Shell programming if statement | detailed + examples

  table of Contents

1. Basic grammar

1.1 if

1.2 if else 

1.3 if elif

2. Examples

2.1 if statement

2.2 if else statement

2.3 if elif statement

Three, summary

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In Shell programming, if statements are often used when judging. However, there are some differences between if statements in Shell and C/C++/Java and other languages, which are explained below with examples.

1. Basic grammar

The if statement mainly has the following forms. 

1.1 if

(1) Form one

if condition; then
    符合 condition 的执行语句
fi

Note: At the end, the if is reversed and written fi as the end sign. 

(2) Form two

You can write then on one line with if, or write it in separate lines, as shown below:

if condition
then
    符合 condition 的执行语句
fi

1.2 if else 

A single if else statement is as follows: 

if condition
then
    符合 condition 的执行语句
else
    不符合 condition 的执行语句
fi

Here then can also be written in one line with if. 

1.3 if elif

Note: In Shell, else if is abbreviated as elif, elif must also have then, as shown below: 

if condition_1
then
    符合 condition_1 的执行语句
elif condition_2
then
    符合 condition_2 的执行语句
else 
    不符合 condition_1 和 condition_2 的执行语句
fi

Of course, there are more combinations, which are not explained here. 

2. Examples

2.1 if statement

#!/bin/bash

file="/root"

#形式一
if [ -d $file ]; then
    echo "$file is directory!"
fi

#形式二
if [ -d $file ]
then
    echo "$file is directory!"
fi

2.2 if else statement

#!/bin/bash

file="/root"
if [ -d $file ]
then
    echo "$file is directory!"
else
    echo "$file is not directory!"
fi

2.3 if elif statement

#!/bin/bash

file="/root"
if [ -f $file ]
then
    echo "$file is regular file!"
elif [ -d $file ]
then
    echo "$file is directory!"
else
    echo "$file is not regular file and directory"
fi

Three, summary

The judgment logic of if statement is common to various programming languages. In Shell, pay attention to the use of fi at the end of the if statement (if written in reverse), else if should be written as elif, and don't forget then when writing if and elif.