Linux Shell condition test

ArticleDirectory

• File status test

1. File condition test
-R readable
-W writable
-X executable
-D directory
-F: General Files
-Block B Devices
-C character device
-U file SUID
-L Symbolic Link
-S file is greater than 0, not empty
-E: whether the file exists

2. String condition test
= The two strings are the same
! = Two strings are different
-Z is an empty string
-N non-empty string

3. Numerical condition test
-Equal EQ
-Ne is not equal
-GT greater
-Ge is greater than or equal
-Lt is less
-Le is less than or equal

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File status test

-B filename	Returns true if filename exists and is a block file (0 is returned)
-C filename	Returns true if filename exists and is a character file
-D pathname	Returns true if pathname exists and is a directory.
-E pathname	Returns true if a file or directory specified by pathname exists.
-F filename	Returns true if filename exists and is a regular file.
-G pathname	If the file or directory specified by pathname exists and the sgid bit is set, the system returns the true
-H filename	Returns true (or-l filename) When filename exists and is a symbolic link file)
-K pathname	If the file or directory specified by pathname exists and the "Sticky" bit is set, the system returns the true
-P filename	Returns true if filename exists and is a named pipe.
-R pathname	Returns true if the file or directory specified by pathname exists and is readable.
-S filename	Returns true if filename exists and the file size is greater than 0
-S filename	Returns true if filename exists and is a socket
-T FD	Returns true if FD is a file descriptor associated with the terminal device.
-U pathname	If a file or directory specified by pathname exists and SUID bit is set, the system returns true
-W pathname	Returns true if a file or directory specified by pathname exists and can be written.
-X pathname	Returns true if the file or directory specified by pathname exists and is executable.
-O pathname	Returns true (uppercase letter O) when the pathname exists and is owned by a user with a valid user ID of the current process)
-G pathname	If the user group of a user whose pathname exists and belongs to the valid user ID of the current process
File1-nt file2	When file1 is better than file2, true is returned.
File1-ot file2	If file1 is earlier than file2, true is returned.
F1-Ef F2	Files F1 and F2 are hard links to the same file

Example: If [-B/dev/hda]; then Echo "yes"; else echo "no"; Fi // print Yes

Test-C/dev/hda; echo $? // Print 1 to indicate that the return value of the test command is 1, And/dev/hda is not a character device.

[-W/etc/passwd]; echo $? // Check whether the passwd file is writable for the current user

Logical operators during testing

-	Logic and, the operator is true on both sides, the result is true, otherwise it is false.
-O	Logic or, if either side of the operator is true, the result is true. Otherwise, the result is false.
!	Logical No. The condition is false and the result is true.

Example: [-W result.txt-a-W score.txt]; echo $? // Test whether two files can be written.

Common string tests

-Z string	Returns true if string is an empty string (0 in length ).
-N string	Returns true if the string is a non-empty string.
Str1 = str2	Returns true if the string str1 is equal to the string str2
Str1 = str2	Same as =
Str1! = Str2	Returns true if str1 and str2 are not equal.
Str1 <str2	Sort by dictionary. The string str1 is prior to the string str2.
Str1> str2	Sort by dictionary, and the string str1 is after the string str2

Example: name = "zqf"; [$ name = "zqf"]; echo $? // Print 0 to indicate that the variable name value is equal to the string "zqf"

Common numerical tests

Int1-EQ int2	If int1 is equal to int2, true is returned.
Int1-ne int2	If int1 is not equal to int2, true is returned.
Int1-lt int2	If int1 is smaller than int2, true is returned.
Int1-Le int2	If int1 is less than or equal to int2, true is returned.
Int1-GT int2	If int1 is greater than int2, true is returned.
Int1-ge int2	If int1 is greater than or equal to int2, true is returned.

Test in:

<	Less than (used in double brackets)	("$ A" <"$ B "))
<=	Less than or equal to (used in double brackets)	("$ A" <= "$ B "))
>	Greater than (used in double brackets)	("$ A"> "$ B "))
> =	Greater than or equal to (used in double brackets)	("$ A"> = "$ B "))

Example: x = 1; [$ X-EQ 1]; echo $? // Print 0 to indicate that the value of variable X is equal to the number 1X = A; [$ X-EQ "1"] // The shell prints the error message [: A: integer expression expected

Test, [], [[]

Because shell and we usuallyProgramming LanguageDifferent, it is more often to interact with it and always call others. So some belongProgramThe concept of language itself is hard to understand in shell. "Basic Skills" are not good, and it is easier to be "sleepy". I am just one.

Take Bash as an example (other compatible with Shell ):

1. Test and [are Bash Internal commands. The coreutils software package of the GNU/Linux system usually includes the/usr/bin/test and/usr/bin/[commands. If we do not need to specify the absolute path, we usually use Bash commands.
2. [[It is a keyword of bash programming language!

 
$ LS-L/usr/bin/[/usr/bin/test
-Rwxr-XR-x 1 Root 37400 September 18 15:25/usr/bin /[
-Rwxr-XR-x 1 Root 33920 September 18 15:25/usr/bin/test
$ Type [[[Test
[Is a shell builtin
[Is a shell keyword
Test is a shell builtin
 

In most cases, these three functions are common. However, commands and keywords are always different. What is the difference between commands and keywords?

If it is a command, it is interpreted by shell as a combination of parameters. For example, ">" <"is interpreted as a redirection symbol by shell. The keyword is not like this.

Use & and | in [

[Use-A and-O to represent the logic and logic or.

[Wildcard characters can be used in [

 
Arch = istrap
[[$ Arch = I * 86] & Echo "arch is x86! "
 

[[Match strings or wildcards without quotation marks

 [[$ arch_com = i386 | $ arch = I * 86] & 
 CAT> $ tfs_repo 
  
    [tfs-i386] 
   
 name = gtes11.3 prelim1 
   
 baseurl =$ {baseurl} i386/
   
 enabled = 1 
   
 EOF