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7-Shell_ conditional Operation Test and if statement

To make Shell scripts "intelligent", the first problem is how to distinguish between different situations to determine what to do. For example, send a warning when the disk utilization exceeds 95%, automatically create a backup directory when it does not exist, and do not continue the installation when the configuration of the source code compiler fails.

The Shell environment returns the status value ($?) after the command is executed. To determine whether the execution is successful, if the return value is 0, it is successful, and if the value is other, it means failure.

Using a special testing tool-test command, you can test specific conditions and judge whether the condition is true according to the return value (return value 0 is true)

Use two forms of the test command

A:test conditional expression

B: [conditional expression]

The effects of the above two methods are exactly the same, and the latter is commonly used. But the latter needs to note that there is at least one space between the square brackets "[", "]" and the conditional expression.

1. File testing

File testing refers to judging whether the file corresponds to the file or the target according to the given path, or whether the file has read, writeable, executable and other permissions. Common operations are as follows:

After performing the conditional test operation, through the predefined variable "$?" The return status of the test command can be obtained to determine whether the condition is true.

For example, the following operation determines whether the / media/cdrom/ directory exists, and the return value is 1.

If the test condition does not exist, the test return value will not be 0 (usually 1), as shown in the figure:

two。 Integer value comparison

Mainly according to the given two values, judge the relationship between the first and the second number, such as whether it is greater than, less than, or equal to the second number. Common options are as follows:

Integer values are more often used in Shell scripts. For example, determine the number of logged-in users, the number of open processes, disk utilization, and whether the software version meets the requirements. In actual use, a value is often obtained through variable references, command substitution, and so on.

For example, to determine the number of currently logged-in users and output "Too may" when more than five, you can execute the following statement. Among them, the number of logged-in users can be queried by "who | wc-l" and replaced by command.

0

Another example: to determine the size of the current available memory (buffers/cache) and output a specific value when it is lower than 2048MB, you can do the following. The "free-m" command outputs memory information in MB units.

3. String comparison

It is usually used to check whether the user input and system environment meet the conditions. In the Shell script that provides interactive operation, it can also be used to determine whether the location entered by the user meets the requirements. Common options are as follows:

For example: judge the locale of the current system, and output the prompt message "Not en.US" when it is found that it is not "en.US":

Another example: in Shell scripts, users are often required to type "yes" or "no" to confirm a task. The following operation shows the process of confirming the interaction, and of course, further operations are performed according to the value of the variable "ACK" in actual use.

4. Logic test

Is to judge the dependence between two or more conditions, when the system task depends on many different conditions, according to whether these conditions are established at the same time or as long as one of them is established, there needs to be a testing process. Common options are as follows:

The & &: logic and means "and". The return value of the entire test command is 0 only if both conditions are true. When using the test command, "& &" can be changed to "- a"

| |: logical OR, which means "or". Only if one of the two conditions is true, the return value of the whole test command is 0. When using the test command, "| |" can be changed to "- o"

! Logical no, which means "no". The return value of the entire test command is 0 only if the specified condition is not true.

"&", "| |" are also usually used to interval different command operations, and their effects are similar.

For example, to determine whether the current kernel version of the Linux system is greater than 2.4, you can do the following.

Use if conditional statements

In Shell scripts, if statements are the most commonly used method of process control, which is used to perform different actions according to specific conditional test results (if... So. )

1. Separate if statement

The "branch" of the if statement refers to the execution statement (one or more) corresponding to different test results. For the single-branch selection structure, the corresponding "command sequence" is executed only when the conditional test is established, otherwise no action is performed, and the single-branch structure is as follows:

In the above statement structure, conditional test operations are either "[conditional expression]" statements or other executable command sentences; a command sequence refers to one or more executable command lines, as well as nested if statements or other process control statements.

The execution flow of a single branch: first judge the result of the conditional test statement, and the return value is 0, indicating that the condition is valid, then execute the command sequence after then, and continue to execute other script code until the end of fi judgment; if the return value is other, ignore the command sequence after then and directly jump to fi to execute other script code.

Case study:

two。 Double-branched if statement

For the double-branch structure, it is required to execute different command sequences according to the two cases of "condition is established" and "condition is not true".

The execution flow of the double-branch if statement: first judge the result of the conditional test operation, if it is established, execute the command sequence 1 after then, ignore the command sequence 2 after else, until you meet the end judgment of fi; if not, ignore then and subsequent command sequence 1, jump directly to the command sequence 2 after else, until you meet the end judgment of fi.

Case study:

3. Multi-branch if statement

Because the if statement can perform operations separately according to the establishment of the test results, it can be used in nesting and make multiple judgments. For example: first, judge whether a student has a few scores, and if several, then judge whether it is higher than 90, and so on. The format is as follows:

Case study:

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