Shell learning notes -- if condition judgment and judgment Condition

Preface:

WhateverProgramming LanguageConditional judgment is indispensable. Shell is no exception.

If list then
Do something here
Elif list then
Do another thing here
Else
Do something else here
Fi

Ex1:

#! /Bin/sh

System = 'uname-S' # obtain the operating system type.

If [$ system = "Linux"]; then # If it is Linux, print the Linux string
Echo "Linux"
Elif [$ system = "FreeBSD"]; then
Echo "FreeBSD"
Elif [$ system = "Solaris"]; then
Echo "Solaris"
Else
Echo "what? "
FI # ifend

It is basically the same as other scripting languages. There is no big difference. However, it is worth noting that. [] Condition judgment.

1 string judgment

Str1 = str2: true when two strings have the same content and length
Str1! = Str2: true when str1 and str2 are different
-N str1: true when the length of a string is greater than 0 (the string is not empty)
-Z str1: true (empty string) when the string length is 0)
Str1 is true when str1 is not empty

2. digit judgment

Int1-EQ int2 two numbers are equal to true
Int1-ne int2 two numbers are not true
Int1-GT int2 int1 greater than int2 true
Int1-ge int2 int1 greater than or equal to int2 true
Int1-lt int2 int1 smaller than int2 true
Int1-Le int2 int1 smaller than or equal to int2 true

3. File judgment

-R file user readable to true
-W file users can write to true
-X file user executable is true
-F file: the regular file is true.
-D file: the directory is true.
-C file: the special character file is true.
-B file: The Block special file is true.
-S file is true if the file size is not 0
-T file: this parameter is true when the specified device is a terminal in the file descriptor (1 by default ).

3. complex logic judgment

-A and
-O or
! Non

End

Although the syntax is simple, its functions become powerful when used in shell.

 

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Appendix:

[-A file] true if the file exists.
[-B file] If the file exists and is a special block file, it is true.
[-C file] If the file exists and is a special word file, it is true.
[-D file] If the file exists and is a directory, it is true.
[-E file] If the file exists, it is true.
[-F file] If the file exists and is a normal file, it is true.
[-G file] If the file exists and the sgid has been set, it is true. [-H file] If the file exists and is a symbolic connection, it is true.
[-K file] true if the file exists and the stick bits have been set.
[-P file] If the file exists and is a name pipe (f If O), it is true.
[-R file] true if the file exists and is readable.
[-S file] If the file exists and the size is not 0, it is true.
[-T fd] It is true if the file descriptor FD is opened and points to a terminal.
[-U file] If the file exists and SUID (Set User ID) is set, it is true.
[-W file] true if the file exists and is writable.
[-X file] true if the file exists and is executable.
[-O file] true if the file exists and belongs to a valid user ID.
[-G file] true if the file exists and belongs to a valid user group.
[-L file] If the file exists and is a symbolic connection, it is true.
[-N file] If file exists and has been mod, it is true if IED since it was last read.
[-S file] If the file exists and is a socket, it is true.
[File1-nt file2] If file1 has been changed more recently than file2, or if file1 exists and file2 does not is true.
[File1-ot file2] True if file1 is older than file2, or if file2 exists and file1 does not exist.
[File1-Ef file2] True if file1 and file2 point to the same device and node number.
[-O optionname] true if the shell option "optionname" is enabled.
[-Z string] "string" is true if its length is zero.
If the length of [-N string] or [String] "string" is non-zero, it is true.
[String1 = string2] If the two strings are the same. "=" May be used instead of "=" for strict POSIX compliance is true.
[String1! = String2] true if the strings are not equal.
[String1 <string2] True if "string1" sorts before "string2" lexicographically in the current locale.
[String1> string2] True if "string1" sorts after "string2" lexicographically in the current locale.
[Arg1 op arg2] "OP" is one of-EQ,-ne,-lt,-Le,-GT or-ge. these arithmetic binary operators return true if "arg1" is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to "arg2 ", respectively. "arg1" and "arg2" are integers.