linux下的bash与sh 详解以及例子[转]

linux下的bash与sh 详解以及例子[转]

1、bash的POSIX标准

在一般的linux系统当中（如redhat），
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)

[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（具体区别见下面的“Things bash has that sh does not”）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（引自Bash FAQ）：

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 $(                $(cat filename)
        the ${#param} parameter value length operator
        the ${!param} indirect parameter expansion operator
        the ${!param*} prefix expansion operator
        the ${paramffset[:length]} parameter substring operator
        the ${param/pat[/string]} parameter pattern substitution operator
        expansions to perform substring removal (${p%[%]w}, ${p#[#]w})
        expansion of positional parameters beyond $9 with ${num}
        variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, REPLY,
                   TIMEFORMAT, PPID, PWD, OLDPWD, SHLVL, RANDOM, SECONDS,
                   LINENO, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE, HOSTNAME,
                   ENV, PS3, PS4, DIRSTACK, PIPESTATUS, HISTSIZE, HISTFILE,
                   HISTFILESIZE, HISTCONTROL, HISTIGNORE, GLOBIGNORE, GROUPS,
                   PROMPT_COMMAND, FCEDIT, FIGNORE, IGNOREEOF, INPUTRC,
                   SHELLOPTS, OPTERR, HOSTFILE, TMOUT, FUNCNAME, histchars,
                   auto_resume
        DEBUG trap
        ERR trap
        variable arrays with new compound assignment syntax
        redirections: <>, &>, >|, <<<, [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





2、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：

选项：
-i        If the -i option is present, the shell is interactive.
-l        Make bash act as if it had been invoked as a login shell (see
INVOCATION below).

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
$ENV

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


3、关于ssh

非常值得一提的是，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 wangweiyu@127.0.0.1 'echo $-'
wangweiyu@127.0.0.1's password:
hBc
[wangweiyu@ComSeOp ~]$ echo $-
himBH
[wangweiyu@ComSeOp ~]$ ssh wangweiyu@127.0.0.1 'echo $0'
wangweiyu@127.0.0.1's password:
bash
[wangweiyu@ComSeOp ~]$ 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”，所以其会先载入“~/.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' # 随便输出一些字符

[wangweiyu@ComSeOp ~]$ ssh wangweiyu@127.0.0.1 'echo $-'
wangweiyu@127.0.0.1's password:
xxx # .bashrc 被执行
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' # 随便输出一些字符
[wangweiyu@ComSeOp ~]$ scp file wangweiyu@127.0.0.1:/tmp
wangweiyu@127.0.0.1's password:
xxx # 输出xxx，随后scp失败
[wangweiyu@ComSeOp ~]$ echo $?
1
[wangweiyu@ComSeOp ~]$ ls /tmp/
[wangweiyu@ComSeOp ~]$