9.3. 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.
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 "${variable:-3}" # 3 exit 0The default parameter construct finds use in providing "missing" command-line arguments in scripts.
DEFAULT_FILENAME=generic.data filename=${1:-$DEFAULT_FILENAME} # If not otherwise specified, the following command block operates #+ on the file "generic.data". # # Commands follow.See also Example 3-4, Example 28-2, 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 ${username=`whoami`} # Variable "username" is now set to `whoami`.- ${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.
Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, as above.
Example 9-16. 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 9-17. 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 15-9). 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:
Example 9-18. 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 9-19. 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 9-20. 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}
As above, if Replacement is omitted, then all occurrences of Pattern are replaced by nothing, that is, deleted.
Example 9-21. 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 9-22. 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