9.2. Manipulating Strings

String Length

${#string}

expr length $string

expr "$string" : '.*'

stringZ=abcABC123ABCabc echo ${#stringZ} # 15 echo `expr length $stringZ` # 15 echo `expr "$stringZ" : '.*'` # 15

Length of Matching Substring at Beginning of String

expr match "$string" '$substring'

$substring is a regular expression.

expr "$string" : '$substring'

$substring is a regular expression.

stringZ=abcABC123ABCabc # |------| echo `expr match "$stringZ" 'abc[A-Z]*.2'` # 8 echo `expr "$stringZ" : 'abc[A-Z]*.2'` # 8

Index

expr index $string $substring

Numerical position in $string of first character in $substring that matches.

stringZ=abcABC123ABCabc echo `expr index "$stringZ" C12` # 6 # C position. echo `expr index "$stringZ" 1c` # 3 # 'c' (in #3 position) matches before '1'.

This is the near equivalent of strchr() in C.

Substring Extraction

${string:position}

Extracts substring from $string at $position.

If the string parameter is "*" or "@", then this extracts the positional parameters, [1] starting at position.

${string:position:length}

Extracts $length characters of substring from $string at $position.

stringZ=abcABC123ABCabc # 0123456789..... # 0-based indexing. echo ${stringZ:0} # abcABC123ABCabc echo ${stringZ:1} # bcABC123ABCabc echo ${stringZ:7} # 23ABCabc echo ${stringZ:7:3} # 23A # Three characters of substring.

If the string parameter is "*" or "@", then this extracts a maximum of length positional parameters, starting at position.

echo ${*:2} # Echoes second and following positional parameters. echo ${@:2} # Same as above. echo ${*:2:3} # Echoes three positional parameters, starting at second.

expr substr $string $position $length

Extracts $length characters from $string starting at $position.

stringZ=abcABC123ABCabc # 123456789...... # 1-based indexing. echo `expr substr $stringZ 1 2` # ab echo `expr substr $stringZ 4 3` # ABC

expr match "$string" '\($substring\)'

Extracts $substring at beginning of $string, where $substring is a regular expression.

expr "$string" : '\($substring\)'

Extracts $substring at beginning of $string, where $substring is a regular expression.

stringZ=abcABC123ABCabc echo `expr match "$stringZ" '\(.[b-c]*[A-Z]..[0-9]\)'` # abcABC1 echo `expr "$stringZ" : '\(.[b-c]*[A-Z]..[0-9]\)'` # abcABC1 # Both of the above forms are equivalent.

Substring Removal

${string#substring}

Strips shortest match of $substring from front of $string.

${string##substring}

Strips longest match of $substring from front of $string.

stringZ=abcABC123ABCabc # |----| # |----------| echo ${stringZ#a*C} # 123ABCabc # Strip out shortest match between 'a' and 'C'. echo ${stringZ##a*C} # abc # Strip out longest match between 'a' and 'C'.

${string%substring}

Strips shortest match of $substring from back of $string.

${string%%substring}

Strips longest match of $substring from back of $string.

stringZ=abcABC123ABCabc # || # |------------| echo ${stringZ%b*c} # abcABC123ABCa # Strip out shortest match between 'b' and 'c', from back of $stringZ. echo ${stringZ%%b*c} # a # Strip out longest match between 'b' and 'c', from back of $stringZ.

Example 9-10. Converting graphic file formats, with filename change

#!/bin/bash # cvt.sh: # Converts all the MacPaint image files in a directory to "pbm" format. # Uses the "macptopbm" binary from the "netpbm" package, #+ which is maintained by Brian Henderson ([email protected]). # Netpbm is a standard part of most Linux distros. OPERATION=macptopbm SUFFIX=pbm # New filename suffix. if [ -n "$1" ] then directory=$1 # If directory name given as a script argument... else directory=$PWD # Otherwise use current working directory. fi # Assumes all files in the target directory are MacPaint image files, # + with a ".mac" suffix. for file in $directory/* # Filename globbing. do filename=${file%.*c} # Strip ".mac" suffix off filename #+ ('.*c' matches everything #+ between '.' and 'c', inclusive). $OPERATION $file > $filename.$SUFFIX # Redirect conversion to new filename. rm -f $file # Delete original files after converting. echo "$filename.$SUFFIX" # Log what is happening to stdout. done exit 0

Substring Replacement

${string/substring/replacement}

Replace first match of $substring with $replacement.

${string//substring/replacement}

Replace all matches of $substring with $replacement.

stringZ=abcABC123ABCabc echo ${stringZ/abc/xyz} # xyzABC123ABCabc # Replaces first match of 'abc' with 'xyz'. echo ${stringZ//abc/xyz} # xyzABC123ABCxyz # Replaces all matches of 'abc' with # 'xyz'.

${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.

stringZ=abcABC123ABCabc echo ${stringZ/#abc/XYZ} # XYZABC123ABCabc # Replaces front-end match of 'abc' with 'xyz'. echo ${stringZ/%abc/XYZ} # abcABC123ABCXYZ # Replaces back-end match of 'abc' with 'xyz'.