Test conditions for Linux shell programming

Executes the command and uses the command state return value to determine certain states

echo $?

0 stands for Success, not 0 for unsuccessful

To test an expression

Numerical comparison

-eq equals

The return value is 1, indicating 2 is not equal to 3

-ne Not equal to

The return value is 0 description 2 is not equal to 3

You can also use brackets directly without the need to enter the test keyword

A return value of 1 means 2 equals 2

-gt left is greater than right

2 on the left is greater than 1 on the right

-lt left is less than right

Left 2 not less than right 1

The rest is Le (less than equals) and GE (greater than or equal)

string comparison

Note: When comparing strings, it is best to use double brackets, if there is a variable, it is best to enclose it in double quotation marks.

Whether > is greater than < is less than

Why is a less than B? Because the characters are in the comparison, the ASCII value is used

! = does not equal

The following comparison is rarely seen, called pattern matching.

Whether the left-hand string can be matched to the right-hand match

That's even a match. Two times, first time success, second failure

If we're going to make it this second time, you can write.

-Z "STRINGS" to determine whether the STRINGS string is empty

-N "STRINGS" to determine whether the STRINGS string is not empty

Because the variable name value is NULL, the return value of-n is 1.

Script return value

The default is the state return value of the last command in the script, and the administrator can customize the script return value

EXIT[N] n is the value returned, and the shell script exits the program when it encounters exit

File test

To see if a file exists-E

See if a file is a block device-B

To see if a file is a character device file

-C

See if a file is a normal file

-F

To see if a file is a catalog file

-D

To see if a file is a symbolic link file

-h/-l

to see if a file is a pipe file

-P

to see if a file is a socket file

-S

to see if a file exists and is readable for the current user

-R

to see if a file exists and is writable for the current user

-W

to see if a file exists and can be executed for the current user

-X

See if a file exists and has SGID

-G

See if the file has Suid-u

See if the file has Sticky-k

See if a file exists and has content-s

There are files, but there is no content in the file

To see if the file has been modified since it was last read

-N

View whether the current user is the owner of the specified file-o

View whether the current user belongs to the specified file group-G

Binocular test

-ef two files are two hard-linked files that point to the same file

Is the file on the left of-nt newer than the file on the right? Time is dominated by the most recent modification time

Is the file on the left of-ot older than the file on the right? Time is dominated by the most recent modification time

Aaaa.text than Fstab.1 New

Fstab.1 files older than Aaaa.text

Combination Test

Command1 && Command 2

If the first command is wrong, you don't run the second command.

If the first command is right, the second command continues to run

Command1 | | Command2

If the first command is wrong, the second command continues to run

If the first command is true, the second command is not run.

&& is equivalent to -a note that if it is a-a argument then the expression is either a single square bracket

|| Equivalent to-O

Non-command

!command1

Test conditions for Linux shell programming