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Linux命令行与shell脚本编程大全

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本文字数： 3.9k 阅读时长 ≈ 4 分钟

Basic

  • indicate the shell you use in the first line of your script, for example

​ #!/bin/bash

  • to run your script without occurring the error command ××× not found, you can

    • add the container folder of your script to the PATH

      or

    • use absolute path

  • while you can still find that you don’t have the privilege to run the script, you can use chmod command

    • chmod u+x your_script

  • display message

    • use echo, but need to pay attention to the quotation in the sentence
    • echo has many parameters
      1. use -n => do not output the trailing newline
      2. …..
    • use man echo to learn more

  • use variables

    1. environment variables

      1. $VAR => show the value of VAR
      2. \$ => display character $ in a string

    2. user variables

      1. var1=10 => no blank space allow
      2. $var1 => to use var1

    3. command substitution

      1. use back quote, testing=`data`

      2. use $(), testing=$(data)

        shell will run data and pip the output to testing

        pay attention, subshell cannot use the varables created by the caller, for example date can not use variable VAR

  • Redirection

    1. output redirection

      redirection output to a file, command > outputfile(over write if exist already), command >> outputfile (append if exist already)

    2. input redirection

      command < inputfile

      wc (word count) => count the data in plain text input

    3. 内联输入重定向

      <<

  • Pip

    1. command1 | command2, the output of command1 will be piped to command2 as its input

      actually these two commands run exactly the same time, not in order

  • Math Calculation

    1. expr

      nearly useless, very complicated

    2. $[ operation ]

      bash shell only support integer calculation, while z shell provides complete floating point operation

    3. use bc in you script

      bc is built-in shell calculator, allowing floating point operation

      combine bc , | and $() , probably also with <<

      variable=$(echo "options; expression" | bc)

      for example

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      var1=$(echo "scale=4; 3.44 / 5" | bc)
      echo The answer is $var1

      OR with <<

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#!/bin/bash

var1=10.46
var2=43.67
var3=33.2
var4=71

var5=$(bc << EOF
scale = 4
a1 = ( $var1 * $var2 )
b1 = ( $var3 * $var4 )
a1 + b1
EOF
)

echo The final answer for this mess is $var5

  • exit script

    use exit num to quit your script with a status number num ranging from 0 to 255

Use Structured command

  • use if-then

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    if command
    then
    	commands
    fi

    different from other languages, if 语句之后的对象是一个等式，这个等式的求值结果为TRUE 或者FALSE，bash shell的if 语句不是这么做的

    bash shell will run the command followed the if , and if the exit status code is 0, which indicates the command runs successfully, then the commands after then will be excuted

  • use if-then-else

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    if command
    then
    	commands
    else
    	commands
    fi

  • use nested if-else or elif

  • use test

    combine test and if-else allow you implement if else like other language

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    if test condition
    then
    	commands
    fi

    test support

    1. numeric comparison
    2. string comparison
    3. file comparison

    there is also a alternative way to use condition in if-else

    [ condition ] => must keep the blank space

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    if [ condition ]
    then
    	commands
    fi

    • numeric comparison

      comparison Description
      n1 -eq n2 equal?
      -ge bigger or equal?
      -gt bigger?
      -le less or equal?
      -lt less?
      -ne not equal? 
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      if [ $value1 -gt 5 ]
      then
      	echo "The test value $value1 is greater than 5"
      fi
      #
      if [ $value1 -eq $value2 ]
      then
      	echo "The values are equal"
      else
      	echo "The values are different"
      fi

      can’t use floating point number

    • string comparison

      comparison Description
      str2 = str2 equal?
      != not equal?
      < less?
      > bigger
      -n str1 len(str1) != 0?
      -z str1 len(str1) == 0?

      -n and -z can be used to check whether a variable contains data

      when using > and < need to use escape character \> and \< or they will be regarded as redirection

    • file comparison

      comparison Description
      -d file file exist and is a dictionary?
      -e file exist?
      -f file exist and is a file?
      -r file exist and file readable?
      -s file exist and is no empty?
      -w file exist and writable
      -x file exist and excutable
      -O file own by current user?
      -G file file是否存在并且默认组与当前用户相同
      file1 -nt file2 file1 is newer than file2?
      file1 -ot file2 file1 is older than file2?

  • compound condition test

    [ condition1 ] && [ condition2 ]

    [ condition2 ] || [ condision2 ]

  • Advance features of if-else

    1. use double brackets [[]]

      pattern matching

    2. use double brace (())

      more operators

      ++

      --

      !

      ~

      **

      and more

  • case

    substitution for long if-else

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    case variable in
    pattern1 | pattern2) commands1;;
    pattern3) commands2;;
    *) default commands;;
    esac


    # for example
    case $USER in
    rich | barbara)
    	echo "Welcome, $USER"
    	echo "Please enjoy your visit";;
    testing)
    	echo "Special testing account";;
    jessica)
    	echo "Do not forget to log off when you're done";;
    *)
    	echo "Sorry, you are not allowed here";;
    esac

more structured commands