一個程序演示全部的shell編程知識

Shell
演示程序

#!/bin/sh -x
#由反引號括起來的也是一條命令，Shell先執行該命令，而後將輸出結果馬上代換到當前命令行中。例如定義一個變量存放date命令的輸出：
echo `date`
#命令代換也能夠用$()表示：
echo $(date)

#若是一個變量叫作VARNAME，用${VARNAME}能夠表示它的值，在不引發歧義的狀況下也能夠用$VARNAME表示它的值。經過如下例子比較這兩種表示法的不一樣：
echo $SHELL
echo $SHELLabc
echo ${SHELL}abc

#單引號用於保持引號內全部字符的字面值，即便引號內的\和回車也不例外，可是字符串中不能出現單引號
echo '$SHELL'
echo "$SHELL"
echo '$SHELL
          hello world'
#雙引號用於保持引號內全部字符的字面值（回車也不例外），除如下狀況外：
#$加變量名能夠取變量的值
#反引號仍表示命令替換
#\$表示$的字面值
#\`表示`的字面值
#\"表示"的字面值
#\\表示\的字面值
echo "$SHELL
          hello world"
echo "\$SHELL
          hello world"

#命令test或[能夠測試一個條件是否成立，若是測試結果爲真，則該命令的Exit Status爲0，若是測試結果爲假，則命令的Exit Status爲1
VAR=2
test $VAR -gt 1
echo $?

test $VAR -gt 3
echo $?

[ $VAR -gt 3 ]
echo $?
#存在Desktop目錄且VAR等於abc
VAR=abc
[ -d Desktop -a $VAR = 'abc' ]
echo $?

#在Shell中用if、then、elif、else、fi這幾條命令實現分支控制
echo "Is it morning? Please answer yes or no."
read YES_OR_NO
if [ "$YES_OR_NO" = "yes" ]; then
        echo "Good morning!"
elif [ "$YES_OR_NO" = "no" ]; then
        echo "Good afternoon!"
else
        echo "Sorry, $YES_OR_NO not recognized. Enter yes or no."
        exit 1
fi

#C語言的case只能匹配整型或字符型常量表達式，而Shell腳本的case能夠匹配字符串和Wildcard，每一個匹配分支能夠有若干條命令，末尾爲;;
echo "Is it morning? Please answer yes or no."
read YES_OR_NO
case "$YES_OR_NO" in
        yes|y|Yes|YES)
        echo "Good Morning!";;
        [nN]*)
        echo "Good Afternoon!";;
        *)
        echo "Sorry, $YES_OR_NO not recognized. Enter yes or no."
        exit 1;;
esac

#FRUIT是一個循環變量，第一次循環$FRUIT的取值是apple，第二次取值是banana，第三次取值是pear
for FRUIT in apple banana pear; do
          echo "I like $FRUIT"
done

#while的用法和C語言相似
echo "Enter password:"
read TRY
while [ "$TRY" != "secret" ]; do
        echo "Sorry, try again"
        read TRY
done

COUNTER=1
while [ "$COUNTER" -lt 10 ]; do
        echo "COUNTER is $COUNTER"
        COUNTER=$(($COUNTER+1))
done

#$0     至關於C語言main函數的argv[0]
echo "The program $0 is now running"
#$一、$2...       這些稱爲位置參數（Positional Parameter），至關於C語言main函數的argv[1]、argv[2]...
echo "The first parameter is $1"
echo "The second parameter is $2"
$@      表示參數列表"$1" "$2" ...，例如能夠用在for循環中的in後面
for PARAM in $@; do
        echo "PARAM $PARAM"
done

echo "The parameter list is $@"
#位置參數能夠用shift命令左移
shift
echo "The first parameter is $1"
echo "The second parameter is $2"
echo "The parameter list is $@"

#數就像是迷你腳本，調用函數時能夠傳任意個參數，在函數內一樣是用$0、$一、$2等變量來提取參數
#函數中的位置參數至關於函數的局部變量，改變這些變量並不會影響函數外面的$0、$一、$2等變量
#函數中能夠用return命令返回，若是return後面跟一個數字則表示函數的Exit Status

is_directory()
{
        DIR_NAME=$1
        if [ ! -d $DIR_NAME ]; then
                return 1
        else
                return 0
        fi
}

for DIR in "$@"; do
        if is_directory "$DIR"
        then :
        else
                echo "$DIR doesn't exist. Creating it now..."
                mkdir $DIR > /dev/null 2>&1
                if [ $? -ne 0 ]; then
                        echo "Cannot create directory $DIR"
                        exit 1
                fi
        fi
done

sh與bash區別
在咱們所使用的系統當中，使用sh調用執行腳本，至關於打開了bash的POSIX標準模式 （等效於bash的 --posix 參數）

通常的，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：

[wwy@sf-watch 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

[wwy@sf-watch test]$ bash -x ./t2.sh # 使用bash 調用，不會出問題
+ diff /dev/fd/63 /dev/fd/62
++ echo xxx
++ echo yyy
1c1
< xxx
---
> yyy
[wwy@sf-watch test]$ sh ./t2.sh # 而用sh調用，報錯以下
./t2.sh: line 3: syntax error near unexpected token `('
./t2.sh: line 3: `diff <(echo xxx) <(echo yyy)'
[wwy@sf-watch test]$ echo $?

2

可是，在咱們的linux系統中，sh是bash的一個軟連接：

[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

那爲何上面的例子中還會出現問題呢？緣由在於： bash程序執行，當「$0」是「sh」的時候，則要求下面的代碼遵循必定的規範，當不符合規範的語法存在時，則會報錯， 因此能夠這樣理解， 「sh」並非一個程序，而是一種標準（POSIX），這種標準，在必定程度上保證了腳本的跨系統性（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別：

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

調用相關：

在腳本的調用方面（interactive、login相關），bash與sh也是存在差別 如下是詳細說明（假如被調用執行的腳本名字叫xxx.sh）

BASH： 一、 交互式的登陸shell （bash –il xxx.sh） 載入的信息：

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

二、非交互式的登陸shell （bash –l xxx.sh） 載入的信息：

/etc/profile
~/.bash_profile （ -> ~/.bashrc -> /etc/bashrc）
~/.bash_login
~/.profile
$BASH_ENV

三、交互式的非登陸shell （bash –i xxx.sh） 載入的信息：

~/.bashrc （ -> /etc/bashrc）

四、非交互式的非登陸shell （bash xxx.sh） 載入的信息：

$BASH_ENV

SH：

一、交互式的登陸shell 載入的信息：

/etc/profile
~/.profile

二、非交互式的登陸shell 載入的信息：

/etc/profile
~/.profile

三、交互式的非登陸shell 載入的信息：

$ENV

四、非交互式的非登陸shell 載入的信息： nothing

由此能夠看出，最主要的區別在於相關配置文件的是否載入， 而這些配置的是否載入，也就致使了不少默認選項的差別 （具體請仔細查看~/.bash_profile 等文件） 如：

[wangweiyu@ComSeOp ~]$ grep ulimit /etc/profile 
ulimit -S -c unlimited > /dev/null 2>&1

即，若是/etc/profile沒有被載入，則不會產生core dump

值得一提的是，使用ssh遠程執行命令， 遠端sshd進程經過「bash –c」的方式來執行命令（即「非交互式的非登陸shell」） 因此這一點，和登陸以後再在本地執行執行命令，就存在了必定的差別

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」表明「交互式shell」 「$0」的顯示結果爲「-bash」，bash前面多個「-」，表明「登陸shell」 沒有「i「和「-」的，是「非交互式的非登陸shell」

另外還有一點，雖然ssh遠程執行的命令是「非交互式的非登陸shell」，但在執行命令以前，ssh的那一次登陸自己是「交互式的登陸shell」，因此其會先讀一下「~/.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 wangweiyu@127.0.0.1 'echo $-'
wangweiyu@127.0.0.1 's password: 
xxx
hBc

這一點，衍生出一個關於scp的問題，scp在傳輸數據以前，會先進行一次ssh登陸， 而當.bashrc文件有輸出的時候，則會致使scp失敗！緣由是解析返回的數據包出現混亂

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 wangweiyu@127.0.0.1 :/tmp
wangweiyu@127.0.0.1 's password: 
xxx
[wangweiyu@ComSeOp ~]$ echo $?
1
[wangweiyu@ComSeOp ~]$ ls /tmp/
[wangweiyu@ComSeOp ~]$