在学习shell脚本的过程中发现一个问题，使用./与sh 执行脚本的结果不同，如下：

#!/bin/bash                                                                     
read -p "please input: " tmp    
echo ${tmp}
[ "${tmp}" == "Y" ] || [ "${tmp}" == "y" ] && echo -e "OK,continue" && exit 0
[ "${tmp}" == "N" ] || [ "${tmp}" == "n" ] && echo -e "Oh,interrrupt" && exit 0
echo -e "I dont't know what you choice is" && exit 0

很是费解，明明在脚本中都声明了调用/bin/bash了，怎么结果还不同呢？于是查找资料，总结如下。

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shell简介

第一个流行的shell是由Steven Bourne 发展出来的，为了纪念它就称为Bourne shell，或直接简称为sh

而Linux使用的版本为Bourne Again SHell 简称为bash，这个shell是Bourne shell 的增强版本。

为什么bash与sh的不同

在Ubuntu中我们可以看到

我们打开man bash  可以看到

如果使用sh名称调用bash，它将尝试模拟启动尽可能接近历史版本sh的行为，而且符合POSIX标准。

dash是什么呢

从Ubuntu 6.10开始，默认使用dash(theDebian Almquist Shell)而不是bash(the GNUBourne-Again Shell). 但Login Shell还是bash. 原因是dash更快、更高效，而且它符合POSIX规范。Ubuntu在启动的时候会运行很多shell脚本，使用dash可以加快启动速度。

所以说，sh就是一个低版本的 bash，它有一些功能不完善，我们需要注意尽量使用bash。

bash与sh的不同之处

1.bash 中的echo -e 可以启用反斜杠的转义

   sh中没有。

2.以后用到再翻译。。。

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bash与sh不同的原文

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