Linux Bash 基础总结

 1            Overview

The shell is a program that takes commands from the keyboard and gives them to the operating system to perform.

 

    4 types of shells:

    - bash (Bourne Again SHell)

    - csh (C SHell)

    - ksh (KornSHell)

    - zsh

 2            Variable

    Define a variable

        VAR="Hello World"

        VAR=123

        Note: There is no space before and after "="

 

    Use a variable

        $VAR

        ${VAR}

 

    Set variable readonly

        readonly VAR

 

    Unset a variable

        unset VAR

 

    ''      -   Keep the char original.

    ""      -   Translate the special char include "$" and "\".

    ``      -   Execute the command.

 

    sudo yum install -y $(cat rpm_requirements.txt)

 

 3            String

    A string can be defined with "", '' or nothing.

 

    Get the length of a string

        string="abcd"

        echo ${#string} # output is 4

 

    Cut out a string

        echo ${var:0:5} # start from 0, length is 5, var[0,5)

 

    Substitute

        ${VAR/a/A} # Substitute the first 'a' with 'A'

        ${VAR//a/A} # Substitute all 'a' with 'A'

        ${VAR//[ |:]/-} # Substitute all ' ' or ':' with '-'

 

 4            Array

    Define an array

        array_name=(value0 value1 value2 value3)

 

        array_name=(

        value0

        value1

        value2

        value3

        )

 

        array_name[0]=value0

        array_name[1]=value1

        array_name[n]=valuen

 

    Get an element of  an array

        ${array_name[n]}

 

    Get all elements of an array

        ${array_name[@]}

 

    Get length of an array

        ${#array_name[@]}

        ${#array_name[*]}

    Get length of element n

        ${#array_name[n]}

 

    Through an array

        for data in ${array[@]}; do 

            echo ${data}

        done

 

 5            Function

    Define firstly and then use it.

    Two ways to define a function:

 

        foo() {

        }

 

        function foo {

        }

 

        function foo() {

        }

 

    Use a function

        foo

    Use a function with parameter

        var="Hello world"

        foo ${var} # There will be two parameters, $1="Hello" $2="world"

        foo "Hello world" # There will be one parameter, $1="Hello world"

 

    Return value is returned by using "return xxx". xxx is in [0,255], saved in $?.

    Return a string is invalid.

    If no "return xxx" in function, return the result of last command.

 

    Two ways to get the return value:

        foo

        i=$?

 

        foo() {

            echo 3

        }

        i=`foo`

 

    Use keyword "local" to define a local variable in function.

    Otherwise, the varibale in function is global.

 

    Use keyword "exit" in function will exit the script.

 

 6            Parameters

    $0 - bash name

    $1 - first parameter

    $2 - second parameter

    $@ - all parameters

    $* - all paramters

    $# - number of paramters

 

    shift

    shift will shift parameter to left,

    "shift" default "shift 1"

 

 7            if

  ## FORMAT

    if [ condition ]; then

        cmd

    elif [ condition ]; then

        cmd

    else

        cmd

    fi

 

  ## EXAMPLE

    if [ -e file1 ] # file1 exists

    if [ -f file1 ] # file1 exists and is a regular file

    if [ -s file1 ] # file1 exists and size > 0

    if [ -L file1 ] # file1 exists and is a link

    if [ -r file1 ] # file1 exists and is readable

    if [ -w file1 ] # file1 exists and is writable

    if [ -x file1 ] # file1 exists and is executable

    if [ -d dir1 ] # dir1 exist

    if [ file1 -nt file2 ] # file1 new than file2

    if [ file1 -ot file2 ] # file1 old than file2

 

    if [ -z string ] # string length is 0

    if [ -n string ] # string length is not 0

    if [ "$str1"x = "$str2"x ] # str1 == str2

    if [ "$str1"x == "$str2"x ] # str1 == str2

    if [ "$str1"x != "$str2"x ] # str1 != str2

 

    if [ $num1 -eq $num2 ] # num1 == num2

    if [ $num1 -nq $num2 ] # num1 != num2

    if [ $num1 -lt $num2 ] # num1 < num2

    if [ $num1 -le $num2 ] # num1 <= num2

    if [ $num1 -gt $num2 ] # num1 > num2

    if [ $num1 -ge $num2 ] # num1 >= num2

 

    Note:

    1. use []

    2. after '[' and before ']' have a space

 

    if [ -e file1 -a -e file2 ]

    if [ -e file1 -o -e file2 ]

    if [ ! -e file1 ]

 

    if  [[ exp1 && exp2 ]]

    if  [[ exp1 || exp2 ]]

 

 8            for

    for ((i=0; i<10; i++)); do

        echo $i

    done

 

    for ((i=0; i<10; i=i+2)); do

        echo $i

    done

 

    for i in {1..5}; do

        echo $i

    done

 

    for i in {1..100..2}; do

        echo $i

    done

 

    for i in $(ls); do

        echo $i

    done

 

 9            while

    while [ condition ]; do

        cmd

    done

 

 10        case

    case $Command in

    gzip)

        tar czfP /backup/backupfile-`date +%F-%H-%M-%S`.tar.gz /etc/

        ;;

    bzip2)

        tar cjfP /backup/backupfile-`date +%F-%H-%M-%S`.tar.bz2 /etc/

        ;;

    xz)

        tar cJfP /backup/backupfile-`date +%F-%H-%M-%S`.tar.xz /etc/

        ;;

    *)

        echo "Usage:`basename $0`{gzip | bzip2 | xz }."

        ;;

    esac

 

 11        Redirect IO

    ls > log

    ls >> log

    ls 2>&1 > log

   

    wc -l < file

    wc -l

        first line

        second line

    delimiter

 

 12        Calculation

    For int calculation:

        val=`expr 2 + 2`

        val=`expr $a + $b`

        val=`expr $a - $b`

        val=`expr $a \* $b`

        val=`expr $b / $a`

        val=`expr $b % $a`

 

        Note: There should be space before and after operator.

 

 13        Utils

    Get current shell directory

        SHELL_FOLDER=$(dirname $(readlink -f "$0"))

 

    $? stores the return code.