UNIX System Management: Shell Programming Branch statements (1)

By completing this chapter, you will be able to do the following things:

Describes the effect of a return value in a conditional branch statement.

Use the test command to parse the return value of a command.

Use the IF and case structures in a shell program.

&http://www.aliyun.com/zixun/aggregation/37954.html ">nbsp;1. return value

Shell variables? To save the return value of the last executed command:

0: Command executed successfully (TRUE)

Non 0: The command was terminated due to an error (false)

Example：

$ true $ false

$ echo $? $ echo $?

0 1

$ LS $ cp

$ echo $? USAGE:CP F1 F2

0 CP [-r] F1 ... fn d1

$ echo $? $echo $?

0 1

$echo $?

0

All commands at the end of the UNIX operating system produce a return value. This return value is usually used to determine whether the command ended normally (return 0) or an error (returns a value other than 0). You can also see what is wrong by returning a value other than 0. For example, a syntax error usually returns 0 for the 1,true command, and the false command returns 1.

Most of the judgment statements in the shell program are controlled by the process by analyzing the return value. A special variable is defined in the shell? "is used to save the return value at the end of the previous command.

You can observe the return value of the previous command in the following ways:

echo $?

When you perform a conditional judgment (less than, greater than, equal to), the return value indicates whether the condition is true (returns 0) or False (returns non-0).

The conditional judgment statement is described in the next sections.

2.test command

Syntax：

Test expression or [expression]

The test command tests the expression and sets the return value.

Value return value of expression

True 0

False not 0 (usually 1)

The test command is capable of testing the following objects:

Integer

String

File

The test command is used to evaluate an expression and produce a return value. It makes a logical expression with parameters and evaluates the return value of the expression, the test command does not produce standard output, you must judge the result of the test command by the return value, and if the expression is true, the return value is 0, and if the expression is false, the return value is 1.

The test command can be used alone, and then you can see the return value, but it is used most often in the if and while structures to provide conditional flow control.

The test command can also be replaced with [expression]. This approach can improve readability, especially when dealing with numbers or strings.

Note: You must have spaces around the [and] symbols.