9.1. 内部变量

内建 变量

影响 bash 脚本行为的变量

$BASH

Bash 二进制文件本身的路径

bash$ echo $BASH /bin/bash

$BASH_ENV

一个 环境变量，指向一个 Bash 启动文件，当脚本被调用时将被读取

$BASH_SUBSHELL

一个变量，指示 子 shell 级别。这是 Bash 的新功能，版本 3。

有关用法，请参见 示例 21-1。

$BASHPID

当前 Bash 实例的进程 ID。这与 $$ 变量不同，但通常给出相同的结果。

bash4$ echo $$ 11015 bash4$ echo $BASHPID 11015 bash4$ ps ax | grep bash4 11015 pts/2 R 0:00 bash4

但是 ...

#!/bin/bash4 echo "\$\$ outside of subshell = $$" # 9602 echo "\$BASH_SUBSHELL outside of subshell = $BASH_SUBSHELL" # 0 echo "\$BASHPID outside of subshell = $BASHPID" # 9602 echo ( echo "\$\$ inside of subshell = $$" # 9602 echo "\$BASH_SUBSHELL inside of subshell = $BASH_SUBSHELL" # 1 echo "\$BASHPID inside of subshell = $BASHPID" ) # 9603 # Note that $$ returns PID of parent process.

$BASH_VERSINFO[n]

一个 6 元素 数组，包含有关已安装 Bash 版本的版本信息。这类似于$BASH_VERSION，如下所示，但更详细一些。

# Bash version info: for n in 0 1 2 3 4 5 do echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}" done # BASH_VERSINFO[0] = 3 # Major version no. # BASH_VERSINFO[1] = 00 # Minor version no. # BASH_VERSINFO[2] = 14 # Patch level. # BASH_VERSINFO[3] = 1 # Build version. # BASH_VERSINFO[4] = release # Release status. # BASH_VERSINFO[5] = i386-redhat-linux-gnu # Architecture # (same as $MACHTYPE).

$BASH_VERSION

系统上安装的 Bash 版本

bash$ echo $BASH_VERSION 3.2.25(1)-release

tcsh% echo $BASH_VERSION BASH_VERSION: Undefined variable.

检查 $BASH_VERSION 是确定正在运行哪个 shell 的好方法。$SHELL 不一定给出正确的答案。

$CDPATH

一个冒号分隔的搜索路径列表，可用于 cd 命令，功能类似于二进制文件的 $PATH 变量。该$CDPATH变量可以在本地 ~/.bashrc 文件中设置。

bash$ cd bash-doc bash: cd: bash-doc: No such file or directory bash$ CDPATH=/usr/share/doc bash$ cd bash-doc /usr/share/doc/bash-doc bash$ echo $PWD /usr/share/doc/bash-doc

$DIRSTACK

目录栈中的顶部值 [1]（受 pushd 和 popd 影响）

此内建变量对应于 dirs 命令，但是 dirs 显示目录栈的全部内容。

$EDITOR

脚本调用的默认编辑器，通常是 vi 或 emacs。

$EUID

“有效” 用户 ID 号

当前用户已假定的身份的识别号，可能是通过 su 的方式。

该$EUID不一定与 $UID 相同。

$FUNCNAME

当前函数的名称

xyz23 () { echo "$FUNCNAME now executing." # xyz23 now executing. } xyz23 echo "FUNCNAME = $FUNCNAME" # FUNCNAME = # Null value outside a function.

另请参见 示例 A-50。

$GLOBIGNORE

文件名模式列表，在 通配 中排除匹配。

$GROUPS

当前用户所属的组

这是当前用户的组 ID 号列表（数组），记录在 /etc/passwd 和/etc/group.

root# echo $GROUPS 0 root# echo ${GROUPS[1]} 1 root# echo ${GROUPS[5]} 6

$HOME

用户的主目录，通常是/home/username（参见 示例 10-7）

$HOSTNAME

hostname 命令在启动脚本中于启动时分配系统主机名。但是，gethostname()函数设置 Bash 内部变量$HOSTNAME。另请参见 示例 10-7。

$HOSTTYPE

主机类型

与 $MACHTYPE 类似，标识系统硬件。

bash$ echo $HOSTTYPE i686

$IFS

内部字段分隔符

此变量确定 Bash 在解释字符串时如何识别字段或单词边界。

$IFS 默认为 空白字符（空格、制表符和换行符），但可以更改，例如，解析逗号分隔的数据文件。请注意，$* 使用保存在$IFS中的第一个字符。请参见 示例 5-1。

bash$ echo "$IFS" (With $IFS set to default, a blank line displays.) bash$ echo "$IFS" | cat -vte ^I$ $ (Show whitespace: here a single space, ^I [horizontal tab], and newline, and display "$" at end-of-line.) bash$ bash -c 'set w x y z; IFS=":-;"; echo "$*"' w:x:y:z (Read commands from string and assign any arguments to pos params.)

设置$IFS以消除 路径名中的空白字符。

IFS="$(printf '\n\t')" # Per David Wheeler.

$IFS处理空白字符的方式与其他字符不同。 示例 9-1. $IFS 和空白字符 #!/bin/bash # ifs.sh var1="a+b+c" var2="d-e-f" var3="g,h,i" IFS=+ # The plus sign will be interpreted as a separator. echo $var1 # a b c echo $var2 # d-e-f echo $var3 # g,h,i echo IFS="-" # The plus sign reverts to default interpretation. # The minus sign will be interpreted as a separator. echo $var1 # a+b+c echo $var2 # d e f echo $var3 # g,h,i echo IFS="," # The comma will be interpreted as a separator. # The minus sign reverts to default interpretation. echo $var1 # a+b+c echo $var2 # d-e-f echo $var3 # g h i echo IFS=" " # The space character will be interpreted as a separator. # The comma reverts to default interpretation. echo $var1 # a+b+c echo $var2 # d-e-f echo $var3 # g,h,i # ======================================================== # # However ... # $IFS treats whitespace differently than other characters. output_args_one_per_line() { for arg do echo "[$arg]" done # ^ ^ Embed within brackets, for your viewing pleasure. } echo; echo "IFS=\" \"" echo "-------" IFS=" " var=" a b c " # ^ ^^ ^^^ output_args_one_per_line $var # output_args_one_per_line `echo " a b c "` # [a] # [b] # [c] echo; echo "IFS=:" echo "-----" IFS=: var=":a::b:c:::" # Same pattern as above, # ^ ^^ ^^^ #+ but substituting ":" for " " ... output_args_one_per_line $var # [] # [a] # [] # [b] # [c] # [] # [] # Note "empty" brackets. # The same thing happens with the "FS" field separator in awk. echo exit

（非常感谢 St�phane Chazelas 的澄清和以上示例。）

另请参见 示例 16-41、示例 11-8 和 示例 19-14，了解如何使用$IFS.

$IGNOREEOF

忽略 EOF：shell 在注销之前将忽略多少个文件结束符（control-D）。

$LC_COLLATE

通常在 .bashrc 或/etc/profile文件中设置，此变量控制文件名扩展和模式匹配中的排序规则。如果处理不当，LC_COLLATE可能会在 文件名通配 中导致意外结果。

从 Bash 2.05 版本开始，文件名通配不再区分括号中字符范围内的字母大小写。例如，ls [A-M]* 将匹配File1.txt和file1.txt。要恢复括号匹配的习惯行为，请设置LC_COLLATE为C通过export LC_COLLATE=C在/etc/profile和/或~/.bashrc.

$LC_CTYPE

此内部变量控制 通配 和模式匹配中的字符解释。

$LINENO

此变量是 shell 脚本中出现此变量的行号。它仅在出现它的脚本中具有意义，主要用于调试目的。

# *** BEGIN DEBUG BLOCK *** last_cmd_arg=$_ # Save it. echo "At line number $LINENO, variable \"v1\" = $v1" echo "Last command argument processed = $last_cmd_arg" # *** END DEBUG BLOCK ***

$MACHTYPE

机器类型

标识系统硬件。

bash$ echo $MACHTYPE i686

$OLDPWD

旧工作目录（“OLD-Print-Working-Directory”，您之前所在的目录）。

$OSTYPE

操作系统类型

bash$ echo $OSTYPE linux

$PATH

二进制文件的路径，通常是/usr/bin/, /usr/X11R6/bin/, /usr/local/bin等等。

当给定命令时，shell 会自动在 path 中列出的目录中执行哈希表搜索以查找可执行文件。路径存储在 环境变量，$PATH中，这是一个目录列表，用冒号分隔。通常，系统将$PATH定义存储在/etc/profile和/或 ~/.bashrc 中（参见 附录 H）。

bash$ echo $PATH /bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin:/sbin:/usr/sbin

PATH=${PATH}:/opt/bin将/opt/bin目录附加到当前路径。在脚本中，以这种方式临时将目录添加到路径可能很方便。当脚本退出时，这将恢复原始$PATH（子进程（例如脚本）可能无法更改父进程（shell）的环境）。

当前的 “工作目录”，./通常从$PATH中省略，作为一种安全措施。

$PIPESTATUS

数组变量，保存最后执行的前台 管道的 退出状态。

bash$ echo $PIPESTATUS 0 bash$ ls -al | bogus_command bash: bogus_command: command not found bash$ echo ${PIPESTATUS[1]} 127 bash$ ls -al | bogus_command bash: bogus_command: command not found bash$ echo $? 127

该$PIPESTATUS数组的成员保存管道中执行的每个相应命令的退出状态。$PIPESTATUS[0]保存管道中第一个命令的退出状态，$PIPESTATUS[1]第二个命令的退出状态，依此类推。

该$PIPESTATUS变量在登录 shell 中可能包含错误的 0 值（在 Bash 3.0 之前的版本中）。 tcsh% bash bash$ who | grep nobody | sort bash$ echo ${PIPESTATUS[*]} 0 脚本中包含的以上行将产生预期的0 1 0输出。 感谢 Wayne Pollock 指出这一点并提供以上示例。

该$PIPESTATUS变量在某些情况下会给出意外的结果。 bash$ echo $BASH_VERSION 3.00.14(1)-release bash$ $ ls | bogus_command | wc bash: bogus_command: command not found 0 0 0 bash$ echo ${PIPESTATUS[@]} 141 127 0 Chet Ramey 将以上输出归因于 ls 的行为。如果 ls 写入一个输出未被读取的管道，则SIGPIPE会杀死它，并且其 退出状态 为 141。否则，其退出状态为预期的 0。对于 tr 也是如此。

$PIPESTATUS是一个 “易变” 变量。需要在相关管道之后立即捕获它，在任何其他命令介入之前。 bash$ $ ls | bogus_command | wc bash: bogus_command: command not found 0 0 0 bash$ echo ${PIPESTATUS[@]} 0 127 0 bash$ echo ${PIPESTATUS[@]} 0

在$PIPESTATUS未给出所需信息的情况下，pipefail 选项可能很有用。

$PPID

该$PPID进程的 是其父进程的进程 ID（pid）。[2]

将其与 pidof 命令进行比较。

$PROMPT_COMMAND

一个变量，保存要在主提示符之前执行的命令，$PS1将被显示。

$PS1

这是主提示符，在命令行中可见。

$PS2

辅助提示符，当需要额外输入时可见。它显示为 “>”。

$PS3

三级提示符，在 select 循环中显示（参见 示例 11-30）。

$PS4

四级提示符，当使用 -x [详细跟踪] 选项 调用脚本时，在每行输出的开头显示。它显示为 “+”。

作为调试辅助，在$PS4.

P4='$(read time junk < /proc/$$/schedstat; echo "@@@ $time @@@ " )' # Per suggestion by Erik Brandsberg. set -x # Various commands follow ...

$PWD

工作目录（您当前所在的目录）

中嵌入诊断信息可能很有用。这是 pwd 内建命令的类似物。

#!/bin/bash E_WRONG_DIRECTORY=85 clear # Clear the screen. TargetDirectory=/home/bozo/projects/GreatAmericanNovel cd $TargetDirectory echo "Deleting stale files in $TargetDirectory." if [ "$PWD" != "$TargetDirectory" ] then # Keep from wiping out wrong directory by accident. echo "Wrong directory!" echo "In $PWD, rather than $TargetDirectory!" echo "Bailing out!" exit $E_WRONG_DIRECTORY fi rm -rf * rm .[A-Za-z0-9]* # Delete dotfiles. # rm -f .[^.]* ..?* to remove filenames beginning with multiple dots. # (shopt -s dotglob; rm -f *) will also work. # Thanks, S.C. for pointing this out. # A filename (`basename`) may contain all characters in the 0 - 255 range, #+ except "/". # Deleting files beginning with weird characters, such as - #+ is left as an exercise. (Hint: rm ./-weirdname or rm -- -weirdname) result=$? # Result of delete operations. If successful = 0. echo ls -al # Any files left? echo "Done." echo "Old files deleted in $TargetDirectory." echo # Various other operations here, as necessary. exit $result

$REPLY

当未向 read 提供变量时的默认值。也适用于 select 菜单，但仅提供所选变量的项目编号，而不是变量本身的值。

#!/bin/bash # reply.sh # REPLY is the default value for a 'read' command. echo echo -n "What is your favorite vegetable? " read echo "Your favorite vegetable is $REPLY." # REPLY holds the value of last "read" if and only if #+ no variable supplied. echo echo -n "What is your favorite fruit? " read fruit echo "Your favorite fruit is $fruit." echo "but..." echo "Value of \$REPLY is still $REPLY." # $REPLY is still set to its previous value because #+ the variable $fruit absorbed the new "read" value. echo exit 0

$SECONDS

脚本已运行的秒数。

#!/bin/bash TIME_LIMIT=10 INTERVAL=1 echo echo "Hit Control-C to exit before $TIME_LIMIT seconds." echo while [ "$SECONDS" -le "$TIME_LIMIT" ] do # $SECONDS is an internal shell variable. if [ "$SECONDS" -eq 1 ] then units=second else units=seconds fi echo "This script has been running $SECONDS $units." # On a slow or overburdened machine, the script may skip a count #+ every once in a while. sleep $INTERVAL done echo -e "\a" # Beep! exit 0

$SHELLOPTS

已启用的 shell 选项列表，一个只读变量。

bash$ echo $SHELLOPTS braceexpand:hashall:histexpand:monitor:history:interactive-comments:emacs

$SHLVL

Shell 级别，Bash 的嵌套深度。[3] 如果在命令行中 $SHLVL 为 1，则在脚本中它将递增为 2。

此变量不受子 shell 的影响。当您需要指示子 shell 嵌套时，请使用 $BASH_SUBSHELL。

$TMOUT

如果$TMOUT环境变量设置为非零值time，则 shell 提示符将在$time秒后超时。这将导致注销。

从 Bash 2.05b 版本开始，现在可以在脚本中将$TMOUT与 read 结合使用。

# Works in scripts for Bash, versions 2.05b and later. TMOUT=3 # Prompt times out at three seconds. echo "What is your favorite song?" echo "Quickly now, you only have $TMOUT seconds to answer!" read song if [ -z "$song" ] then song="(no answer)" # Default response. fi echo "Your favorite song is $song."

还有其他更复杂的方法可以在脚本中实现定时输入。一种替代方法是设置一个定时循环，以便在超时时向脚本发出信号。这也需要一个信号处理例程来 trap（参见 示例 32-5）由定时循环生成的interrupt（哇！）。

示例 9-2. 定时输入

#!/bin/bash # timed-input.sh # TMOUT=3 Also works, as of newer versions of Bash. TIMER_INTERRUPT=14 TIMELIMIT=3 # Three seconds in this instance. # May be set to different value. PrintAnswer() { if [ "$answer" = TIMEOUT ] then echo $answer else # Don't want to mix up the two instances. echo "Your favorite veggie is $answer" kill $! # Kills no-longer-needed TimerOn function #+ running in background. # $! is PID of last job running in background. fi } TimerOn() { sleep $TIMELIMIT && kill -s 14 $$ & # Waits 3 seconds, then sends sigalarm to script. } Int14Vector() { answer="TIMEOUT" PrintAnswer exit $TIMER_INTERRUPT } trap Int14Vector $TIMER_INTERRUPT # Timer interrupt (14) subverted for our purposes. echo "What is your favorite vegetable " TimerOn read answer PrintAnswer # Admittedly, this is a kludgy implementation of timed input. # However, the "-t" option to "read" simplifies this task. # See the "t-out.sh" script. # However, what about timing not just single user input, #+ but an entire script? # If you need something really elegant ... #+ consider writing the application in C or C++, #+ using appropriate library functions, such as 'alarm' and 'setitimer.' exit 0

另一种替代方法是使用 stty。

示例 9-3. 再次，定时输入

#!/bin/bash # timeout.sh # Written by Stephane Chazelas, #+ and modified by the document author. INTERVAL=5 # timeout interval timedout_read() { timeout=$1 varname=$2 old_tty_settings=`stty -g` stty -icanon min 0 time ${timeout}0 eval read $varname # or just read $varname stty "$old_tty_settings" # See man page for "stty." } echo; echo -n "What's your name? Quick! " timedout_read $INTERVAL your_name # This may not work on every terminal type. # The maximum timeout depends on the terminal. #+ (it is often 25.5 seconds). echo if [ ! -z "$your_name" ] # If name input before timeout ... then echo "Your name is $your_name." else echo "Timed out." fi echo # The behavior of this script differs somewhat from "timed-input.sh." # At each keystroke, the counter resets. exit 0

也许最简单的方法是使用 read 的-t选项。

示例 9-4. 定时 read

#!/bin/bash # t-out.sh [time-out] # Inspired by a suggestion from "syngin seven" (thanks). TIMELIMIT=4 # 4 seconds read -t $TIMELIMIT variable <&1 # ^^^ # In this instance, "<&1" is needed for Bash 1.x and 2.x, # but unnecessary for Bash 3+. echo if [ -z "$variable" ] # Is null? then echo "Timed out, variable still unset." else echo "variable = $variable" fi exit 0

$UID

用户 ID 号

当前用户的用户识别号，记录在 /etc/passwd 中

这是当前用户的真实 ID，即使她通过 su 暂时假定了另一个身份。$UID是一个只读变量，不受命令行或脚本中的更改的影响，并且是 id 内建命令的对应项。

示例 9-5. 我是 root 吗？

#!/bin/bash # am-i-root.sh: Am I root or not? ROOT_UID=0 # Root has $UID 0. if [ "$UID" -eq "$ROOT_UID" ] # Will the real "root" please stand up? then echo "You are root." else echo "You are just an ordinary user (but mom loves you just the same)." fi exit 0 # ============================================================= # # Code below will not execute, because the script already exited. # An alternate method of getting to the root of matters: ROOTUSER_NAME=root username=`id -nu` # Or... username=`whoami` if [ "$username" = "$ROOTUSER_NAME" ] then echo "Rooty, toot, toot. You are root." else echo "You are just a regular fella." fi

另请参见 示例 2-3。

变量$ENV, $LOGNAME, $MAIL, $TERM, $USER和$USERNAME不是 Bash 内建。但是，这些通常在 Bash 或 login 启动文件之一中设置为 环境变量。$SHELL，用户的登录 shell 的名称，可以从/etc/passwd或 “init” 脚本中设置，它同样不是 Bash 内建命令。 tcsh% echo $LOGNAME bozo tcsh% echo $SHELL /bin/tcsh tcsh% echo $TERM rxvt bash$ echo $LOGNAME bozo bash$ echo $SHELL /bin/tcsh bash$ echo $TERM rxvt

位置参数

$0, $1, $2等等。

位置参数，从命令行传递到脚本，传递给函数，或 set 到变量（参见 示例 4-5 和 示例 15-16）

$#

命令行参数的数量 [4] 或位置参数的数量（参见 示例 36-2）

$*

所有位置参数，视为单个单词

“$*” 必须用引号括起来。

$@

与 $* 相同，但每个参数都是带引号的字符串，也就是说，参数在没有解释或扩展的情况下完整传递。这意味着，除其他事项外，参数列表中的每个参数都被视为一个单独的单词。

当然，“$@” 应该用引号括起来。

示例 9-6. arglist：使用 $* 和 $@ 列出参数

#!/bin/bash # arglist.sh # Invoke this script with several arguments, such as "one two three" ... E_BADARGS=85 if [ ! -n "$1" ] then echo "Usage: `basename $0` argument1 argument2 etc." exit $E_BADARGS fi echo index=1 # Initialize count. echo "Listing args with \"\$*\":" for arg in "$*" # Doesn't work properly if "$*" isn't quoted. do echo "Arg #$index = $arg" let "index+=1" done # $* sees all arguments as single word. echo "Entire arg list seen as single word." echo index=1 # Reset count. # What happens if you forget to do this? echo "Listing args with \"\$@\":" for arg in "$@" do echo "Arg #$index = $arg" let "index+=1" done # $@ sees arguments as separate words. echo "Arg list seen as separate words." echo index=1 # Reset count. echo "Listing args with \$* (unquoted):" for arg in $* do echo "Arg #$index = $arg" let "index+=1" done # Unquoted $* sees arguments as separate words. echo "Arg list seen as separate words." exit 0

在 shift 之后，$@保存剩余的命令行参数，缺少之前的$1，它已丢失。

#!/bin/bash # Invoke with ./scriptname 1 2 3 4 5 echo "$@" # 1 2 3 4 5 shift echo "$@" # 2 3 4 5 shift echo "$@" # 3 4 5 # Each "shift" loses parameter $1. # "$@" then contains the remaining parameters.

该$@特殊参数可用作过滤输入到 shell 脚本的工具。cat "$@" 构造接受来自脚本的输入，可以来自stdin或从作为参数提供给脚本的文件。参见 示例 16-24 和 示例 16-25。

该$*和$@参数有时会显示不一致且令人困惑的行为，具体取决于 $IFS 的设置。

示例 9-7. 不一致的$*和$@行为

#!/bin/bash # Erratic behavior of the "$*" and "$@" internal Bash variables, #+ depending on whether or not they are quoted. # Demonstrates inconsistent handling of word splitting and linefeeds. set -- "First one" "second" "third:one" "" "Fifth: :one" # Setting the script arguments, $1, $2, $3, etc. echo echo 'IFS unchanged, using "$*"' c=0 for i in "$*" # quoted do echo "$((c+=1)): [$i]" # This line remains the same in every instance. # Echo args. done echo --- echo 'IFS unchanged, using $*' c=0 for i in $* # unquoted do echo "$((c+=1)): [$i]" done echo --- echo 'IFS unchanged, using "$@"' c=0 for i in "$@" do echo "$((c+=1)): [$i]" done echo --- echo 'IFS unchanged, using $@' c=0 for i in $@ do echo "$((c+=1)): [$i]" done echo --- IFS=: echo 'IFS=":", using "$*"' c=0 for i in "$*" do echo "$((c+=1)): [$i]" done echo --- echo 'IFS=":", using $*' c=0 for i in $* do echo "$((c+=1)): [$i]" done echo --- var=$* echo 'IFS=":", using "$var" (var=$*)' c=0 for i in "$var" do echo "$((c+=1)): [$i]" done echo --- echo 'IFS=":", using $var (var=$*)' c=0 for i in $var do echo "$((c+=1)): [$i]" done echo --- var="$*" echo 'IFS=":", using $var (var="$*")' c=0 for i in $var do echo "$((c+=1)): [$i]" done echo --- echo 'IFS=":", using "$var" (var="$*")' c=0 for i in "$var" do echo "$((c+=1)): [$i]" done echo --- echo 'IFS=":", using "$@"' c=0 for i in "$@" do echo "$((c+=1)): [$i]" done echo --- echo 'IFS=":", using $@' c=0 for i in $@ do echo "$((c+=1)): [$i]" done echo --- var=$@ echo 'IFS=":", using $var (var=$@)' c=0 for i in $var do echo "$((c+=1)): [$i]" done echo --- echo 'IFS=":", using "$var" (var=$@)' c=0 for i in "$var" do echo "$((c+=1)): [$i]" done echo --- var="$@" echo 'IFS=":", using "$var" (var="$@")' c=0 for i in "$var" do echo "$((c+=1)): [$i]" done echo --- echo 'IFS=":", using $var (var="$@")' c=0 for i in $var do echo "$((c+=1)): [$i]" done echo # Try this script with ksh or zsh -y. exit 0 # This example script written by Stephane Chazelas, #+ and slightly modified by the document author.

$@ 和 $* 参数仅在双引号之间时有所不同。

示例 9-8.$*和$@当$IFS为空时

#!/bin/bash # If $IFS set, but empty, #+ then "$*" and "$@" do not echo positional params as expected. mecho () # Echo positional parameters. { echo "$1,$2,$3"; } IFS="" # Set, but empty. set a b c # Positional parameters. mecho "$*" # abc,, # ^^ mecho $* # a,b,c mecho $@ # a,b,c mecho "$@" # a,b,c # The behavior of $* and $@ when $IFS is empty depends #+ on which Bash or sh version being run. # It is therefore inadvisable to depend on this "feature" in a script. # Thanks, Stephane Chazelas. exit

其他特殊参数

$-

传递给脚本的标志（使用 set）。参见 示例 15-16。

这最初是一个 ksh 构造，被 Bash 采纳，但不幸的是，它在 Bash 脚本中似乎无法可靠地工作。它的一种可能用途是让脚本 自测它是否是交互式的。

$!

PID（进程 ID），在后台运行的最后一个作业

LOG=$0.log COMMAND1="sleep 100" echo "Logging PIDs background commands for script: $0" >> "$LOG" # So they can be monitored, and killed as necessary. echo >> "$LOG" # Logging commands. echo -n "PID of \"$COMMAND1\": " >> "$LOG" ${COMMAND1} & echo $! >> "$LOG" # PID of "sleep 100": 1506 # Thank you, Jacques Lederer, for suggesting this.

使用$!进行作业控制

possibly_hanging_job & { sleep ${TIMEOUT}; eval 'kill -9 $!' &> /dev/null; } # Forces completion of an ill-behaved program. # Useful, for example, in init scripts. # Thank you, Sylvain Fourmanoit, for this creative use of the "!" variable.

或者，或者

# This example by Matthew Sage. # Used with permission. TIMEOUT=30 # Timeout value in seconds count=0 possibly_hanging_job & { while ((count < TIMEOUT )); do eval '[ ! -d "/proc/$!" ] && ((count = TIMEOUT))' # /proc is where information about running processes is found. # "-d" tests whether it exists (whether directory exists). # So, we're waiting for the job in question to show up. ((count++)) sleep 1 done eval '[ -d "/proc/$!" ] && kill -15 $!' # If the hanging job is running, kill it. } # -------------------------------------------------------------- # # However, this may not not work as specified if another process #+ begins to run after the "hanging_job" . . . # In such a case, the wrong job may be killed. # Ariel Meragelman suggests the following fix. TIMEOUT=30 count=0 # Timeout value in seconds possibly_hanging_job & { while ((count < TIMEOUT )); do eval '[ ! -d "/proc/$lastjob" ] && ((count = TIMEOUT))' lastjob=$! ((count++)) sleep 1 done eval '[ -d "/proc/$lastjob" ] && kill -15 $lastjob' } exit

$_

特殊变量，设置为先前执行的命令的最后一个参数。

示例 9-9. 下划线变量

#!/bin/bash echo $_ # /bin/bash # Just called /bin/bash to run the script. # Note that this will vary according to #+ how the script is invoked. du >/dev/null # So no output from command. echo $_ # du ls -al >/dev/null # So no output from command. echo $_ # -al (last argument) : echo $_ # :

$?

命令、函数或脚本本身的 退出状态（参见 示例 24-7）

$$

脚本本身的进程 ID（PID）。[5] 该$$变量经常在脚本中用于构造 “唯一” 临时文件名（参见 示例 32-6、示例 16-31 和 示例 15-27）。这通常比调用 mktemp 更简单。