Shell conditional expression

F statement format

If condition Then Command Else Command Fi, don't forget the end.

If statements forget the final fi Test. sh: line 14: syntax error: unexpected end of fi

Three conditional expressions of if

If Command Then If Function Then	If the command is successfully executed, 0 is returned (such as grep, matching is found) Execution failed. Non-0 (grep, no matching found) is returned)
If [expression_r_r] Then	If the expression is true, 0 is returned. If, the value 0 is directed to then.
If test expression_r_r_r Then	If the expression result is false, non-0 is returned. If, the non-0 value is directed to then.

[] & -- Quick if

[-F "/etc/shadow"] & Echo "this computer uses shadow passwors"

& Can be understood as then If the expression on the left is true, execute the Statement on the right.

 

Functional Differences Between shell if and C if

Shell if	C language if
0 is true, go to then	On the contrary, if the value is not 0, then is used.
Direct if Required: If [I-Ne 0] However, if If [STR] If the string is not 0	Support direct if If (I)

 
====================================== Use command as the if condition ==== ======================================

Use multiple commands or functions as the if condition

Echo-n "input :" Read user If Multiple commands, which are equivalent to "and" (and) Grep $ user/etc/passwd>/tmp/null Who-u | grep $ user Commands on thenBothExecution successful, return value $? 0,0 is true, Run then Echo "$ user has logged" Else command execution failed, $? 1, run else Echo "$ user has not logged" Fi

# Sh test. Sh Input: MACG MACG pts/0 May 15 :55. 2075 (192.168.1.100) MACG has logged # Sh test. Sh Input: ddd Ddd has not logged

Use a function as the if condition (the function is equivalent to command, and the function has the advantage that its return value can be customized)

If Use a function as the if condition, Getyn The value of Reture in the then function is 0. Echo "your answer is yes" The Return Value of the else function is false if it is not 0. Echo "Your Anser is no" Fi

If command is equivalent to command + If $?

$ VI testsh. Sh #! /Bin/sh If Cat 111-tmp.txt | grep ting1 Then Echo found Else Echo "No found" Fi	$ VI testsh. Sh #! /Bin/sh Cat 111-tmp.txt | grep ting1 If [$? -EQ 0] Then Echo $? Echo found Else Echo $? Echo "No found" Fi
$ Sh testsh. Sh No found	$ Sh testsh. Sh 1 No found
$ VI 111-tmp.txt That is 222 File Thisting1 is 111 File $ Sh testsh. Sh Thisting1 is 111 File Found	$ VI 111-tmp.txt That is 222 File Thisting1 is 111 File $ Sh testsh. Sh Thisting1 is 111 File 0 Found

========================================================== Use a conditional expression as the if condition ====================================

Traditional if statements -- use conditional expressions as if conditions
If [conditional expression]
Then
Command
Command
Command
Else
Command
Command
Fi

Conditional expressions

• File expression

If [-F file] If the file exists
If [-d...] If the directory exists
If [-s file] If the file exists and is not empty
If [-r file] If the file exists and is readable
If [-W file] If the file exists and can be written
If [-X file] If the file exists and can be executed

• Integer variable expression

If [int1-EQ int2] If int1 is equal to int2
If [int1-ne int2] is not equal
If [int1-ge int2] If> =
If [int1-GT int2] If>
If [int1-Le int2] If <=
If [int1-lt int2] If <

• String variable expression

If [$ A = $ B] If string1 is equal to string2
A string can be equal by a value.
If [$ string1! = $ String2] If string1 is not equal to string2
If [-N $ string] If the string is not empty (not 0), 0 (true) is returned)
If [-Z $ string] is null
If [$ Sting] If the string is not null, 0 is returned (similar to-N)

Variable referenced by conditional expressions must contain $

If [A = B]; then Echo equal Else Echo no equal Fi

[MACG @ machome ~] $ Sh test. Sh Input: 5 Input B: 5 No equal (equal to the expression that does not compare $ A and $ B, but compares with a and B, natural! = B)

Correction:

If [$ A = $ B]; then Echo equal Else Echo no equal Fi

[MACG @ machome ~] $ Sh test. Sh Input: 5 Input B: 5 Equal

-Eq-ne-lt-NT can only be used as an integer. It is not applicable to strings. A string equals to a value =

[MACG @ machome ~] $ VI test. Sh Echo-n "input your choice :" Read VaR If [$ var-EQ "yes"] Then Echo $ VaR Fi [MACG @ machome ~] $ Sh-X test. Sh Input your choice: Y Test. sh: Line 3: Test: Y: integer expression_r_r_r expected Expected integer form, I .e.-EQ does not support strings

= The value is assigned elsewhere. If [] is the string equals. Shell does not contain =, which is equal to the C language.

A string without spaces. You can add "" or no space.

[MACG @ machome ~] $ VI test. Sh Echo "input :" Read Echo "input is $" If [$ A = 123]; then Echo processing 123 Fi

[MACG @ machome ~] $ Sh test. Sh Input: 123 Output is 123 2017123