UNIX System Management: Shell Programming Branch statements (2)

8.if statement

Syntax: (for one-way judgment branches)

If

List A

Then

List B

Fi

Example：

If

Test-s Funfile

Then

Echo Funfile exists

Fi

echo Hello

An If structure is a process control method based on the return value of a command. If the test command returns a value of 0, a specified list of commands is executed, and if the command returned with a value other than 0, the specified list of commands is ignored and not executed.

The example above illustrates a common format for the IF structure: Each command list consists of a shell command of one or more UNIX systems, each of which is separated by a carriage return or semicolon, and the last command executed in List a determines the result of the IF statement.

The procedure for if structure execution is as follows:

1.list a command is executed.

2. If the last command in List a returns a value of 0 (True), execute the command in List B, and then continue with the command after FI.

3. If the return value of the last command in List A is not 0 (false), skip to fi and continue with the command after FI.

The test command is often used as a process control, and it can use any UNIX command because all UNIX commands produce a return value, as the following example illustrates:

If

grep kingkong/etc/passwd >/dev/http://www.aliyun.com/zixun/aggregation/19527.html ">null

Then

Echo found KingKong

Fi

The If structure can also provide process control when a program fails. As shown in the following example:

If

[$# NE 3]

Then

echo Incorrect syntax

echo usage:cmd arg1 arg2 Arg3

Exit 99

Fi