9.2. 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 Length
- ${#string}
- expr length $string
- expr "$string" : '.*'
stringZ=abcABC123ABCabc
echo ${#stringZ} # 15
echo `expr length $stringZ` # 15
echo `expr "$stringZ" : '.*'` # 15 |
Example 9-10. Inserting a blank line between paragraphs in a text file
#!/bin/bash
# paragraph-space.sh
# Inserts a blank line between paragraphs of a single-spaced text file.
# Usage: $0 <FILENAME
MINLEN=45 # May need to change this value.
# Assume lines shorter than $MINLEN characters
#+ terminate a paragraph.
while read line # For as many lines as the input file has...
do
echo "$line" # Output the line itself.
len=${#line}
if [ "$len" -lt "$MINLEN" ]
then echo # Add a blank line after short line.
fi
done
exit 0 |
Length of Matching Substring at Beginning of String
- expr match "$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,
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
echo `expr "$stringZ" : '\(.......\)'` # abcABC1
# All of the above forms give an identical result. |
- expr match "$string" '.*\($substring\)'
Extracts $substring
at end of
$string, where
$substring is a regular
expression.
- expr "$string" : '.*\($substring\)'
Extracts $substring
at end of $string,
where $substring is a regular
expression.
stringZ=abcABC123ABCabc
# ======
echo `expr match "$stringZ" '.*\([A-C][A-C][A-C][a-c]*\)'` # ABCabc
echo `expr "$stringZ" : '.*\(......\)'` # ABCabc |
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 9-11. 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
# Exercise:
# --------
# As it stands, this script converts *all* the files in the current
#+ working directory.
# Modify it to work *only* on files with a ".mac" suffix. |
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'. |
9.2.1. Manipulating strings using awk
A Bash script may invoke the string manipulation facilities of
awk as an alternative to using its
built-in operations.
Example 9-12. Alternate ways of extracting substrings
#!/bin/bash
# substring-extraction.sh
String=23skidoo1
# 012345678 Bash
# 123456789 awk
# Note different string indexing system:
# Bash numbers first character of string as '0'.
# Awk numbers first character of string as '1'.
echo ${String:2:4} # position 3 (0-1-2), 4 characters long
# skid
# The awk equivalent of ${string:pos:length} is substr(string,pos,length).
echo | awk '
{ print substr("'"${String}"'",3,4) # skid
}
'
# Piping an empty "echo" to awk gives it dummy input,
#+ and thus makes it unnecessary to supply a filename.
exit 0 |
