Linux学习之道：linux下的bash与sh 详解以及例子

约定不等于承诺〃 2021-08-27 20:15 435阅读 0赞

关于bash与sh的话题（限于一般linux），以下个人的一些总结及理解，有理解错误的地方还望指点  
  
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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：

但是，在我们的linux系统中，sh是bash的一个软链接：

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）：

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 <td style="word-wrap:break-word; font-size:14px; line-height:1.6em; font-family:song,Verdana; padding:4px; border:1px solid rgb(0,0,0); overflow:hidden"> Things bash has that sh does not: &nbsp; &nbsp; &nbsp; &nbsp; long invocation options &nbsp; &nbsp; &nbsp; &nbsp; [+-]O invocation option &nbsp; &nbsp; &nbsp; &nbsp; -l invocation option &nbsp; &nbsp; &nbsp; &nbsp; `!' reserved word to invert pipeline return value &nbsp; &nbsp; &nbsp; &nbsp; `time' reserved word to time pipelines and shell builtins &nbsp; &nbsp; &nbsp; &nbsp; the `function' reserved word &nbsp; &nbsp; &nbsp; &nbsp; the `select' compound command and reserved word &nbsp; &nbsp; &nbsp; &nbsp; arithmetic for command: for ((expr1 ; expr2; expr3 )); do list; done &nbsp; &nbsp; &nbsp; &nbsp; new $'...' and $"..." quoting &nbsp; &nbsp; &nbsp; &nbsp; the $(...) form of command substitution &nbsp; &nbsp; &nbsp; &nbsp; the $(&lt;filename) form of command substitution, equivalent to &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; $(cat filename) &nbsp; &nbsp; &nbsp; &nbsp; the ${#param} parameter value length operator &nbsp; &nbsp; &nbsp; &nbsp; the ${!param} indirect parameter expansion operator &nbsp; &nbsp; &nbsp; &nbsp; the ${!param*} prefix expansion operator &nbsp; &nbsp; &nbsp; &nbsp; the ${param<img src="http://bbs.chinaunix.net/static/image/smiley/default/icon_surprised.gif" alt="" style="word-wrap:break-word">ffset[:length]} parameter substring operator &nbsp; &nbsp; &nbsp; &nbsp; the ${param/pat[/string]} parameter pattern substitution operator &nbsp; &nbsp; &nbsp; &nbsp; expansions to perform substring removal (${p%[%]w}, ${p#[#]w}) &nbsp; &nbsp; &nbsp; &nbsp; expansion of positional parameters beyond $9 with ${num} &nbsp; &nbsp; &nbsp; &nbsp; variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, REPLY, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; &nbsp; TIMEFORMAT, PPID, PWD, OLDPWD, SHLVL, RANDOM, SECONDS, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; &nbsp; LINENO, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE, HOSTNAME, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; &nbsp; ENV, PS3, PS4, DIRSTACK, PIPESTATUS, HISTSIZE, HISTFILE, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; &nbsp; HISTFILESIZE, HISTCONTROL, HISTIGNORE, GLOBIGNORE, GROUPS, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; &nbsp; PROMPT_COMMAND, FCEDIT, FIGNORE, IGNOREEOF, INPUTRC, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; &nbsp; SHELLOPTS, OPTERR, HOSTFILE, TMOUT, FUNCNAME, histchars, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; &nbsp; auto_resume &nbsp; &nbsp; &nbsp; &nbsp; DEBUG trap &nbsp; &nbsp; &nbsp; &nbsp; ERR trap &nbsp; &nbsp; &nbsp; &nbsp; variable arrays with new compound assignment syntax &nbsp; &nbsp; &nbsp; &nbsp; redirections: &lt;&gt;, &amp;&gt;, &gt;|, &lt;&lt;&lt;, [n]&lt;&amp;word-, [n]&gt;&amp;word- &nbsp; &nbsp; &nbsp; &nbsp; prompt string special char translation and variable expansion &nbsp; &nbsp; &nbsp; &nbsp; auto-export of variables in initial environment &nbsp; &nbsp; &nbsp; &nbsp; command search finds functions before builtins &nbsp; &nbsp; &nbsp; &nbsp; bash return builtin will exit a file sourced with `.' &nbsp; &nbsp; &nbsp; &nbsp; builtins: cd -/-L/-P, exec -l/-c/-a, echo -e/-E, hash -d/-l/-p/-t. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; &nbsp;export -n/-f/-p/name=value, pwd -L/-P, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; &nbsp;read -e/-p/-a/-t/-n/-d/-s/-u, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; &nbsp;readonly -a/-f/name=value, trap -l, set +o, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; &nbsp;set -b/-m/-o option/-h/-p/-B/-C/-H/-P, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; &nbsp;unset -f/-v, ulimit -i/-m/-p/-q/-u/-x, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; &nbsp;type -a/-p/-t/-f/-P, suspend -f, kill -n, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; &nbsp;test -o optname/s1 == s2/s1 &lt; s2/s1 &gt; s2/-nt/-ot/-ef/-O/-G/-S &nbsp; &nbsp; &nbsp; &nbsp; bash reads ~/.bashrc for interactive shells, $ENV for non-interactive &nbsp; &nbsp; &nbsp; &nbsp; bash restricted shell mode is more extensive &nbsp; &nbsp; &nbsp; &nbsp; bash allows functions and variables with the same name &nbsp; &nbsp; &nbsp; &nbsp; brace expansion &nbsp; &nbsp; &nbsp; &nbsp; tilde expansion &nbsp; &nbsp; &nbsp; &nbsp; arithmetic expansion with $((...)) and `let' builtin &nbsp; &nbsp; &nbsp; &nbsp; the `[[...]]' extended conditional command &nbsp; &nbsp; &nbsp; &nbsp; process substitution &nbsp; &nbsp; &nbsp; &nbsp; aliases and alias/unalias builtins &nbsp; &nbsp; &nbsp; &nbsp; local variables in functions and `local' builtin &nbsp; &nbsp; &nbsp; &nbsp; readline and command-line editing with programmable completion &nbsp; &nbsp; &nbsp; &nbsp; command history and history/fc builtins &nbsp; &nbsp; &nbsp; &nbsp; csh-like history expansion &nbsp; &nbsp; &nbsp; &nbsp; other new bash builtins: bind, command, compgen, complete, builtin, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp;declare/typeset, dirs, enable, fc, help, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp;history, logout, popd, pushd, disown, shopt, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp;printf &nbsp; &nbsp; &nbsp; &nbsp; exported functions &nbsp; &nbsp; &nbsp; &nbsp; filename generation when using output redirection (command &gt;a*) &nbsp; &nbsp; &nbsp; &nbsp; POSIX.2-style globbing character classes &nbsp; &nbsp; &nbsp; &nbsp; POSIX.2-style globbing equivalence classes &nbsp; &nbsp; &nbsp; &nbsp; POSIX.2-style globbing collating symbols &nbsp; &nbsp; &nbsp; &nbsp; egrep-like extended pattern matching operators &nbsp; &nbsp; &nbsp; &nbsp; case-insensitive pattern matching and globbing &nbsp; &nbsp; &nbsp; &nbsp; variable assignments preceding commands affect only that command, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; even for builtins and functions &nbsp; &nbsp; &nbsp; &nbsp; posix mode and strict posix conformance &nbsp; &nbsp; &nbsp; &nbsp; redirection to /dev/fd/N, /dev/stdin, /dev/stdout, /dev/stderr, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; /dev/tcp/host/port, /dev/udp/host/port &nbsp; &nbsp; &nbsp; &nbsp; debugger support, including `caller' builtin and new variables &nbsp; &nbsp; &nbsp; &nbsp; RETURN trap &nbsp; &nbsp; &nbsp; &nbsp; the `+=' assignment operator Things sh has that bash does not: &nbsp; &nbsp; &nbsp; &nbsp; uses variable SHACCT to do shell accounting &nbsp; &nbsp; &nbsp; &nbsp; includes `stop' builtin (bash can use alias stop='kill -s STOP') &nbsp; &nbsp; &nbsp; &nbsp; `newgrp' builtin &nbsp; &nbsp; &nbsp; &nbsp; turns on job control if called as `jsh' &nbsp; &nbsp; &nbsp; &nbsp; $TIMEOUT (like bash $TMOUT) &nbsp; &nbsp; &nbsp; &nbsp; `^' is a synonym for `|' &nbsp; &nbsp; &nbsp; &nbsp; new SVR4.2 sh builtins: mldmode, priv Implementation differences: &nbsp; &nbsp; &nbsp; &nbsp; redirection to/from compound commands causes sh to create a subshell &nbsp; &nbsp; &nbsp; &nbsp; bash does not allow unbalanced quotes; sh silently inserts them at EOF &nbsp; &nbsp; &nbsp; &nbsp; bash does not mess with signal 11 &nbsp; &nbsp; &nbsp; &nbsp; sh sets (euid, egid) to (uid, gid) if -p not supplied and uid &lt; 100 &nbsp; &nbsp; &nbsp; &nbsp; bash splits only the results of expansions on IFS, using POSIX.2 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; field splitting rules; sh splits all words on IFS &nbsp; &nbsp; &nbsp; &nbsp; sh does not allow MAILCHECK to be unset (?) &nbsp; &nbsp; &nbsp; &nbsp; sh does not allow traps on SIGALRM or SIGCHLD &nbsp; &nbsp; &nbsp; &nbsp; bash allows multiple option arguments when invoked (e.g. -x -v); &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; sh allows only a single option argument (`sh -x -v' attempts &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; to open a file named `-v', and, on SunOS 4.1.4, dumps core. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; On Solaris 2.4 and earlier versions, sh goes into an infinite &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; loop.) &nbsp; &nbsp; &nbsp; &nbsp; sh exits a script if any builtin fails; bash exits only if one of &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; the POSIX.2 `special' builtins fails </td> 
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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）

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：

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：

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：

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：