The IF statement for the shell script

First, preface

In structured programming, there are three main structures: sequential, branching, and cyclic. This shows the importance of branch judgment.

And in shell programming often encountered in the case of judgment, such as: Determine whether the file exists, the directory exists, and so on.

II. structure of the IF statement

if conditions  Then     ... elif conditions    ... Else     ... fi

Iii. branch judgment in the shell

There are several judgments in the shell:

1. String judgments

STR1 =! = str2 When two strings of content are not true at the same time -n str1 when the length of the string is greater than 0 is true -z str1 when the string length is 0 is true str1 when the string is not empty is true 

2. Digital Judgment

N1------le n2 N1 less than equals N2 is true

3. Document Judgment

file The file can be read as true  -w file file can be written  as a true file for real files executable as normal   file files for a directory file  as true file file for characters special files  as true for block special files for true file   The file file is true for the linked file  when the device specified by the descriptor is terminal

4. Combination

-A and -o or !  Non -

The IF statement for the shell script