Carding the basic knowledge of shell script & lt; III & gt;: conditional Test and comparison, process Control if

I. conditional testing and comparison

1. Test test expressions are commonly used

Test-f file & & echo true | | echo false

2. [test expression] is very commonly used

[- f file] & & echo true | | echo false

3. [[test expression]]

II. Process control

1. Flow control statement if

Example 1

#! / bin/bash

# get 0 in uid=0 (root)

Id=id | awk-F'[= (]'{print $2}')

Echo "your user id is:$id"

If [$id-eq 0]

Then

Echo "root"

Else

Echo "not root"

Fi

Example 2: determine the logged-in user

#! / bin/bash

# $# is the number of parameters added to shell

If [$#-eq 1] # or [[$# = = 1]] or ($# = = 1)

Then

If who | grep $1 > / dev/null

Then

Echo $1 is active.

Else

Echo $1 is not active.

Fi

Else

Echo "Usage: $0"

Exit 1

Fi

~

The result after execution

[root@localhost shell] # sh if-if.sh root

Root is active.

[root@localhost shell] # sh if-if.sh zabbix

Zabbix is not active.

[root@localhost shell] # sh if-if.sh

Usage: if-if.sh

[root@localhost shell] #

Instance 3 if-elif..else-fi

#! / bin/bash

# # if-elif..else-fi

Read-p "how lod are you?" Age

# conditional testing using the shell arithmetic operator (())

If ((age120)); then # [age

< 0 || age >

120]]

Echo "out of range!"

Exit 1

Fi

If ((age > = 0&&age=13&&age=20&&age=30&&age)