Shell Script Language

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The basic grammer of shell script.

ref: shell tutorial

Intro

不同的Shell实现会有不同的方言，不过这很少见。 这里只介绍最标准的Shell Script语法，, 能够被最常用的Shell(如Bash)接受

Variable

  • assign variables in bash: foo=bar

    • Note that foo = bar will not work since it is interpreted as calling the foo program with arguments = and bar. In general, in shell scripts the space character will perform argument splitting.
      • 因此不要加空格

  • access the value of the variable: $foo

    • 等价于 ${foo}， 花括号可以精确地界定变量名称的范围。

  • 可以用read命令从标准输入接受数据并赋值:read val

    example:

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    #! /usr/bin/env bash
    echo -n "Enter your name:"
    read name
    echo "hello $name"
    exit 0

String

  • Strings in bash can be defined with ' and " delimiters, but they are not equivalent.

  • Strings delimited with ' are literal strings and will not substitute variable values whereas " delimited strings will.

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    foo=bar
    echo "$foo"
    # prints bar
    echo '$foo'
    # prints $foo

Array

  • array initialization −

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    array_name=(value1 ... valuen)

  • assign

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    array_name[index]=value

  • access Array Values

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    ${array_name[index]}

Quoting Mechanism

metacharacters

Unix Shell provides various metacharacters which have special meaning while using them in any Shell Script and causes termination of a word unless quoted.

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* ? [ ] ' " \ $ ; & ( ) | ^ < > new-line space tab

quoting

The following table lists the four forms of quoting −

Sr.No. Quoting & Description
1 Single quote
All special characters between these quotes lose their special meaning.
2 Double quote
Most special characters between these quotes lose their special meaning with these exceptions −$`$'"\
3 Backslash
Any character immediately following the backslash loses its special meaning.
4 Back quote (aka backtick)
Everything you type between backticks is evaluated (executed) by the shell before the main command, and the output of that execution is used by that command,

  • The Single Quotes: 其内容不转义, 相当于在每个字符前加 backslash

  • The Double Quotes： 其内容转义

  • The Backslash: 取消其后面的一个字符的转义

  • The Backquotes：将其内容视作 command 并执行， 与后文的CMD substitution类似

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    var=`command`
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    DATE=`date`

    echo $DATE

    #等价于后文的CMD substitution
    echo $(DATE)

常见语法

条件语句

  • As with most programming languages, bash supports control flow techniques including if, case, while and for.

if

syntax:

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if [ expression ] 
then 
   Statement(s) to be executed if expression is true 
fi
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if [ expression 1 ]
then
   Statement(s) to be executed if expression 1 is true
elif [ expression 2 ]
then
   Statement(s) to be executed if expression 2 is true
elif [ expression 3 ]
then
   Statement(s) to be executed if expression 3 is true
else
   Statement(s) to be executed if no expression is true
fi

紧凑形式： ; (同一行上多个命令的分隔符)

example:

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if [ -f ~/.bashrc ]; then 
. ~/.bashrc fi
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#!/bin/sh
read -p "Is this morning? Please answer yes or no: " answer
if [ "$answer" = "yes" ]; then
 echo “Good morning”
elif [ "$answer" = "no" ]; then
 echo “Good afternoon” 
else
 echo “Sorry, $answer not recognized. Enter yes or no”
 exit 1 
fi
exit 0

case

syntax:

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case word in
   pattern1 | pattern2)
      Statement(s) to be executed if pattern1 matches
      ;;
   pattern3)
      Statement(s) to be executed if pattern2 matches
      ;;
   pattern4)
      Statement(s) to be executed if pattern3 matches
      ;;
   *)
     Default condition to be executed
     ;;
esac

example:

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#!/bin/sh

option="${1}" 
case ${option} in 
   -f) FILE="${2}" 
      echo "File name is $FILE"
      ;; 
   -d) DIR="${2}" 
      echo "Dir name is $DIR"
      ;; 
   *)  
      echo "`basename ${0}`:usage: [-f file] | [-d directory]" 
      exit 1 # Command to come out of the program with status 1
      ;; 
esac 
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#!/bin/sh

read -p "Is this morning? Please answer yes or no." answer
case "$answer" in
    yes | y | Yes | YES) 
        echo “Good morning!” 
        ;; 
    no | n | No | NO) 
        echo “Good afternoon!” 
        ;; 
    *)
        echo “Sorry, $answer not recognized.” 
        ;;
esac 
exit 0

select in && case in

select in语句自带循环

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select variable in value_list
do
  statements
done
  • variable: 表示变量
  • value_list: 取值列表
  • in: Shell关键字

select in 通常和 case in 一起使用，在用户输入不同的编号时可以做出不同的反应

example:

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#!/bin/sh
clear
select item in Continue Finish
do
    case "$item" in
    Continue) 
        ;;
    Finish) 
        break 
        ;;
    *) 
        echo "Wrong choice! Please select again!" 
        ;;
    esac 
done

该命令的while版本：

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while [ "$item" != "Finish" ]; 
do 
    read item
    case "$item" in
        "Continue") 
            ;;
        "Finish")
            ;;
        *) echo "Wrong choice! Please select again!"
            ;;
    esac 
done

循环语句

break: 从for/while/until循环退出

for

syntax：

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for var in word1 word2 ... wordN
do
   Statement(s) to be executed for every word.
done

example:

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for FILE in $HOME/.bash*
do
   echo $FILE
done
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for f in *.png             
do
	mv -n $f $f.kk
done

while

syntax:

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while condition 
do
	statements 
done

example:

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quit=n
while [ "$quit" != "y" ]; do read menu_choice
case "$menu_choice" in
a) echo "a, continue...";;
b) echo "b, continue...";;
q|Q) quit=y;;
*) echo "Sorry, choice not recognized.";;
esac done
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a=0
LIMIT=10
while [ $a -le $LIMIT ] 
do
    a=$(($a+1))
        if [ $a -gt 2 ] 
        then
            break # Skip entire rest of loop. 
        fi
    echo -n "$a"
done

命令组合

分号串联：

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command1 ; command2 ; ...

条件组合: 

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statement1 && statement2 && statement3 && ...

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statement1 || statement2 || statement3 ||

语句块

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{
	statement1 
	statement2 
	...
}

或

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{ 
	statement1; statement2 ; ... ; 
}

test expression 或 [ expression ]

  • [ (aka test) command the and [[ ... ]] test construct are used to evaluate expressions

  • [是一条命令， 与test等价，大多数shell都支持。在现代的大多数sh实现中，[与test是builtin命令

    • []将其operand直接当作argument

  • [[，是关键字，许多shell(如ash bsh)并不支持这种方式

    • [[]]将其operand进行参数引用，算术扩展和CMD substitution， 不需要手动转义等

test

test expression:

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test 1 -lt 2
echo $?
0
test 1 -gt 2
echo $?
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[ expression ]:

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echo "Starting program at $(date)" # Date will be substituted

echo "Running program $0 with $# arguments with pid $$"

for file in "$@"; do
    grep foobar "$file" > /dev/null 2> /dev/null
    # When pattern is not found, grep has exit status 1
    # We redirect STDOUT and STDERR to a null register since we do not care about them
    if [[ $? -ne 0 ]]; then
        echo "File $file does not have any foobar, adding one"
        echo "# foobar" >> "$file"
    fi
done

与文件有关的条件测试

文件测试运算符的形式及功能

option parameter function
-r 文件名 如文件存在并且是用户可读的，则测试条件为真
-w 文件名 如文件存在并且是用户可写的，则测试条件为真
-x 文件名 如文件存在并且是用户可执行的，则测试条件为真
-f 文件名 如文件存在并且是普通文件，则测试条件为真
-d 文件名 如文件存在并且是目录文件，则测试条件为真
-p 文件名 如文件存在并且是命名的FIFO文件，则测试条件为真
-b 文件名 如文件存在并且是块特殊文件，则测试条件为真
-c 文件名 如文件存在并且是字符特殊文件，则测试条件为真
-s 文件名 如文件存在并且文件长度大于0，则测试条件为真
-t 文件描述符 如文件被打开且文件描述符是与终端设备相关的，则测试条件为真，默认文件描述符是1

字符串测试

option parameter function
-z s1 如果字符串s1的长度为0，则测试条件为真
-n s1 如果字符串s1的长度大于0，则测试条件为真
s1 如果字符串s1不是空字符串，则测试条件为真
=或== s1=s2 如果s1等于s2，则测试条件为真,“=”前后应有空格
!= s1!=s2 如果s1不等于s2，则测试条件为真
< s1<s2 如果按字典顺序s1在s2之前，则测试条件为真
> s1>s2 如果按自定顺序s1在s2之后，则测试条件为真

数值测试

parameter function
n1 -eq n2 如果整数n1等于n2，则测试条件为真
n1 -ne n2 如果整数n1不等于n2，则测试条件为真
n1 -lt n2 如果如果n1小于n2,则测试条件为真
n1 -le n2 如果如果n1小于或等于n2,则测试条件为真
n1 -gt n2 如果n1大于n2,则测试条件为真
n1 -ge n2 如果n1大于或等于n2,则测试条件为真

逻辑操作

parameter function
! expr 逻辑表达式求反
expr1 –a expr2 两个逻辑表达式“And“
expr1 –o expr2 两个逻辑表达式“Or“

Function

syntax

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function_name(){
	list of commands
	[ return value ]
}

example:

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mcd () {
    mkdir -p "$1"
    cd "$1"
}

return

  • exit: 不仅会退出函数, 还会退出执行该函数的shell
  • return code:仅仅退出函数. 和command的return code同

Function Call from Prompt

  • 令shell加载函数定义：

    • 可以将函数定义在主目录下的.profile，这样每次登陆后，在命令提示符后面输入函数名字就可以立即调用:

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      func para1 para2

    • 将函数定义写在一个文件( say test.sh ), 然后执行它

  • 令shell删除函数定义：

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    unset -f function_name
    • 该命令也可用来令shell删除变量定义
    • 反之，set可以用来定义变量

    output

  • Commands will often return output using STDOUT, errors through STDERR

return code

  • Commands have Return Code to report errors in a more script-friendly manner.
    • 0 usually means everything went OK; anything different from 0 means an error occurred.
    • return code， 你可以指定返回任何值

return code as bool value

  • Return codes can be used to conditionally execute commands using && (and operator) and || (or operator) .

  • Commands can also be separated within the same line using a semicolon ;.

  • The true program will always have a 0 return code and the false command will always have a 1 return code.

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    false || echo "Oops, fail"
    # Oops, fail

    true || echo "Will not be printed"
    #

    true && echo "Things went well"
    # Things went well

    false && echo "Will not be printed"
    #

    true ; echo "This will always run"
    # This will always run

    false ; echo "This will always run"
    # This will always run