Manipulating Strings#

Bash supports a surprising number of string manipulation operations. Unfortunately, these tools lack a unified focus. Some are a subset of parameter substitution, and others fall under the functionality of the UNIX expr command. This results in inconsistent command syntax and overlap of functionality, not to mention confusion.

String Operations#

ExpressionMeaning
${#string}Length of $string
${string:position}Extract substring from $string at $position
${string:position:length}Extract $length characters substring from $string at $position [zero-indexed,
${string#substring}Strip shortest match of $substring from front of $string
${string##substring}Strip longest match of $substring from front of $string
${string%substring}Strip shortest match of $substring from back of $string
${string%%substring}Strip longest match of $substring from back of $string
${string/substring/replacement}Replace first match of $substring with $replacement
${string//substring/replacement}Replace all matches of $substring with $replacement
${string/#substring/replacement}If $substring matches front end of $string, substitute $replacement for $substring
${string/%substring/replacement}If $substring matches back end of $string, substitute $replacement for $substring
expr match “$string” ‘$substring’Length of matching $substring* at beginning of $string
expr “$string” : ‘$substring’Length of matching $substring* at beginning of $string
expr index “$string” $substringNumerical position in $string of first character in $substring* that matches [0 if no match, first character counts as position 1]
expr substr $string $position $lengthExtract$length characters from $string starting at $position [0 if
expr match “$string” ‘($substring)’Extract$substring*, searching from beginning of $string
expr “$string” : ‘($substring)’Extract$substring* , searching from beginning of $string
expr match “$string” ‘.*($substring)’Extract$substring*, searching from end of $string
expr “$string” : ‘.*($substring)’Extract$substring*, searching from end of $string

Parameter Substitution and Expansion#

ExpressionMeaning
${var}Value of var (same as $var)
${var-$DEFAULT}If var not set, evaluate expression as $DEFAULT *
${var:-$DEFAULT}If var not set or is empty, evaluate expression as $DEFAULT *
${var=$DEFAULT}If var not set, evaluate expression as $DEFAULT *
${var:=$DEFAULT}If var not set or is empty, evaluate expression as $DEFAULT *
${var+$OTHER}If var set, evaluate expression as $OTHER, otherwise as null string
${var:+$OTHER}If var set, evaluate expression as $OTHER, otherwise as null string
${var?$ERR_MSG}If var not set, print $ERR_MSG and abort script with an exit status of 1.*
${var:?$ERR_MSG}If var not set, print $ERR_MSG and abort script with an exit status of 1.*
${!varprefix*}Matches all previously declared variables beginning with varprefix
${!varprefix@}Matches all previously declared variables beginning with varprefix

Samples#

  • Replace file name from abc_noteN to abc_N
touch abc_note1 abc_note2 abc_note3

for f in $(find . -type f -name *abc_note*); 
do 
    mv -- $f "${f//note}" 
done ;

ls abc*
# expected output
# abc_1   abc_2   abc_3
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