Self-study Linux shell12.1-structured command if-then

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12.1-Structured Command If-then

Many programs require some logical process control over the commands in shell scripts. A class of commands will cause the script to skip certain commands depending on the condition. Such commands are often called structured commands (structured command).

1. Basic Structured If-then Statements

Structured commands allow you to change the order in which programs are executed. There are a lot of structured commands in the Bash shell, and the basic Structured command is the If-then statement, and the If-then statement can have two forms

The first type:

1 if     command2then3      commands4 fi

The second type:

1 if   command;then2   commands3 fi

• The IF statement of the bash Shell runs the command after the IF.
• If the exit status code for the command is 0 (the command runs successfully), the command in the then section is executed.
  If the exit status code for the command is a different value, then the command in the then section will not be executed, and the bash shell will continue to execute the next command in the script.
• Then the section can have more than one command, and these commands are treated as a block
  If the exit status code for this command is 0 (the command runs successfully), all commands in the then section are executed.
  If the exit status code for the command is a different value, then all the commands in the then section will not be executed, and the bash shell will continue to execute the next command in the script.
• The FI statement is used to represent the end of the If-then statement.

Example 1

Example 2

Example 3

Example 4

The If statement line uses the grep command to find out whether a user name is currently being used on the system in the/etc/passwd file. If a user uses that login name, the script displays some textual information and lists the bash files for that user's home directory.

However, if you set the TestUser variable to a user who does not exist on the system, nothing is displayed.

2. If-then-else statements

1 if   command2then3    command4else5      Command6 fi

• Commands in the then section are executed when the command in the IF statement returns 0 exit status code
• Commands in the Else section are executed when the command in the IF statement returns a non-0 exit status code
• Else section can contain more than one command

Example 1

Example 2

3. Nesting if

• The bash Shell executes the IF statement sequentially, and only the then part of the first statement that returns exit status code 0 is executed
• In the Elif statement, the Else statement immediately following it belongs to the ELIF code block.

1 ifCommand2  Then3 Command4 elifCommand5  Then6 Command7 elifCommand8  Then9 CommandTen Else One Command AFi

Self-study Linux shell12.1-structured command if-then