoes the result of `whoami`, if variable $username is still unset.

  | `${parameter-default}` and `${parameter:-default}` are almost equivalent. The extra : makes a difference only when `parameter` has been declared, but is null.

      #!/bin/bash
      # param-sub.sh

      #  Whether a variable has been declared
      #+ affects triggering of the default option
      #+ even if the variable is null.

      username0=
      echo "username0 has been declared, but is set to null."
      echo "username0 = ${username0-`whoami`}"
      # Will not echo.

      echo

      echo username1 has not been declared.
      echo "username1 = ${username1-`whoami`}"
      # Will echo.

      username2=
      echo "username2 has been declared, but is set to null."
      echo "username2 = ${username2:-`whoami`}"
      #                            ^
      # Will echo because of :- rather than just - in condition test.
      # Compare to first instance, above.

      #

      # Once again:

      variable=
      # variable has been declared, but is set to null.

      echo "${variable-0}"    # (no output)
      echo "${variable:-1}"   # 1
      #               ^

      unset variable

      echo "${variable-2}"    # 2
      echo "${variable:-3}"   # 3

      exit 0

  The default parameter construct finds use in providing "missing" command-line arguments in scripts.

      DEFAULT_FILENAME=generic.data
      filename=${1:-$DEFAULT_FILENAME}
      #  If not otherwise specified, the following command block operates
      #+ on the file "generic.data".
      #  Begin-Command-Block
      #  ...
      #  ...
      #  ...
      #  End-Command-Block

      #  From "hanoi2.bash" example:
      DISKS=${1:-E_NOPARAM}   # Must specify how many disks.
      #  Set $DISKS to $1 command-line-parameter,
      #+ or to $E_NOPARAM if that is unset.

  See also Example 3-4, Example 31-2, and Example A-6.

  Compare this method with using an and list to supply a default command-line argument.

`${parameter=default}`, `${parameter:=default}`

  If parameter not set, set it to default.

  Both forms nearly equivalent. The : makes a difference only when `$parameter` has been declared and is null, [1] as above.

      echo ${var=abc}   # abc
      echo ${var=xyz}   # abc
      # $var had already been set to abc, so it did not change.

`${parameter+alt_value}`, `${parameter:+alt_value}`

  If parameter set, use `alt_value`, else use null string.

  Both forms nearly equivalent. The : makes a difference only when `parameter` has been declared and is null, see below.

      echo "###### \${parameter+alt_value} ########"
      echo

      a=${param1+xyz}
      echo "a = $a"      # a =

      param2=
      a=${param2+xyz}
      echo "a = $a"      # a = xyz

      param3=123
      a=${param3+xyz}
      echo "a = $a"      # a = xyz

      echo
      echo "###### \${parameter:+alt_value} ########"
      echo

      a=${param4:+xyz}
      echo "a = $a"      # a =

      param5=
      a=${param5:+xyz}
      echo "a = $a"      # a =
      # Different result from   a=${param5+xyz}

      param6=123
      a=${param6:+xyz}
      echo "a = $a"      # a = xyz

`${parameter?err_msg}`, `${parameter:?err_msg}`

  If parameter set, use it, else print err_msg and abort the script with an exit status of 1.

  Both forms nearly equivalent. The : makes a difference only when `parameter` has been declared and is null, as above.

Example 10-7. Using parameter substitution and error messages

    #!/bin/bash

    #  Check some of the system's environmental variables.
    #  This is good preventative maintenance.
    #  If, for example, $USER, the name of the person at the console, is not set,
    #+ the machine will not recognize you.

    : ${HOSTNAME?} ${USER?} ${HOME?} ${MAIL?}
      echo
      echo "Name of the machine is $HOSTNAME."
      echo "You are $USER."
      echo "Your home directory is $HOME."
      echo "Your mail INBOX is located in $MAIL."
      echo
      echo "If you are reading this message,"
      echo "critical environmental variables have been set."
      echo
      echo

    # ------------------------------------------------------

    #  The ${variablename?} construction can also check
    #+ for variables set within the script.

    ThisVariable=Value-of-ThisVariable
    #  Note, by the way, that string variables may be set
    #+ to characters disallowed in their names.
    : ${ThisVariable?}
    echo "Value of ThisVariable is $ThisVariable".

    echo; echo

    : ${ZZXy23AB?"ZZXy23AB has not been set."}
    #  Since ZZXy23AB has not been set,
    #+ then the script terminates with an error message.

    # You can specify the error message.
    # : ${variablename?"ERROR MESSAGE"}

    # Same result with:   dummy_variable=${ZZXy23AB?}
    #                     dummy_variable=${ZZXy23AB?"ZXy23AB has not been set."}
    #
    #                     echo ${ZZXy23AB?} >/dev/null

    #  Compare these methods of checking whether a variable has been set
    #+ with "set -u" . . .

    echo "You will not see this message, because script already terminated."

    HERE=0
    exit $HERE   # Will NOT exit here.

    # In fact, this script will return an exit status (echo $?) of 1.

Example 10-8. Parameter substitution and "usage" messages

    #!/bin/bash
    # usage-message.sh

    : ${1?"Usage: $0 ARGUMENT"}
    #  Script exits here if command-line parameter absent,
    #+ with following error message.
    #    usage-message.sh: 1: Usage: usage-message.sh ARGUMENT

    echo "These two lines echo only if command-line parameter given."
    echo "command-line parameter = \"$1\""

    exit 0  # Will exit here only if command-line parameter present.

    # Check the exit status, both with and without command-line parameter.
    # If command-line parameter present, then "$?" is 0.
    # If not, then "$?" is 1.

Parameter substitution and/or expansion. The following expressions are the complement to the match `in` expr string operations (see Example 16-9). These particular ones are used mostly in parsing file path names.

Variable length / Substring removal

`${#var}`

  `String length` (number of characters in `$var`). For an array, ${#array} is the length of the first element in the array.

  | Exceptions: ${#*} and ${#@} give the number of positional parameters. For an array, ${#array[*]} and ${#array[@]} give the number of elements in the array.

  Example 10-9. Length of a variable

      #!/bin/bash
      # length.sh

      E_NO_ARGS=65

      if [ $# -eq 0 ]  # Must have command-line args to demo script.
      then
        echo "Please invoke this script with one or more command-line arguments."
        exit $E_NO_ARGS
      fi

      var01=abcdEFGH28ij
      echo "var01 = ${var01}"
      echo "Length of var01 = ${#var01}"
      # Now, let's try embedding a space.
      var02="abcd EFGH28ij"
      echo "var02 = ${var02}"
      echo "Length of var02 = ${#var02}"

      echo "Number of command-line arguments passed to script = ${#@}"
      echo "Number of command-line arguments passed to script = ${#*}"

      exit 0

`${var#Pattern}`, `${var##Pattern}`

  ${var#Pattern} Remove from `$var` the shortest part of `$Pattern` that matches the `front end` of `$var`.

  ${var##Pattern} Remove from `$var` the longest part of `$Pattern` that matches the `front end` of `$var`.

  A usage illustration from Example A-7:

      # Function from "days-between.sh" example.
      # Strips leading zero(s) from argument passed.

      strip_leading_zero () #  Strip possible leading zero(s)
      {                     #+ from argument passed.
        return=${1#0}       #  The "1" refers to "$1" -- passed arg.
      }                     #  The "0" is what to remove from "$1" -- strips zeros.

  Manfred Schwarb's more elaborate variation of the above:

      strip_leading_zero2 () # Strip possible leading zero(s), since otherwise
      {                      # Bash will interpret such numbers as octal values.
        shopt -s extglob     # Turn on extended globbing.
        local val=${1##+(0)} # Use local variable, longest matching series of 0's.
        shopt -u extglob     # Turn off extended globbing.
        _strip_leading_zero2=${val:-0}
                             # If input was 0, return 0 instead of "".
      }

  Another usage illustration:

      echo `basename $PWD`        # Basename of current working directory.
      echo "${PWD##*/}"           # Basename of current working directory.
      echo
      echo `basename $0`          # Name of script.
      echo $0                     # Name of script.
      echo "${0##*/}"             # Name of script.
      echo
      filename=test.data
      echo "${filename##*.}"      # data
                                  # Extension of filename.

`${var%Pattern}`, `${var%%Pattern}`

  ${var%Pattern} Remove from `$var` the shortest part of `$Pattern` that matches the `back end` of `$var`.

  ${var%%Pattern} Remove from `$var` the longest part of `$Pattern` that matches the `back end` of `$var`.

Version 2 of Bash added additional options.

Example 10-10. Pattern matching in parameter substitution

    #!/bin/bash
    # patt-matching.sh

    # Pattern matching  using the # ## % %% parameter substitution operators.

    var1=abcd12345abc6789
    pattern1=a*c  # * (wild card) matches everything between a - c.

    echo
    echo "var1 = $var1"           # abcd12345abc6789
    echo "var1 = ${var1}"         # abcd12345abc6789
                                  # (alternate form)
    echo "Number of characters in ${var1} = ${#var1}"
    echo

    echo "pattern1 = $pattern1"   # a*c  (everything between 'a' and 'c')
    echo "--------------"
    echo '${var1#$pattern1}  =' "${var1#$pattern1}"    #         d12345abc6789
    # Shortest possible match, strips out first 3 characters  abcd12345abc6789
    #                                     ^^^^^               |-|
    echo '${var1##$pattern1} =' "${var1##$pattern1}"   #                  6789
    # Longest possible match, strips out first 12 characters  abcd12345abc6789
    #                                    ^^^^^                |----------|

    echo; echo; echo

    pattern2=b*9            # everything between 'b' and '9'
    echo "var1 = $var1"     # Still  abcd12345abc6789
    echo
    echo "pattern2 = $pattern2"
    echo "--------------"
    echo '${var1%pattern2}  =' "${var1%$pattern2}"     #     abcd12345a
    # Shortest possible match, strips out last 6 characters  abcd12345abc6789
    #                                     ^^^^                         |----|
    echo '${var1%%pattern2} =' "${var1%%$pattern2}"    #     a
    # Longest possible match, strips out last 12 characters  abcd12345abc6789
    #                                    ^^^^                 |-------------|

    # Remember, # and ## work from the left end (beginning) of string,
    #           % and %% work from the right end.

    echo

    exit 0

Example 10-11. Renaming file extensions:

    #!/bin/bash
    # rfe.sh: Renaming file extensions.
    #
    #         rfe old_extension new_extension
    #
    # Example:
    # To rename all *.gif files in working directory to *.jpg,
    #          rfe gif jpg

    E_BADARGS=65

    case $# in
      0|1)             # The vertical bar means "or" in this context.
      echo "Usage: `basename $0` old_file_suffix new_file_suffix"
      exit $E_BADARGS  # If 0 or 1 arg, then bail out.
      ;;
    esac

    for filename in *.$1
    # Traverse list of files ending with 1st argument.
    do
      mv $filename ${filename%$1}$2
      #  Strip off part of filename matching 1st argument,
      #+ then append 2nd argument.
    done

    exit 0

Variable expansion / Substring replacement

  These constructs have been adopted from ksh.

`${var:pos}`

  Variable `var` expanded, starting from offset `pos`.

`${var:pos:len}`

  Expansion to a max of `len` characters of variable `var`, from offset `pos`. See Example A-13 for an example of the creative use of this operator.

`${var/Pattern/Replacement}`

  First match of `Pattern`, within `var` replaced with `Replacement`.

  If `Replacement` is omitted, then the first match of `Pattern` is replaced by nothing, that is, deleted.

`${var//Pattern/Replacement}`

  Global replacement. All matches of `Pattern`, within `var` replaced with `Replacement`.

  As above, if `Replacement` is omitted, then all occurrences of `Pattern` are replaced by nothing, that is, deleted.

  Example 10-12. Using pattern matching to parse arbitrary strings

      #!/bin/bash

      var1=abcd-1234-defg
      echo "var1 = $var1"

      t=${var1#*-*}
      echo "var1 (with everything, up to and including first - stripped out) = $t"
      #  t=${var1#*-}  works just the same,
      #+ since # matches the shortest string,
      #+ and * matches everything preceding, including an empty string.
      # (Thanks, Stephane Chazelas, for pointing this out.)

      t=${var1##*-*}
      echo "If var1 contains a \"-\", returns empty string...   var1 = $t"

      t=${var1%*-*}
      echo "var1 (with everything from the last - on stripped out) = $t"

      echo

      # -------------------------------------------
      path_name=/home/bozo/ideas/thoughts.for.today
      # -------------------------------------------
      echo "path_name = $path_name"
      t=${path_name##/*/}
      echo "path_name, stripped of prefixes = $t"
      # Same effect as   t=`basename $path_name` in this particular case.
      #  t=${path_name%/}; t=${t##*/}   is a more general solution,
      #+ but still fails sometimes.
      #  If $path_name ends with a newline, then `basename $path_name` will not work,
      #+ but the above expression will.
      # (Thanks, S.C.)

      t=${path_name%/*.*}
      # Same effect as   t=`dirname $path_name`
      echo "path_name, stripped of suffixes = $t"
      # These will fail in some cases, such as "../", "/foo////", # "foo/", "/".
      #  Removing suffixes, especially when the basename has no suffix,
      #+ but the dirname does, also complicates matters.
      # (Thanks, S.C.)

      echo

      t=${path_name:11}
      echo "$path_name, with first 11 chars stripped off = $t"
      t=${path_name:11:5}
      echo "$path_name, with first 11 chars stripped off, length 5 = $t"

      echo

      t=${path_name/bozo/clown}
      echo "$path_name with \"bozo\" replaced  by \"clown\" = $t"
      t=${path_name/today/}
      echo "$path_name with \"today\" deleted = $t"
      t=${path_name//o/O}
      echo "$path_name with all o's capitalized = $t"
      t=${path_name//o/}
      echo "$path_name with all o's deleted = $t"

      exit 0

`${var/#Pattern/Replacement}`

  If prefix of `var` matches `Pattern`, then substitute `Replacement` for `Pattern`.

`${var/%Pattern/Replacement}`

  If suffix of `var` matches `Pattern`, then substitute `Replacement` for `Pattern`.

  Example 10-13. Matching patterns at prefix or suffix of string

      #!/bin/bash
      # var-match.sh:
      # Demo of pattern replacement at prefix / suffix of string.

      v0=abc1234zip1234abc    # Original variable.
      echo "v0 = $v0"         # abc1234zip1234abc
      echo

      # Match at prefix (beginning) of string.
      v1=${v0/#abc/ABCDEF}    # abc1234zip1234abc
                              # |-|
      echo "v1 = $v1"         # ABCDEF1234zip1234abc
                              # |----|

      # Match at suffix (end) of string.
      v2=${v0/%abc/ABCDEF}    # abc1234zip123abc
                              #              |-|
      echo "v2 = $v2"         # abc1234zip1234ABCDEF
                              #               |----|

      echo

      #  ----------------------------------------------------
      #  Must match at beginning / end of string,
      #+ otherwise no replacement results.
      #  ----------------------------------------------------
      v3=${v0/#123/000}       # Matches, but not at beginning.
      echo "v3 = $v3"         # abc1234zip1234abc
                              # NO REPLACEMENT.
      v4=${v0/%123/000}       # Matches, but not at end.
      echo "v4 = $v4"         # abc1234zip1234abc
                              # NO REPLACEMENT.

      exit 0

`${!varprefix*}`, `${!varprefix@}`

  Matches names of all previously declared variables beginning with `varprefix`.

      # This is a variation on indirect reference, but with a * or @.
      # Bash, version 2.04, adds this feature.

      xyz23=whatever
      xyz24=

      a=${!xyz*}         #  Expands to *names* of declared variables
      # ^ ^   ^           + beginning with "xyz".
      echo "a = $a"      #  a = xyz23 xyz24
      a=${!xyz@}         #  Same as above.
      echo "a = $a"      #  a = xyz23 xyz24

      echo "---"

      abc23=something_else
      b=${!abc*}
      echo "b = $b"      #  b = abc23
      c=${!b}            #  Now, the more familiar type of indirect reference.
      echo $c            #  something_else

### Notes

[1] | If $parameter is null in a non-interactive script, it will terminate with a 127 exit status (the Bash error code for "command not found").

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