Linux self-study _shell script if detailed

1) What is the function of the IF condition control statement?

The shell script is also a weak scripting language and supports if conditional control statements.

2) If the structure syntax

Single Branch

If

Then

Fi

Dual Branch

If

Then

Else

Fi

Multi-Branch

If

Then

Elif

Then

Else

Fi

3) Scope of application

Determine if business services are functioning properly

Example

Determine if the LFTP is installed if not installed

VI install_lftp.sh

#!/bin/bash

Rpm-qa |grep lftp 1>/dev/null

If [$?-ne 0];then # #若上调命令执性不成功说明没有安装lftp, install

Yum-y Install lftp &>/dev/null &&echo "lftp installed."

Fi

: Wq

Expand "" One of the usages

When the variable is a number, it is to be judged by the numbers, and the state of the software is analyzed by the result.

Integer comparison

-eq equals, such as: if ["$a"-eq "$b"]

-ne not equal to, such as: if ["$a"-ne "$b"]

-GT greater than, such as: if ["$a"-gt "$b"]

-ge greater than equals, such as: if ["$a"-ge "$b"]

-lt less than, such as: if ["$a"-lt "$b"]

-le less than equals, such as: if ["$a"-le "$b"]

Linux self-study _shell script if detailed