Shell Script Raiders (learning notes)--1.3 Command logical execution order

Source: Internet
Author: User

You can use the semicolon ";", the double ampersand "&&" and the Double vertical bar "| |" in Linux To connect multiple commands.

1.3.1 Semicolon;

When multiple commands want to be executed at the same time on a single line, a semicolon ";" can be used after each command. There is no logical relationship between multiple commands, and all commands written are executed, even if a command has errors and does not affect other commands.

ls das; Echo " Hdakl "

Ls:cannot access das:no such file or directory


1.3.2 &&

Logic and. Command1 && Command2, Command2 is executed only if Command1 is executed correctly, Command1 is not performed if Command2 is not executed correctly. How to judge correctly, bash internal will pass the predefined variable "$?" To judge.

Echo " Hdakl " ls


Ls:cannot access ds:no such file or directory

ls Echo " Hdakl "

Ls:cannot access das:no such file or directory

1.3.3 | |

Logical OR. Command1 | | Command2, Command2 will not be executed unless Command1 is executed correctly command2,command1 execution is performed correctly. | And && are short-circuiting symbols, which have a logical relationship between the commands around the symbol.

ls Echo " Hdakl "

Ls:cannot access das:no such file or directory


Echo " Hdakl " ls ds   


general use of && and | | , it's basically logical and logical, or :command1 && Command2 | | Command3. Because in practice, Command2 and Command3 should all be commands that you want to execute. If Command1 is executed correctly, $? equals 0, executes Command2, and then the situation executes Command3, if the Command1 error executes, $? is not equal to 0, does not execute Command2, but then according to this non-0, judged the | | The right side should be executed.

Shell Script Raiders (learning notes)--1.3 command logical execution order

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