在我們所使用的系統當中，使用sh呼叫執行指令碼，相當於打開了bash的POSIX標準模式 （等效於bash的 --posix 引數） 一般的，sh是bash的“子集” （不是子集的部分，具體區別見下的“Things sh has that bash does not”） 例子： [[email protected] test]$ cat t2.sh  #!/bin/bash diff <(echo xxx) <(echo yyy) # 此語法包含bash的特性，不屬於sh的POSIX標準     [[email protected] test]$ bash -x ./t2.sh # 使用bash 呼叫，不會出問題 + diff /dev/fd/63 /dev/fd/62 ++ echo xxx ++ echo yyy 1c1 < xxx --- >    yyy [[email protected] test]$ sh ./t2.sh    # 而用sh呼叫，報錯如下 ./t2.sh: line 3: syntax error near unexpected token `(' ./t2.sh: line 3: `diff <(echo xxx) <(echo yyy)' [[email protected] test]$ echo $?  2 但是，在我們的linux系統中，sh是bash的一個軟連結： [[email protected]

mon]$ which sh  /bin/sh [[email protected] mon]$ ls -l /bin/sh  lrwxrwxrwx  1 root root 4 Mar 21  2007 /bin/sh -> bash 那為什麼上面的例子中還會出現問題呢？原因在於： bash程式執行，當“$0”是“sh”的時候， 則要求下面的程式碼遵循一定的規範，當不符合規範的語法存在時，則會報錯， 所以可以這樣理解， “sh”並不是一個程式，而是一種標準（POSIX）， 這種標準，在一定程度上保證了指令碼的跨系統性（跨UNIX系統） 下面的內容詳細的說明了bash與sh在語法等方面的具體差異： 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 $(<filename) form of command substitution, equivalent to       $(cat filename)    the ${#param} parameter value length operator    the ${!param} indirect parameter expansion operator    the ${!param*} prefix expansion operator    the ${param:offset[: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 呼叫相關： 在指令碼的呼叫方面（interactive、login相關），bash與sh也是存在差異 以下是詳細說明（假如被呼叫執行的指令碼名字叫xxx.sh） BASH： 1、   互動式的登入shell （bash –il xxx.sh） 載入的資訊： /etc/profile ~/.bash_profile（ ->  ~/.bashrc  ->  /etc/bashrc） ~/.bash_login ~/.profile 2、非互動式的登入shell （bash –l xxx.sh） 載入的資訊： /etc/profile ~/.bash_profile （ ->  ~/.bashrc  ->  /etc/bashrc） ~/.bash_login ~/.profile $BASH_ENV 3、互動式的非登入shell （bash –i xxx.sh） 載入的資訊： ~/.bashrc （ ->  /etc/bashrc） 4、非互動式的非登入shell （bash xxx.sh） 載入的資訊： $BASH_ENV SH： 1、互動式的登入shell 載入的資訊： /etc/profile ~/.profile 2、非互動式的登入shell 載入的資訊： /etc/profile ~/.profile 3、互動式的非登入shell 載入的資訊： $ENV 4、非互動式的非登入shell 載入的資訊： nothing 由此可以看出，最主要的區別在於相關配置檔案的是否載入， 而這些配置的是否載入，也就導致了很多預設選項的差異 （具體請仔細檢視~/.bash_profile 等檔案） 如： [[email protected] ~]$ grep ulimit /etc/profile     ulimit -S -c unlimited > /dev/null 2>&1 即，如果/etc/profile沒有被載入，則不會產生core dump 值得一提的是，使用ssh遠端執行命令， 遠端sshd程序通過“bash –c”的方式來執行命令（即“非互動式的非登入shell”） 所以這一點，和登入之後再在本地執行執行命令，就存在了一定的差異 如： [[email protected] ~]$ ssh [email protected] 'echo $-' [email protected]'s password:  hBc [[email protected] ~]$ echo $- himBH [[email protected] ~]$ ssh [email protected] 'echo $0' [email protected]'s password:  bash [[email protected] ~]$ echo $0 -bash 注： “$-” 中含有“i”代表“互動式shell” “$0”的顯示結果為“-bash”，bash前面多個“-”，代表“登入shell” 沒有“i“和“-”的，是“非互動式的非登入shell” 另外還有一點，雖然ssh遠端執行的命令是“非互動式的非登入shell”，但在執行命令之前，ssh的那一次登入本身是“互動式的登入shell”，所以其會先讀一下“~/.bash_profile” 如： [[email protected] ~]$ cat .bashrc  # .bashrc # User specific aliases and functions # Source global definitions if [ -f /etc/bashrc ]; then         . /etc/bashrc fi echo 'xxx' [[email protected] ~]$ ssh [email protected] 'echo $-' [email protected]'s password:  xxx hBc 這一點，衍生出一個關於scp的問題，scp在傳輸資料之前，會先進行一次ssh登入， 而當.bashrc檔案有輸出的時候，則會導致scp失敗！原因是解析返回的資料包出現混亂 如： [[email protected] ~]$ cat .bashrc  # .bashrc # User specific aliases and functions # Source global definitions if [ -f /etc/bashrc ]; then         . /etc/bashrc fi echo 'xxx' [[email protected] ~]$ scp file [email protected]:/tmp [email protected]'s password:  xxx [[email protected] ~]$ echo $? 1 [[email protected] ~]$ ls /tmp/ [[email protected] ~]$

轉自：http://hi.baidu.com/aaronike/blog/item/0072533d1068a5e83c6d97ea.html