10.2. Parameter Substitution. Comtrol DeviceMaster Software Development Kit

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Advanced Bash-Scripting Guide

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."

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# 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.

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.

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Both forms nearly equivalent. The : makes a difference only when

$parameter

has been declared and is null, [51] 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.

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: ${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

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

# 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

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Advanced Bash-Scripting Guide 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

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

$var

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${var##Pattern}

Remove from

$var

the longest part of

$Pattern

that matches the

front end

$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

$var

${var%%Pattern}

Remove from

$var

the longest part of

$Pattern

that matches the

back end

$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.

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Advanced Bash-Scripting Guide 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.

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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

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.

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# 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

# |----|

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

# 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

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