Bash Use And Programming

郑重声明，这个文章不是我写的，是我的同事——刘彬做的学习笔记。我觉得写的太好了，但他又没有发到网上，只好自己贴一下了。所以转载必须带上原作者的名字，以及本段声明。

 

Bash Use And Programming

Contents

• 1   Bash 使用
  • 1.1   按键绑定
  • 1.2   历史记录
  • 1.3   高亮
  • 1.4   Bash Completion
• 2   Bash 编程
  • 2.1   第一行
  • 2.2   Reserved Words (保留字)
  • 2.3   Parameters (参数)
    • 2.3.1   Positional Parameters (位置参数)
    • 2.3.2   Special Parameters (特殊参数)
  • 2.4   Branches And Loops (分支与循环)
    • 2.4.1   Branches
    • 2.4.2   Loops
  • 2.5   Expansion (扩展)
    • 2.5.1   Brace Expansion (大括号扩展)
    • 2.5.2   Tilde Expansion
    • 2.5.3   Parameter Expansion (参数扩展)
    • 2.5.4   Command Substitution (命令替换)
    • 2.5.5   Arithmetic Expansion (数学扩展)
    • 2.5.6   Process Substitution (进程替换)
    • 2.5.7   Word Splitting (单词分割)
    • 2.5.8   Pathname Expansion (路径扩展)
    • 2.5.9   Quote Removal (引号去除)
  • 2.6   Tips
    • 2.6.1   空字符串测试
    • 2.6.2   大小写转换
    • 2.6.3   随机数
    • 2.6.4   read
    • 2.6.5   $cmd 与 eval $cmd
    • 2.6.6   local是个内建命令
    • 2.6.7   赋值语句后跟命令
    • 2.6.8   将结果打到标准输出和错误输出
    • 2.6.9   子shell
    • 2.6.10   :(){ :|: & };:
    • 2.6.11   调试

此文分享我在bash学习，使用，编程中的碰到的一些问题，以及一些体会。

1   Bash 使用

1.1   按键绑定

CTRL 键相关的快捷键:

• Ctrl + a - Jump to the start of the line
• Ctrl + b - Move back a char
• Ctrl + c - Terminate the command
• Ctrl + d - Delete from under the cursor
• Ctrl + e - Jump to the end of the line
• Ctrl + f - Move forward a char
• Ctrl + k - Delete to EOL
• Ctrl + l - Clear the screen
• Ctrl + r - Search the history backwards
• Ctrl + R - Search the history backwards with multi occurrence
• Ctrl + u - Delete backward from cursor // 密码输入错误的时候比较有用
• Ctrl + xx - Move between EOL and current cursor position
• Ctrl + x @ - Show possible hostname completions
• Ctrl + z - Suspend/ Stop the command
• Ctrl + h - backward-delete-char
• Ctrl + w - unix-word-rubout
• Ctrl + p - previous-history
• Ctrl + n - next-history

ALT 键相关的快捷键:

• Alt + < - Move to the first line in the history
• Alt + > - Move to the last line in the history
• Alt + ? - Show current completion list
• Alt + * - Insert all possible completions
• Alt + / - Attempt to complete filename
• Alt + . - Yank last argument to previous command
• Alt + b - Move backward
• Alt + c - Capitalize the word
• Alt + d - Delete word
• Alt + f - Move forward
• Alt + l - Make word lowercase
• Alt + n - Search the history forwards non-incremental
• Alt + p - Search the history backwards non-incremental
• Alt + t - transpose-words
• Alt + u - Make word uppercase
• Alt + back-space - Delete backward from cursor

可通过bash内键命令 bind 修改按键绑定，具体可查帮助: help bind

对于使用readline库的程序，可修改 /etc/inputrc 或 ~/.inputrc 修改绑定关系。

ps: 使用vim的建议交换 CapsLock 与 Esc 键

• X window 下用xmodmap
• console 下用loadkeys
• Gnome的键盘选项里也有’互换CapsLock、Esc”的条目。 ubuntu: system->preferences->keyboard->layouts->layout options->capslock key behaivor->swap esc and capslo

我的 ~/.inputrc

"\e[A": history-search-backward
"\e[B": history-search-forward
"\C-p": history-search-backward
"\C-n": history-search-forward

# mappings for Ctrl-left-arrow and Ctrl-right-arrow for word moving
"\e[1;5C": forward-word
"\e[1;5D": backward-word
"\e[5C": forward-word
"\e[5D": backward-word
"\e\e[C": forward-word
"\e\e[D": backward-word

1.2   历史记录

# 忽略重复的命令
export HISTCONTROL=ignoredups
# 忽略由冒号分割的这些命令
export HISTIGNORE="[   ]*:&:bg:fg:exit"
export HISTFILESIZE=1000000000
export HISTSIZE=1000000
export HISTFILE=~/.lb_bash_history
shopt -s histappend

1.3   高亮

ls

eval "`dircolors -b ~/.dircolors`"
alias ls='ls --color=auto'

grep, fgrep, egrep

export GREP_OPTIONS='--color=auto'

man

export LESS_TERMCAP_mb=$'\E[01;31m'
export LESS_TERMCAP_md=$'\E[01;31m'
export LESS_TERMCAP_me=$'\E[0m'
export LESS_TERMCAP_se=$'\E[0m'
export LESS_TERMCAP_so=$'\E[01;44;33m'
export LESS_TERMCAP_ue=$'\E[0m'
export LESS_TERMCAP_us=$'\E[01;32m'

1.4   Bash Completion

if [ -f /etc/bash_completion ]; then
    . /etc/bash_completion
fi

中文目录拼音补全： chsdir

介绍怎样编写补全脚本： An introduction to bash completion part 1 , An introduction to bash completion part 2

Bash completion for discoverer and retriever

function _discoverer()
{
    local cur prev opts
    COMPREPLY=()
    cur="${COMP_WORDS[COMP_CWORD]}"
    prev="${COMP_WORDS[COMP_CWORD-1]}"

    opts="-r --home -v --version -h --help -w --website -k --keyword -K --linksite-keyword \
-c --category --related -u --userid -m --max-videos -M --max-pages \
-F --linksite-fullsite -C --linksite-category --log-level"

    case "$prev" in
        -w | --website)
        local running="youtube dailymotion myspace aol_movie metacafe msn tudou veoh yahoo youku megavideo ku6 s56 pomoho room6 pandora salloumi nicovideo myvideo tu 66stage blogspot campusist chewymovies fastpasstv freetvonline legalmovies mediateevee movie6 moviesondemand movierumor moviesdesk moviesforfree moviesister movietvonline mymovies nabolister omegatube quickss sevenscreen sidereel tvdash tvpapa watchnewmovies watchmovies watchtvsitcoms wizmovies xoxomovie live-video msn-video sogou-video bing-video-cn bing-video-com baidu-video google-video-cn google-video-com gougou-video soso-video alluc nabolister tvshack tvlinks watchmoviesfree taringa thepiratecity cinemawiki joox tvpapa familyguy freeonlineepisode moviealien tubezoom videofactor redcurtainmovies findthatshow watcheveryshow watchgossipgirl bynik watchtvsitcoms likestreaming allomovies alloshowtv diziport surfthechannel delatv estrenosonline momomesh movie2k ls2megaupload yesfilmes ninjavideo yidio casttv videosurf"

        running+="youporn slutload xnxx gaywatch megaporn xvideos itsallgay tube8 tnaflix keezmovies xhamster gayforit redtube empflix pornhub jerkyourtube youporngay"

        COMPREPLY=( $(compgen -W "$running" -- "$cur") )
        return 0
        ;;
        -r | --home)
        local running="$(dirname $PWD)"
        COMPREPLY=( $(compgen -W "$running" -- "$cur") )
        return 0
        ;;
        *)
        ;;
    esac

    if [[ ("$cur" == -*) || ( ("$cur" = "") && ("$prev" != -*) ) ]] ; then
        COMPREPLY=( $(compgen -W "${opts}" -- ${cur}) )
    fi
}
complete -F _discoverer discoverer

function _retriever()
{
    local cur prev opts
    COMPREPLY=()
    cur="${COMP_WORDS[COMP_CWORD]}"
    prev="${COMP_WORDS[COMP_CWORD-1]}"

    opts="-r --home -v --version -h --help -d --detail -D --download -k --keyid --removal --resource --log-level"
    opts+="  --need-multi-clip --max-total-size --max-clip-num"

    case "$prev" in
        -r | --home)
        local running="$(dirname $PWD)"
        COMPREPLY=( $(compgen -W "$running" -- "$cur") )
        return 0
        ;;
        *)
        ;;
    esac

    if [[ ("$cur" == -*) ]] ; then
        COMPREPLY=( $(compgen -W "${opts}" -- ${cur}) )
    fi
}
complete -F _retriever retriever

2   Bash 编程

2.1   第一行

脚本程序通常第一行以 #! 开头指定一个脚本解释器。如

#!/bin/bash
#!/bin/sh
#!/usr/bin/awk -f
#!/bin/sed -f
#!/usr/bin/expect -f
#!/usr/bin/env perl

注意 : linux 里面，解释器之后只能有一个参数，如有多个，也只做为一个。

例子： test.pl

#!/usr/bin/env LC_CTYPE=zh_CN.UTF-8 perl
print "hello\n"

perl test.pl 可以正常运行，./test.pl则不会正常执行，且不会退出。

运行 ./test.pl 相当于 /usr/bin/env "LC_CTYPE=zh_CN.UTF-8 perl" ./test.pl 。 evn会先设置LC_CTYPE="zh_CN.UTF-8 perl"这样一个环境变量，然后执行 ./test.pl，这就回到了最开始，于是一不断重复这个过程，不退出。

ps: 关于脚本程序第一行的相关信息可见于：man execve

2.2   Reserved Words (保留字)

Bash 有以下保留字：

! case  do done elif else esac fi for function if in select then until while { } time [[ ]]

注意：

• time 是保留字，不是一个内建命令，跟 /usr/bin/time 有本质的区别。 区别于命令的例子： time ls >/dev/null 2>&1 与 /usr/bin/time ls >/dev/null 2>&1
• [[ ]] 是保留字，跟 [ ] (内建命令) 有本质的区别，不能将 [[ ]] 视为一个功能更强大的 [ ]。
  • Word splitting and pathname expansion are not performed on the words between the [[ and ]]; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed.
  • Conditional operators such as -f must be unquoted to be recognized as primaries.
  • ==, !=, =~ 右边的字符串如被引用则进行字符串匹配，而不再是pattern match或正则匹配

ps: 可用 type 查看是否为保留字，内建命令等

2.3   Parameters (参数)

A parameter is an entity that stores values. A variable is a parameter denoted by a name.

2.3.1   Positional Parameters (位置参数)

指 $1, $2, $3 等

注意： $0, $1, .. $9 没有问题，$10 将被解释为 ${1}0。要使用 >= 10 的位置参数，需用 { } 括起来，如 ${15}。

2.3.2   Special Parameters (特殊参数)

From bash manual

The shell treats several parameters specially.  These parameters may only be referenced; assignment to them is not allowed.
*      Expands  to the positional parameters, starting from one.  When the expansion occurs within double quotes, it expands to a single word with the value
       of each parameter separated by the first character of the IFS special variable.  That is, "$*" is equivalent to "$1c$2c...", where  c  is  the  first
       character  of  the  value  of  the IFS variable.  If IFS is unset, the parameters are separated by spaces.  If IFS is null, the parameters are joined
       without intervening separators.
@      Expands to the positional parameters, starting from one.  When the expansion occurs within double quotes, each parameter expands to a separate  word.
       That  is,  "$@"  is  equivalent to "$1" "$2" ...  If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined
       with the beginning part of the original word, and the expansion of the last parameter is joined with the last part of the original word.  When  there
       are no positional parameters, "$@" and $@ expand to nothing (i.e., they are removed).
#      Expands to the number of positional parameters in decimal.
?      Expands to the exit status of the most recently executed foreground pipeline.
-      Expands  to  the  current  option  flags  as  specified upon invocation, by the set builtin command, or those set by the shell itself (such as the -i
       option).
$      Expands to the process ID of the shell.  In a () subshell, it expands to the process ID of the current shell, not the subshell.
!      Expands to the process ID of the most recently executed background (asynchronous) command.
0      Expands to the name of the shell or shell script.  This is set at shell initialization.  If bash is invoked with a file of commands, $0 is set to the
       name  of  that file.  If bash is started with the -c option, then $0 is set to the first argument after the string to be executed, if one is present.
       Otherwise, it is set to the file name used to invoke bash, as given by argument zero.
_      At shell startup, set to the absolute pathname used to invoke the shell or shell script being executed as passed in the environment or argument list.
       Subsequently,  expands to the last argument to the previous command, after expansion.  Also set to the full pathname used to invoke each command exe
       cuted and placed in the environment exported to that command.  When checking mail, this parameter holds the name of the  mail  file  currently  being
       checked.

使用较多的有 $*, $@, $#, $?, $$, $!, $0

* 与 @ 的区别，在别的地方法了是这样。如在数组中 "${array[*]}" 与 "${array[@]}" 。

2.4   Branches And Loops (分支与循环)

2.4.1   Branches

• if list; then list; [ elif list; then list; ] ... [ else list; ] fi

  这是最常用的分支语句，通常用 [ ], [[ ]], (( )) 进行条件测试。事实上一切语句都可以用，如

  if echo "$var" | tr '[:upper:]' '[:lower:]' | grep -q 'regex expression' ; then
      echo matched
  fi
  

• case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac

  case 使用 pattern 匹配进行测试，变得非常好用。 如检查一个字符串是否包含别一个字符串

  case "$1" in )
      *"$2"* ) true ;;
      * ) false ;;
  esca
  

• &&, ||

  如 [ $# -ne 2 ] && exit 1 或 [ $# -eq 2 ] || exit 1

2.4.2   Loops

• for name [ in word ] ; do list ; done

  for host in $(<host_list) ; do ... ; done
  for i in {1..100} ; do ... ; done
  for f in /aaa/bbb/cc* ; do ... ; done
  for v in "$@" ; do ... ; done
  

• for (( expr1 ; expr2 ; expr3 )) ; do list ; done

  跟 c/c++ 中 for 语义一致，好用。如

  for (( i=0; i<100; i++ )) ; do ... ; done
  

• while list; do list; done

  while true ; do ...; done
  while read line ; do ...; done < input_file
  

• until list; do list; done

  语义跟while相反，没见人用过。

2.5   Expansion (扩展)

2.5.1   Brace Expansion (大括号扩展)

$ echo abc{d,e,f}
abcd abce abcf
$ echo {a,b,c}{d,e,f}
ad ae af bd be bf cd ce cf

$ echo {1..10}
1 2 3 4 5 6 7 8 9 10
$ echo {1..10..2}
1 3 5 7 9

ps:

cp filename{,.bak} 好像是某个网站选出来的十大最酷的linux单行命令之一。

2.5.2   Tilde Expansion

$ echo ~
/home/liubin
$ echo ~liubin
/home/liubin
$ echo ~tracker
/home/tracker
$ echo ~no
~no

$ echo ~+
/tmp
$ echo ~-
/home/liubin

2.5.3   Parameter Expansion (参数扩展)

最基本的

• $parameter
• ${parameter}

根据parameter未定义或值为空进行操作/扩展

• ${parameter:-word}

• ${parameter:=word}

• ${parameter:?word}

• ${parameter:+word}

  较为有用，可用于为输入参数置默认值等。如

  out_file=${1:-/dev/null}
  

  size=$(stat -c %s $file)
  if [ ${size:-0} -lt 100 ] ; then
      ...
  fi
  

数组相关

• ${name[subscript]}
• ${name[*]}
• ${name[@]}
• ${#name[*]}
• ${#name[@]}
• ${!name[*]}
• ${!name[@]}

字符串操作

• ${#parameter}

• ${parameter:offset}

• ${parameter:offset:length}

• ${parameter#word}

• ${parameter##word}

• ${parameter%word}

• ${parameter%%word}

  非常有用，如 basename 可近似于 ${path##*/} , dirname 可近似于 ${path$/*}

• ${parameter/pattern/string}

• ${parameter^pattern}

• ${parameter^^pattern}

• ${parameter,pattern}

• ${parameter,,pattern}

  上面四个大小写转换的应该是在 bash 4.0 中引入的。写入脚本中要慎重。

另外

• ${!prefix*}
• ${!prefix@}

2.5.4   Command Substitution (命令替换)

$(command) or `command`

会去掉最后的newline,但中间的会保留。

var=$(<file) 跟var=$(cat file) 是相同的，但要快。

1. ``与$()处理 '\' 的方式是不同的。

   `'sed -e 's/\\/\\\\/g'` 是不可用的，而 $(sed -e 's/\\/\\\\/g') 是可用的
   

   $ echo "`echo '\\'`"
   \
   echo "$(echo '\\')"
   \\
   
   echo "`echo sed -e 's/\\/\\\\/g'`"
   sed -e s/\/\\/g
   $ echo "$(echo sed -e 's/\\/\\\\/g')"
   sed -e s/\\/\\\\/g
   

2. 使用``嵌套时要将里面的 `` 转义。

``是比较旧的方式，现已渐被$()替换

ps: 个人认为 `` 写在脚本中也不好看，符号太小，且易与 ' 混淆，建议不要使用。

2.5.5   Arithmetic Expansion (数学扩展)

$(( expression ))

2.5.6   Process Substitution (进程替换)

<(list) or >(list)

Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files.

例子

$ diff <(printf "%s\n" a b c) <(printf "%s\n" a b c)
$ diff <(printf "%s\n" a b c) <(printf "%s\n" a bb c)
2c2
< b
---
> bb
$ paste <(printf "%s\n" a b c) <(printf "%s\n" a bb c)
a   a
b   bb
c   c

可用于将 list | while read l ; do ...; done 替换为 while read l ; do ...; done < <(list) 这种形式，从而避免 while 循环于子 shell 执行。

2.5.7   Word Splitting (单词分割)

2.5.8   Pathname Expansion (路径扩展)

通过 pattern matching 进行扩展。

From bash manual

*      Matches any string, including the null string.  When the globstar shell option is enabled, and * is used in a filename expansion context,  two  adja
       cent  *s  used  as  a  single pattern will match all files and zero or more directories and subdirectories.  If followed by a /, two adjacent *s will
       match only directories and subdirectories.
?      Matches any single character.
[...]  Matches any one of the enclosed characters.  A pair of characters separated by a hyphen denotes a range expression; any character that sorts  between
       those two characters, inclusive, using the current locale's collating sequence and character set, is matched.  If the first character following the [
       is a !  or a ^ then any character not enclosed is matched.  The sorting order of characters in range expressions is determined by the current  locale
       and  the  value  of the LC_COLLATE shell variable, if set.  A - may be matched by including it as the first or last character in the set.  A ] may be
       matched by including it as the first character in the set.

       Within [ and ], character classes can be specified using the syntax [:class:], where class is one of the following classes defined in the POSIX stan
       dard:
       alnum alpha ascii blank cntrl digit graph lower print punct space upper word xdigit
       A character class matches any character belonging to that class.  The word character class matches letters, digits, and the character _.

       Within [ and ], an equivalence class can be specified using the syntax [=c=], which matches all characters with the same collation weight (as defined
       by the current locale) as the character c.

       Within [ and ], the syntax [.symbol.] matches the collating symbol symbol.

路径扩展是在单词割之后进行，所以如路径名中有空格，该路径也只会做为一个参数传入。

所以，一般来说这种写法 for f in $(ls $dir) ; do ... ; done 不如 for f in $dir/* ; do ...; done

2.5.9   Quote Removal (引号去除)

After the preceding expansions, all unquoted occurrences of the characters , ', and " that did not result from one of the above expansions are removed.

2.6   Tips

2.6.1   空字符串测试

$ var=
$ [ -z "$var" ] ; echo $?
0
$ [ -n "$var" ] ; echo $?
1
$ [ -z $var ] ; echo $?
0
$ [ -n $var ] ; echo $?
0

在用 -n 进行字符串不为空的测试时，一定要将所测试的变量用 "" 引起来。

2.6.2   大小写转换

bash 一般的做法是通过外部命令来实现，或自己写个函数来做。

• tr '[:lower:]' '[:upper:]'
• sed 'y/abcdefghijklmnopqrstuvwxyz/ABCDEFGHIJKLMNOPQRSTUVWXYZ/
• awk '{print toupper($0)}'

在较高版本的bash中可用以下方法

• declare -l, declare -u 然后对变量赋值。
• ${parameter^pattern} ${parameter^^pattern} ${parameter,pattern} ${parameter,,pattern}

2.6.3   随机数

$RANDOM 产生 0 - 32767 之间随机数，要产生更大的随要数，可以以 $RANDOM 为基础做一个函数。 也可以像 hexdump -e '"%u\n"' -n 4 /dev/random 这样去读 /dev/random

2.6.4   read

不对 \ 转义

read 读一行时会对 \ 转义，一般来说是没用的，需加上 -r 参数。

REPLY

read时会去掉头尾的空白，如需要保留，则read后不要跟变量，用 $REPLY 访问读到的内容。

如对一个文本文件进行通过read进行复制可以： while read -r ; do echo "$REPLY" ; doen <input >output

while read line

需防止 while 中的命令去读 stdin

如

printf "%s\n" localhost localhost | while read host ; do ssh $host whoami ; done

这样是有问题的，正确的方法可为

printf "%s\n" localhost localhost | while read host ; do ssh -n $host whoami ; done
printf "%s\n" localhost localhost | while read host ; do ssh $host whoami </dev/null ; done

exec 3< <(printf "%s\n" localhost localhost)
while read -u3 host ; do ssh $host whoami ; done
exec 3<&-

2.6.5   $cmd 与 eval $cmd

见: http://fvue.nl/wiki/Bash:_Why_use_eval_with_variable_expansion%3F

$cmd 一般会有问题，大概是这样理解的 :

$cmd 会最先做一个syntax parser，然后再参数扩展，然后会到 word splitting。 cmd='echo "a                  b"' ， $cmd 在word splitting后会变成 echo '"a' 'b"'

要只被交到 word splitting 的就是按IFS来分的了，如

$ echo $(echo '"a          b"')
"a b"
$ echo "$(echo '"a          b"')"
"a          b"

所以会被分成那个鸟样。

如cmd中有参数，那么 "$cmd" 这样直接执行一般不会有结果，返回值是127，就是找不到命令。

而 eval 是相当于对 扩展好的 cmd 再走了一下解释流程，所以没有问题。

最后结论：不要用 $cmd 要用 eval "$cmd"

2.6.6   local是个内建命令

不要用 local a="a_value" b="${a} + b value"

因为 local 是个命令，跟bash中赋值语句的执行是不一样的。

例子

$ cat t.sh
#!/bin/bash

function t1()
{
    local a=t1 b=$a.bb
    echo "a:[$a] b:[$b]"
}

function t2()
{
    a=t2 b=$a.bb
    echo "a:[$a] b:[$b]"
}

t1
t2
t1
$ ./t.sh
a:[t1] b:[.bb]
a:[t2] b:[t2.bb]
a:[t1] b:[t2.bb]

同理，export也是一样的。

2.6.7   赋值语句后跟命令

命令前有赋值(如 PATH=/usr/bin env )，那个这个赋值只对这个命令本身有效，不会影响当前环境。 所以这个赋值一般要是环境变量才行，不然应该一点用都没有。

常用的形式有 IFS=: read v1 v2 v3

2.6.8   将结果打到标准输出和错误输出

echo hello              # stdout
echo hello 1>&2         # stderr

上面是必需必需的, 还可以通过重定向到以下文件来指定：

/dev/stdout /dev/stderr

/dev/fd/1 /dev/fd/2

/proc/$$/fd/1 /proc/$$/fd/2

/proc/self/fd/1 /proc/self/fd/2

2.6.9   子shell

子shell中的变量对于子shell之外的代码块来说, 是不可见的. 当然, 父进程也不能访问这些变量。

圆括号中的命令列表

(command;command;...; command)

可用于进入一个目录操作再退出

(
cd $dir
.... # do something
)

管道连接的命令

From bash manual

Each command in a pipeline is executed as a separate process (i.e., in a subshell)

常见的一个问题

num=1
seq 1 5 | while read l ; do num=$((num+1)); done
echo $num

可使用进程替换解决

num=1
while read l ; do num=$((num+1)); done < <(seq 1 5)
echo $num

不支持进程替换的可用命名管道解决

mkfifo t_fifo
seq 1 5 >t_fifo &
num=1
while read l ; do num=$((num+1)); done <t_fifo
echo $num

命令替换中的命令列表

$ a=aa
$ echo $(a=bb;echo $a)
bb
$ echo $a
aa

2.6.10   :(){ :|: & };:

fork炸弹，造成死机，危险。

解释

:()
{
    : | : &
}
:

定义了一个名为 ':' 的递归函数，然后调用。

ps: ':' 是一个啥事不干的内建命令，可用于占位，如

if [ $v1 -lt v2 ] ; then
    :
else
    echo do something
fi

2.6.11   调试

• bash -n ./scritp 只做语法检查，不执行
• set -x Print commands and their arguments as they are executed. (set +x 关闭)
• 插入 echo, printf 等打印语句调试
• 对重要的的命令可在调度阶段只 echo 出命令及其参数确定没问题后再改为执行。 如 rm -rf $dir 这样的危险命令也建议先采用这个方法。

 

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