Basic use of shell script

shell Basic use

Shell brief introduction

Shell It's a use. c A program written in a language , It is used by users Linux The bridge . Shell It's a command language , Another programming language .
Shell Script （shell script）, It's for Shell Write the script . What the industry says Shell Usually it means Shell Script . Shell And Shell Script is two different concepts .

Linux Medium shell There are many kinds , Several commonly used :

• Bourne Shell (/usr/bin/sh or /bin/sh)
• Bourne Again Shell (/bin/bash)
• C Shell (/usrbin/csh)
• K Shell (/usr/bin/ksh)
• Shell for Root (/sbin/sh)

Bash（Bourne Again Shell） Because it's easy to use and free , Be in daily work
Widely used . meanwhile ,Bash And most of them Linux By default Shell.

first Shell Script

Create a first.sh file ,Shell Script file to .sh ending , Then start editing the file ：

#!/bin/bash
echo "hello world"

Use after writing ll Command to view the file ：

only
Allow reading and writing , Cannot perform , So add executable permissions to it ：

chmod +x first.sh

After seeing the file turn green , Use ./firsh.sh perform ：

Common grammar

Defining variables

#!/bin/bash
echo "hello world"

greet="hello program"
echo $greet

• Pay attention to defining variables ,= There must be no spaces on either side of the sign .
• Later, when using variables , Add $ Symbol .

if conditional

#!/bin/bash

num=1
if [$num -eq 0]; then
	echo "num is 0"
elif [$num -eq 1]; then
	echo "num is 1"
else
	echo "num is not 0 and 1"
fi

for loop

Suppose you want to print /opt/module All the files under the directory ：

#!/bin/bash

for file in $(ls /opt/module/); do
        echo $file
done

from 1 Add to 100：

#!/bin/bash

sum=0
for i in {
    1..100}; do
        let sum+=i;
done
echo $sum

Check the working state of the machine in the LAN ：

Suppose the network segment is 192.168.199, It is required to print out the working status of all machines under the current network segment .

#!/bin/bash

for i in {
    1..254}; do
        ping -c 2 -i 0.5 192.168.199.$i &>/dev/null
        if [$? -eq 0]; then
                echo "192.168.199.$i is up"
        else
                echo "192.168.199.$i is down"
        fi
done

• ping -c [count] Express ping The number of times .
• ping -i [time] Every time said ping Time interval of .
• /dev/null It means to filter out some useless information printed .
• $? Indicates the execution result of the previous line of instructions .

Use it later and then add .