【转载】Linux中bash与sh的区别

 

【转载】Linux中bash与sh的区别

Author: 匿名  Click: 387   Date: 2011.05.22 @ 11:51:28 am Category: linux

在我们所使用的系统当中，使用sh调用执行脚本，相当于打开了bash的POSIX标准模式 （等效于bash的 --posix 参数）

一般的，sh是bash的“子集” （不是子集的部分，具体区别见下的“Things sh has that bash does not”）

例子：

[wwy@sf-watch test]$ cat t2.sh 

#!/bin/bash

diff <(echo xxx) <(echo yyy) # 此语法包含bash的特性，不属于sh的POSIX标准



    [wwy@sf-watch test]$ bash -x ./t2.sh # 使用bash 调用，不会出问题

+ diff /dev/fd/63 /dev/fd/62

++ echo xxx

++ echo yyy

1c1

< xxx

---

>    yyy

[wwy@sf-watch test]$ sh ./t2.sh    # 而用sh调用，报错如下

./t2.sh: line 3: syntax error near unexpected token `('

./t2.sh: line 3: `diff <(echo xxx) <(echo yyy)'

[wwy@sf-watch test]$ echo $? 

2

但是，在我们的linux系统中，sh是bash的一个软链接：

[wangweiyu@ComSeOp mon]$ which sh 

/bin/sh

[wangweiyu@ComSeOp mon]$ ls -l /bin/sh 

lrwxrwxrwx  1 root root 4 Mar 21  2007 /bin/sh -> bash

那为什么上面的例子中还会出现问题呢？原因在于： bash程序执行，当“$0”是“sh”的时候， 则要求下面的代码遵循一定的规范，当不符合规范的语法存在时，则会报错， 所以可以这样理解， “sh”并不是一个程序，而是一种标准（POSIX）， 这种标准，在一定程度上保证了脚本的跨系统性（跨UNIX系统）

下面的内容详细的说明了bash与sh在语法等方面的具体差异：

Things bash has that sh does not:



   long invocation options

   [+-]O invocation option

   -l invocation option

   `!' reserved word to invert pipeline return value

   `time' reserved word to time pipelines and shell builtins

   the `function' reserved word

   the `select' compound command and reserved word

   arithmetic for command: for ((expr1 ; expr2; expr3 )); do list; done

   new $'...' and $"..." quoting

   the $(...) form of command substitution

   the $(, &>, >|, <<<, [n]<&word-, [n]>&word-

   prompt string special char translation and variable expansion

   auto-export of variables in initial environment

   command search finds functions before builtins

   bash return builtin will exit a file sourced with `.'

   builtins: cd -/-L/-P, exec -l/-c/-a, echo -e/-E, hash -d/-l/-p/-t.

        export -n/-f/-p/name=value, pwd -L/-P,

        read -e/-p/-a/-t/-n/-d/-s/-u,

        readonly -a/-f/name=value, trap -l, set +o,

        set -b/-m/-o option/-h/-p/-B/-C/-H/-P,

        unset -f/-v, ulimit -i/-m/-p/-q/-u/-x,

        type -a/-p/-t/-f/-P, suspend -f, kill -n,

        test -o optname/s1 == s2/s1 < s2/s1 > s2/-nt/-ot/-ef/-O/-G/-S

   bash reads ~/.bashrc for interactive shells, $ENV for non-interactive

   bash restricted shell mode is more extensive

   bash allows functions and variables with the same name

   brace expansion

   tilde expansion

   arithmetic expansion with $((...)) and `let' builtin

   the `[[...]]' extended conditional command

   process substitution

   aliases and alias/unalias builtins

   local variables in functions and `local' builtin

   readline and command-line editing with programmable completion

   command history and history/fc builtins

   csh-like history expansion

   other new bash builtins: bind, command, compgen, complete, builtin,

             declare/typeset, dirs, enable, fc, help,

             history, logout, popd, pushd, disown, shopt,

             printf

   exported functions

   filename generation when using output redirection (command >a*)

   POSIX.2-style globbing character classes

   POSIX.2-style globbing equivalence classes

   POSIX.2-style globbing collating symbols

   egrep-like extended pattern matching operators

   case-insensitive pattern matching and globbing

   variable assignments preceding commands affect only that command,

      even for builtins and functions

   posix mode and strict posix conformance

   redirection to /dev/fd/N, /dev/stdin, /dev/stdout, /dev/stderr,

      /dev/tcp/host/port, /dev/udp/host/port

   debugger support, including `caller' builtin and new variables

   RETURN trap

   the `+=' assignment operator



Things sh has that bash does not:

   uses variable SHACCT to do shell accounting

   includes `stop' builtin (bash can use alias stop='kill -s STOP')

   `newgrp' builtin

   turns on job control if called as `jsh'

   $TIMEOUT (like bash $TMOUT)

   `^' is a synonym for `|'

   new SVR4.2 sh builtins: mldmode, priv



Implementation differences:

   redirection to/from compound commands causes sh to create a subshell

   bash does not allow unbalanced quotes; sh silently inserts them at EOF

   bash does not mess with signal 11

   sh sets (euid, egid) to (uid, gid) if -p not supplied and uid < 100

   bash splits only the results of expansions on IFS, using POSIX.2

      field splitting rules; sh splits all words on IFS

   sh does not allow MAILCHECK to be unset (?)

   sh does not allow traps on SIGALRM or SIGCHLD

   bash allows multiple option arguments when invoked (e.g. -x -v);

      sh allows only a single option argument (`sh -x -v' attempts

      to open a file named `-v', and, on SunOS 4.1.4, dumps core.

      On Solaris 2.4 and earlier versions, sh goes into an infinite

      loop.)

   sh exits a script if any builtin fails; bash exits only if one of

      the POSIX.2 `special' builtins fails

调用相关：

在脚本的调用方面（interactive、login相关），bash与sh也是存在差异 以下是详细说明（假如被调用执行的脚本名字叫xxx.sh）

BASH：

 

1、   交互式的登录shell （bash –il xxx.sh）

载入的信息：

/etc/profile

~/.bash_profile（ ->  ~/.bashrc  ->  /etc/bashrc）

~/.bash_login

~/.profile

 

2、非交互式的登录shell （bash –l xxx.sh）

载入的信息：

/etc/profile

~/.bash_profile （ ->  ~/.bashrc  ->  /etc/bashrc）

~/.bash_login

~/.profile

$BASH_ENV

 

3、交互式的非登录shell （bash –i xxx.sh）

载入的信息：

~/.bashrc （ ->  /etc/bashrc）

 

4、非交互式的非登录shell （bash xxx.sh）

载入的信息：

$BASH_ENV

 

SH：

 

1、交互式的登录shell

载入的信息：

/etc/profile

~/.profile

 

2、非交互式的登录shell

载入的信息：

/etc/profile

~/.profile

 

3、交互式的非登录shell

载入的信息：

$ENV

 

4、非交互式的非登录shell

载入的信息：

nothing

由此可以看出，最主要的区别在于相关配置文件的是否载入， 而这些配置的是否载入，也就导致了很多默认选项的差异 （具体请仔细查看~/.bash_profile 等文件） 如：

[wangweiyu@ComSeOp ~]$ grep ulimit /etc/profile    

ulimit -S -c unlimited > /dev/null 2>&1

即，如果/etc/profile没有被载入，则不会产生core dump

值得一提的是，使用ssh远程执行命令， 远端sshd进程通过“bash –c”的方式来执行命令（即“非交互式的非登录shell”） 所以这一点，和登录之后再在本地执行执行命令，就存在了一定的差异

如：

[wangweiyu@ComSeOp ~]$ ssh [email protected] 'echo $-'

[email protected]'s password: 

hBc

[wangweiyu@ComSeOp ~]$ echo $-

himBH

[wangweiyu@ComSeOp ~]$ ssh [email protected] 'echo $0'

[email protected]'s password: 

bash

[wangweiyu@ComSeOp ~]$ echo $0

-bash

注： “$-” 中含有“i”代表“交互式shell” “$0”的显示结果为“-bash”，bash前面多个“-”，代表“登录shell” 没有“i“和“-”的，是“非交互式的非登录shell”

另外还有一点，虽然ssh远程执行的命令是“非交互式的非登录shell”，但在执行命令之前，ssh的那一次登录本身是“交互式的登录shell”，所以其会先读一下“~/.bash_profile”

如：

[wangweiyu@ComSeOp ~]$ cat .bashrc 

# .bashrc

# User specific aliases and functions

# Source global definitions

if [ -f /etc/bashrc ]; then

        . /etc/bashrc

fi

echo 'xxx'



[wangweiyu@ComSeOp ~]$ ssh [email protected] 'echo $-'

[email protected]'s password: 

xxx

hBc

这一点，衍生出一个关于scp的问题，scp在传输数据之前，会先进行一次ssh登录， 而当.bashrc文件有输出的时候，则会导致scp失败！原因是解析返回的数据包出现混乱

如：

[wangweiyu@ComSeOp ~]$ cat .bashrc 

# .bashrc

# User specific aliases and functions

# Source global definitions

if [ -f /etc/bashrc ]; then

        . /etc/bashrc

fi

echo 'xxx'

[wangweiyu@ComSeOp ~]$ scp file [email protected]:/tmp

[email protected]'s password: 

xxx

[wangweiyu@ComSeOp ~]$ echo $?

1

[wangweiyu@ComSeOp ~]$ ls /tmp/

[wangweiyu@ComSeOp ~]$