高级 Bash 脚本编程指南
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10.1. 字符串操作

Bash 支持非常多的字符串操作。不幸的是，这些工具缺乏统一的焦点。其中一些是参数替换的子集，另一些则属于 UNIX expr 命令的功能。这导致了命令语法的不一致和功能的重叠，更不用说令人困惑了。

字符串长度

${#string}

expr length $string

这些等同于 C 语言中的 strlen()。

expr "$string" : '.*'

stringZ=abcABC123ABCabc

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

示例 10-1. 在文本文件中段落之间插入空行

#!/bin/bash
# paragraph-space.sh
# Ver. 2.1, Reldate 29Jul12 [fixup]

# Inserts a blank line between paragraphs of a single-spaced text file.
# Usage: $0 <FILENAME

MINLEN=60        # Change this value? It's a judgment call.
#  Assume lines shorter than $MINLEN characters ending in a period
#+ terminate a paragraph. See exercises below.

while read line  # For as many lines as the input file has ...
do
  echo "$line"   # Output the line itself.

  len=${#line}
  if [[ "$len" -lt "$MINLEN" && "$line" =~ [*{\.}]$ ]]
# if [[ "$len" -lt "$MINLEN" && "$line" =~ \[*\.\] ]]
# An update to Bash broke the previous version of this script. Ouch!
# Thank you, Halim Srama, for pointing this out and suggesting a fix.
    then echo    #  Add a blank line immediately
  fi             #+ after a short line terminated by a period.
done

exit

# Exercises:
# ---------
#  1) The script usually inserts a blank line at the end
#+    of the target file. Fix this.
#  2) Line 17 only considers periods as sentence terminators.
#     Modify this to include other common end-of-sentence characters,
#+    such as ?, !, and ".

字符串开头匹配子字符串的长度

expr match "$string" '$substring'

$substring是一个正则表达式。

expr "$string" : '$substring'

$substring是一个正则表达式。

stringZ=abcABC123ABCabc
#       |------|
#       12345678

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

索引

expr index $string $substring

$substring 中第一个匹配字符在 $string 中的数值位置。

stringZ=abcABC123ABCabc
#       123456 ...
echo `expr index "$stringZ" C12`             # 6
                                             # C position.

echo `expr index "$stringZ" 1c`              # 3
# 'c' (in #3 position) matches before '1'.

这几乎等同于 C 语言中的 strchr()。

子字符串提取

${string:position}

从$string的$position.

位置提取子字符串。$string如果$position.

${string:position:length}

提取从$length个字符的子字符串，从$string的$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.



# Is it possible to index from the right end of the string?
    
echo ${stringZ:-4}                           # abcABC123ABCabc
# Defaults to full string, as in ${parameter:-default}.
# However . . .

echo ${stringZ:(-4)}                         # Cabc 
echo ${stringZ: -4}                          # Cabc
# Now, it works.
# Parentheses or added space "escape" the position parameter.

# Thank you, Dan Jacobson, for pointing this out.

position 和 length 参数可以是 "参数化的"，也就是说，可以用变量而不是数值常量来表示。

示例 10-2. 生成一个 8 字符的 "随机" 字符串

#!/bin/bash
# rand-string.sh
# Generating an 8-character "random" string.

if [ -n "$1" ]  #  If command-line argument present,
then            #+ then set start-string to it.
  str0="$1"
else            #  Else use PID of script as start-string.
  str0="$$"
fi

POS=2  # Starting from position 2 in the string.
LEN=8  # Extract eight characters.

str1=$( echo "$str0" | md5sum | md5sum )
#  Doubly scramble     ^^^^^^   ^^^^^^
#+ by piping and repiping to md5sum.

randstring="${str1:$POS:$LEN}"
# Can parameterize ^^^^ ^^^^

echo "$randstring"

exit $?

# bozo$ ./rand-string.sh my-password
# 1bdd88c4

#  No, this is is not recommended
#+ as a method of generating hack-proof passwords.

位置提取子字符串。$string参数是 "*" 或 "@"，则这将提取最多$length个位置参数，从$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

提取从$length个字符，从$string位置开始。$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$'

提取从$substring的开头，其中$string, 其中$substring是一个正则表达式。

expr "$string" : '$$substring$'

提取从$substring的开头，其中$string, 其中$substring是一个正则表达式。

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$'

提取从$substring的末尾$string, 其中$substring是一个正则表达式。

expr "$string" : '.*$$substring$'

提取从$substring的末尾$string, 其中$substring是一个正则表达式。

stringZ=abcABC123ABCabc
#                ======

echo `expr match "$stringZ" '.*\([A-C][A-C][A-C][a-c]*\)'`    # ABCabc
echo `expr "$stringZ" : '.*\(......\)'`                       # ABCabc

子字符串删除

${string#substring}

删除$substring前面$string.

${string##substring}

删除最长匹配的$substring前面$string.

stringZ=abcABC123ABCabc
#       |----|          shortest
#       |----------|    longest

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'.



# You can parameterize the substrings.

X='a*C'

echo ${stringZ#$X}      # 123ABCabc
echo ${stringZ##$X}     # abc
                        # As above.

${string%substring}

删除$substring后面$string.

例如

# Rename all filenames in $PWD with "TXT" suffix to a "txt" suffix.
# For example, "file1.TXT" becomes "file1.txt" . . .

SUFF=TXT
suff=txt

for i in $(ls *.$SUFF)
do
  mv -f $i ${i%.$SUFF}.$suff
  #  Leave unchanged everything *except* the shortest pattern match
  #+ starting from the right-hand-side of the variable $i . . .
done ### This could be condensed into a "one-liner" if desired.

# Thank you, Rory Winston.

${string%%substring}

删除最长匹配的$substring后面$string.

stringZ=abcABC123ABCabc
#                    ||     shortest
#        |------------|     longest

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.

此运算符对于生成文件名很有用。

示例 10-3. 转换图形文件格式，并更改文件名

#!/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 (bryanh@giraffe-data.com).
#  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" filename 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.



# *** And here's another way to do it. *** #

#!/bin/bash
# Batch convert into different graphic formats.
# Assumes imagemagick installed (standard in most Linux distros).

INFMT=png   # Can be tif, jpg, gif, etc.
OUTFMT=pdf  # Can be tif, jpg, gif, pdf, etc.

for pic in *"$INFMT"
do
  p2=$(ls "$pic" | sed -e s/\.$INFMT//)
  # echo $p2
    convert "$pic" $p2.$OUTFMT
    done

exit $?

示例 10-4. 将流式音频文件转换为 ogg

#!/bin/bash
# ra2ogg.sh: Convert streaming audio files (*.ra) to ogg.

# Uses the "mplayer" media player program:
#      http://www.mplayerhq.hu/homepage
# Uses the "ogg" library and "oggenc":
#      http://www.xiph.org/
#
# This script may need appropriate codecs installed, such as sipr.so ...
# Possibly also the compat-libstdc++ package.


OFILEPREF=${1%%ra}      # Strip off the "ra" suffix.
OFILESUFF=wav           # Suffix for wav file.
OUTFILE="$OFILEPREF""$OFILESUFF"
E_NOARGS=85

if [ -z "$1" ]          # Must specify a filename to convert.
then
  echo "Usage: `basename $0` [filename]"
  exit $E_NOARGS
fi


##########################################################################
mplayer "$1" -ao pcm:file=$OUTFILE
oggenc "$OUTFILE"  # Correct file extension automatically added by oggenc.
##########################################################################

rm "$OUTFILE"      # Delete intermediate *.wav file.
                   # If you want to keep it, comment out above line.

exit $?

#  Note:
#  ----
#  On a Website, simply clicking on a *.ram streaming audio file
#+ usually only downloads the URL of the actual *.ra audio file.
#  You can then use "wget" or something similar
#+ to download the *.ra file itself.


#  Exercises:
#  ---------
#  As is, this script converts only *.ra filenames.
#  Add flexibility by permitting use of *.ram and other filenames.
#
#  If you're really ambitious, expand the script
#+ to do automatic downloads and conversions of streaming audio files.
#  Given a URL, batch download streaming audio files (using "wget")
#+ and convert them on the fly.

使用子字符串提取构造简单地模拟 getopt。

示例 10-5. 模拟 getopt

#!/bin/bash
# getopt-simple.sh
# Author: Chris Morgan
# Used in the ABS Guide with permission.


getopt_simple()
{
    echo "getopt_simple()"
    echo "Parameters are '$*'"
    until [ -z "$1" ]
    do
      echo "Processing parameter of: '$1'"
      if [ ${1:0:1} = '/' ]
      then
          tmp=${1:1}               # Strip off leading '/' . . .
          parameter=${tmp%%=*}     # Extract name.
          value=${tmp##*=}         # Extract value.
          echo "Parameter: '$parameter', value: '$value'"
          eval $parameter=$value
      fi
      shift
    done
}

# Pass all options to getopt_simple().
getopt_simple $*

echo "test is '$test'"
echo "test2 is '$test2'"

exit 0  # See also, UseGetOpt.sh, a modified version of this script.

---

sh getopt_example.sh /test=value1 /test2=value2

Parameters are '/test=value1 /test2=value2'
Processing parameter of: '/test=value1'
Parameter: 'test', value: 'value1'
Processing parameter of: '/test2=value2'
Parameter: 'test2', value: 'value2'
test is 'value1'
test2 is 'value2'

子字符串替换

${string/substring/replacement}

替换第一个匹配项$substring为$replacement. [2]

${string//substring/replacement}

替换所有匹配项$substring为$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'.

echo  ---------------
echo "$stringZ"               # abcABC123ABCabc
echo  ---------------
                              # The string itself is not altered!

# Can the match and replacement strings be parameterized?
match=abc
repl=000
echo ${stringZ/$match/$repl}  # 000ABC123ABCabc
#              ^      ^         ^^^
echo ${stringZ//$match/$repl} # 000ABC123ABC000
# Yes!          ^      ^        ^^^         ^^^

echo

# What happens if no $replacement string is supplied?
echo ${stringZ/abc}           # ABC123ABCabc
echo ${stringZ//abc}          # ABC123ABC
# A simple deletion takes place.

${string/#substring/replacement}

如果$substring匹配$string，则替换$replacement为$substring.

${string/%substring/replacement}

如果$substring匹配后面$string，则替换$replacement为$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'.

10.1.1. 使用 awk 操作字符串

Bash 脚本可以调用 awk 的字符串操作功能，以替代使用其内置操作。

示例 10-6. 提取和定位子字符串的替代方法

#!/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.

echo "----"

# And likewise:

echo | awk '
{ print index("'"${String}"'", "skid")      # 3
}                                           # (skid starts at position 3)
'   # The awk equivalent of "expr index" ...

exit 0

10.1.2. 进一步参考

有关脚本中字符串操作的更多信息，请参阅第 10.2 节和相关部分的 expr 命令列表。

脚本示例

[1]	这适用于命令行参数或传递给函数的参数。
[2]	请注意`$substring`和`$replacement`可能指的是字面字符串或变量，具体取决于上下文。请参阅第一个用法示例。

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变量操作	向上	参数替换

10.1. 字符串操作

10.1.1. 使用 awk 操作字符串

10.1.2. 进一步参考

注释