Shell programming (Part 1)

1 Overview

The shell is the language we use to communicate with the operating system through the command line.

Shell scripts can be executed directly on the command line, or a set of logic can be organized into a file for easy reuse.
The command line of the terminal can be regarded as a "shell script executed line by line" .

Linux systems generally use bash by default, so the syntax in bash is explained next. The beginning of the file needs to be written #! /bin/bash, indicating that bash is the script interpreter.

2. Hello World example

#! /bin/bash
echo "Hello World!"

Operation mode

作为可执行文件

chmod +x test.sh  # 使脚本具有可执行权限

./test.sh  # 当前路径下执行
Hello World!  # 脚本输出

/xx/test.sh  # 绝对路径下执行
Hello World!  # 脚本输出

用解释器执行

bash test.sh
Hello World!  # 脚本输出

3. Notes

单行注释

# 这是一行注释

echo 'Hello World'  #  这也是注释

多行注释

:<<EOF
第一行注释
第二行注释
第三行注释
EOF

# 其中EOF可以换成其它任意字符串。例如：

:<<abc
第一行注释
第二行注释
第三行注释
abc

:<<!
第一行注释
第二行注释
第三行注释
!

4. Variables

4.1 Basic use

define variables

Note: It is stipulated that no spaces are allowed on both sides of the equal sign, otherwise an error will be reported

name1='shkd'  # 单引号定义字符串
name2="shkd"  # 双引号定义字符串
name3=shkd    # 也可以不加引号，同样表示字符串

use variables

To use variables, you need to add $symbol, or$ the {} symbol.花括号是可选的，主要为了帮助解释器识别变量边界

name=shkd
echo $name  # 输出yxc
echo ${name}  # 输出yxc
echo ${name}acwing  # 输出shkdacwing

read-only variable

Variables can be made read-only using readonly or declare.

name=yxc
readonly name
declare -r name  # 两种写法均可

name=abc  # 会报错，因为此时name只读

delete variable

unsetVariables can be deleted.

name=yxc
unset name
echo $name  # 输出空行

4.2 Variable types

Custom variables (local variables):子进程不能访问的变量
Environment variables (global variables):子进程可以访问的变量

Change custom variables to environment variables:

acs@xxx:~$ name=yxc  # 定义变量
acs@xxx:~$ export name  # 第一种方法
acs@xxx:~$ declare -x name  # 第二种方法

Change the environment variable to a custom variable:

acs@xxx:~$ export name=yxc  # 定义环境变量
acs@xxx:~$ declare +x name  # 改为自定义变量

4.3 Strings

Strings can use single quotes, double quotes, or no quotes.

The difference between single quotes and double quotes:

The content in single quotes will be 原样输出, 不会执行、不会取变量;
content in double quotes 可以执行、可以取变量;

name=yxc  # 不用引号
echo 'hello, $name \"hh\"'  # 单引号字符串，输出 hello, $name \"hh\"
echo "hello, $name \"hh\""  # 双引号字符串，输出 hello, yxc "hh"

get string length

name="yxc"
echo ${
    
    #name}  # 输出3

extract substring

name="hello, yxc"
echo ${name:0:5}  # 提取从0开始的5个字符

5. Default variables

file parameter variable

When executing a shell script, parameters can be passed to the script.$1 is the first parameter, $2 is the second parameter, and so on. 特殊的，$0是文件名（包含路径）. For example:

Create the file test.sh:

#! /bin/bash

echo "文件名："$0
echo "第一个参数："$1
echo "第二个参数："$2
echo "第三个参数："$3
echo "第四个参数："$4

Then execute the script:

acs@9e0ebfcd82d7:~$ chmod +x test.sh 
acs@9e0ebfcd82d7:~$ ./test.sh 1 2 3 4 #执行脚本 并传入参数
文件名：./test.sh
第一个参数：1
第二个参数：2
第三个参数：3
第四个参数：4

Other parameter dependent variables

parameter	illustrate
`$#`	Represents the number of parameters passed in by the file, as in the above example, the value is 4
`$*`	A space-separated string consisting of all parameters, such as the value in the above example`"$1 $2 $3 $4"`
`$@`	Each parameter is a string enclosed in double quotes, as in the above example, the value is`"$1" "$2" "$3" "$4"`
`$$`	Process ID of the script currently running
`$?`	The exit status of the last command (note that it is not stdout, but exit code). 0 means normal exit, other value means error
`$(command)`	Return `command`the stdout of this command (can be nested)
`command`	Return `command`the stdout of this command (not nestable)

6. Array

Multiple values of different types can be stored in an array .只支持一维数组，初始化时不需要指明数组大小。数组下标从0开始

definition

Arrays are denoted by parentheses, between elements 用空格隔开. For example:

array=(1 abc "def" yxc)

It is also possible to directly define the value of an element in an array:

array[0]=1
array[1]=abc
array[2]="def"
array[3]=yxc

read a value

格式

${array[index]}

举例：

array=(1 abc "def" yxc)
echo ${array[0]}
echo ${array[1]}
echo ${array[2]}
echo ${array[3]}

read the entire array

格式

${array[@]}  # 第一种写法
${array[*]}  # 第二种写法

举例：

array=(1 abc "def" yxc)

echo ${array[@]}  # 第一种写法
echo ${array[*]}  # 第二种写法

array length

类似于字符串

${
    
    #array[@]}  # 第一种写法
${
    
    #array[*]}  # 第二种写法

例如

array=(1 abc "def" yxc)

echo ${
    
    #array[@]}  # 第一种写法
echo ${
    
    #array[*]}  # 第二种写法