Bash 脚本高级编程指南
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第 12 章. 命令替换

命令替换 重新分配一个命令 [1] 甚至多个命令的输出；它字面上将命令输出插入到另一个上下文中。 [2]

命令替换的经典形式使用反引号 (`...`)。反引号（反撇号）内的命令生成命令行文本。

script_name=`basename $0`
echo "The name of this script is $script_name."

命令的输出可以用作另一个命令的参数，用于设置变量，甚至用于在 for 循环中生成参数列表。

rm `cat filename`   # "filename" contains a list of files to delete.
#
# S. C. points out that "arg list too long" error might result.
# Better is              xargs rm -- < filename 
# ( -- covers those cases where "filename" begins with a "-" )

textfile_listing=`ls *.txt`
# Variable contains names of all *.txt files in current working directory.
echo $textfile_listing

textfile_listing2=$(ls *.txt)   # The alternative form of command substitution.
echo $textfile_listing2
# Same result.

# A possible problem with putting a list of files into a single string
# is that a newline may creep in.
#
# A safer way to assign a list of files to a parameter is with an array.
#      shopt -s nullglob    # If no match, filename expands to nothing.
#      textfile_listing=( *.txt )
#
# Thanks, S.C.

Note

命令替换调用一个 子 shell。

Caution

命令替换可能导致单词分割。

COMMAND `echo a b`     # 2 args: a and b

COMMAND "`echo a b`"   # 1 arg: "a b"

COMMAND `echo`         # no arg

COMMAND "`echo`"       # one empty arg


# Thanks, S.C.

即使没有单词分割，命令替换也可以删除尾部换行符。

# cd "`pwd`"  # This should always work.
# However...

mkdir 'dir with trailing newline
'

cd 'dir with trailing newline
'

cd "`pwd`"  # Error message:
# bash: cd: /tmp/file with trailing newline: No such file or directory

cd "$PWD"   # Works fine.





old_tty_setting=$(stty -g)   # Save old terminal setting.
echo "Hit a key "
stty -icanon -echo           # Disable "canonical" mode for terminal.
                             # Also, disable *local* echo.
key=$(dd bs=1 count=1 2> /dev/null)   # Using 'dd' to get a keypress.
stty "$old_tty_setting"      # Restore old setting. 
echo "You hit ${#key} key."  # ${#variable} = number of characters in $variable
#
# Hit any key except RETURN, and the output is "You hit 1 key."
# Hit RETURN, and it's "You hit 0 key."
# The newline gets eaten in the command substitution.

#Code snippet by St�phane Chazelas.

Caution

使用 echo 输出一个使用命令替换设置的未被引用的变量，会从重新分配的命令的输出中删除尾部换行符。这可能会导致 неприятных сюрпризов (不愉快的意外 - keep original meaning but no need to translate this part).

dir_listing=`ls -l`
echo $dir_listing     # unquoted

# Expecting a nicely ordered directory listing.

# However, what you get is:
# total 3 -rw-rw-r-- 1 bozo bozo 30 May 13 17:15 1.txt -rw-rw-r-- 1 bozo
# bozo 51 May 15 20:57 t2.sh -rwxr-xr-x 1 bozo bozo 217 Mar 5 21:13 wi.sh

# The newlines disappeared.


echo "$dir_listing"   # quoted
# -rw-rw-r--    1 bozo       30 May 13 17:15 1.txt
# -rw-rw-r--    1 bozo       51 May 15 20:57 t2.sh
# -rwxr-xr-x    1 bozo      217 Mar  5 21:13 wi.sh

命令替换甚至允许使用重定向或 cat 命令将变量设置为文件的内容。

variable1=`<file1`      #  Set "variable1" to contents of "file1".
variable2=`cat file2`   #  Set "variable2" to contents of "file2".
                        #  This, however, forks a new process,
                        #+ so the line of code executes slower than the above version.

#  Note that the variables may contain embedded whitespace,
#+ or even (horrors), control characters.

#  It is not necessary to explicitly assign a variable.
echo "` <$0`"           # Echoes the script itself to stdout.

#  Excerpts from system file, /etc/rc.d/rc.sysinit
#+ (on a Red Hat Linux installation)


if [ -f /fsckoptions ]; then
        fsckoptions=`cat /fsckoptions`
...
fi
#
#
if [ -e "/proc/ide/${disk[$device]}/media" ] ; then
             hdmedia=`cat /proc/ide/${disk[$device]}/media`
...
fi
#
#
if [ ! -n "`uname -r | grep -- "-"`" ]; then
       ktag="`cat /proc/version`"
...
fi
#
#
if [ $usb = "1" ]; then
    sleep 5
    mouseoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot=02"`
    kbdoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot=01"`
...
fi

Caution

除非您有非常好的理由这样做，否则不要将变量设置为长文本文件的内容。 即使是开玩笑，也不要将变量设置为二进制文件的内容。

示例 12-1. 脚本小技巧

#!/bin/bash
# stupid-script-tricks.sh: Don't try this at home, folks.
# From "Stupid Script Tricks," Volume I.

exit 99  ### Comment out this line if you dare.

dangerous_variable=`cat /boot/vmlinuz`   # The compressed Linux kernel itself.

echo "string-length of \$dangerous_variable = ${#dangerous_variable}"
# string-length of $dangerous_variable = 794151
# (Newer kernels are bigger.)
# Does not give same count as 'wc -c /boot/vmlinuz'.

# echo "$dangerous_variable"
# Don't try this! It would hang the script.


#  The document author is aware of no useful applications for
#+ setting a variable to the contents of a binary file.

exit 0

请注意，不会发生缓冲区溢出。这是一个解释型语言（如 Bash）比编译型语言更能提供保护，以防止程序员犯错的例子。

命令替换允许将变量设置为 循环的输出。 这里的关键是获取循环内 echo 命令的输出。

示例 12-2. 从循环生成变量

#!/bin/bash
# csubloop.sh: Setting a variable to the output of a loop.

variable1=`for i in 1 2 3 4 5
do
  echo -n "$i"                 #  The 'echo' command is critical
done`                          #+ to command substitution here.

echo "variable1 = $variable1"  # variable1 = 12345


i=0
variable2=`while [ "$i" -lt 10 ]
do
  echo -n "$i"                 # Again, the necessary 'echo'.
  let "i += 1"                 # Increment.
done`

echo "variable2 = $variable2"  # variable2 = 0123456789

#  Demonstrates that it's possible to embed a loop
#+ inside a variable declaration.

exit 0

命令替换使得扩展 Bash 可用的工具集成为可能。 这仅仅是编写一个程序或脚本，将输出发送到标准输出（像一个行为良好的 UNIX 工具应该做的那样），并将该输出分配给一个变量。

#include <stdio.h>

/*  "Hello, world." C program  */		

int main()
{
  printf( "Hello, world.\n" );
  return (0);
}
bash$ gcc -o hello hello.c
	      

#!/bin/bash
# hello.sh		

greeting=`./hello`
echo $greeting
bash$ sh hello.sh
Hello, world.

Note

$(...) 形式已经取代了反引号用于命令替换。

output=$(sed -n /"$1"/p $file)   # From "grp.sh"	example.
	      
# Setting a variable to the contents of a text file.
File_contents1=$(cat $file1)      
File_contents2=$(<$file2)        # Bash permits this also.

与 `...` 不同，$(...) 形式的命令替换以不同的方式处理双反斜杠。

bash$ echo `echo \\`


bash$ echo $(echo \\)
\

$(...) 形式的命令替换允许嵌套。 [3]

word_count=$( wc -w $(echo * | awk '{print $8}') )

或者，对于更复杂的情况 . . .

示例 12-3. 查找变位词

#!/bin/bash
# agram2.sh
# Example of nested command substitution.

#  Uses "anagram" utility
#+ that is part of the author's "yawl" word list package.
#  http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
#  http://bash.deta.in/yawl-0.3.2.tar.gz

E_NOARGS=86
E_BADARG=87
MINLEN=7

if [ -z "$1" ]
then
  echo "Usage $0 LETTERSET"
  exit $E_NOARGS         # Script needs a command-line argument.
elif [ ${#1} -lt $MINLEN ]
then
  echo "Argument must have at least $MINLEN letters."
  exit $E_BADARG
fi



FILTER='.......'         # Must have at least 7 letters.
#       1234567
Anagrams=( $(echo $(anagram $1 | grep $FILTER) ) )
#          $(     $(  nested command sub.    ) )
#        (              array assignment         )

echo
echo "${#Anagrams[*]}  7+ letter anagrams found"
echo
echo ${Anagrams[0]}      # First anagram.
echo ${Anagrams[1]}      # Second anagram.
                         # Etc.

# echo "${Anagrams[*]}"  # To list all the anagrams in a single line . . .

#  Look ahead to the Arrays chapter for enlightenment on
#+ what's going on here.

# See also the agram.sh script for an exercise in anagram finding.

exit $?

shell 脚本中命令替换的示例

  1. 示例 11-8

  2. 示例 11-27

  3. 示例 9-16

  4. 示例 16-3

  5. 示例 16-22

  6. 示例 16-17

  7. 示例 16-54

  8. 示例 11-14

  9. 示例 11-11

  10. 示例 16-32

  11. 示例 20-8

  12. 示例 A-16

  13. 示例 29-3

  14. 示例 16-47

  15. 示例 16-48

  16. 示例 16-49

说明

[1]

出于命令替换的目的，command 可以是外部系统命令、内建脚本命令，甚至可以是脚本函数。

[2]

从技术上更准确的意义上讲，命令替换 提取命令的标准输出输出，然后使用 = 操作符将其分配给变量。

[3]

事实上，使用反引号进行嵌套也是可能的，但正如 John Default 指出的那样，只能通过转义内部反引号来实现。

word_count=` wc -w \`echo * | awk '{print $8}'\` `

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