Shell Programming--if Statement If-z-n-f-eq-ne-lt__ programming

The use of conditional expressions in shell programming

If condition Then Command Else Command Fi, don't forget this ending.

If statement forgets the end fi Test.sh:line 14:syntax error:unexpected End of fi

Three conditional expressions of IF

If Command Then If Function Then	Command executed successfully, equal to return 0 (such as grep, find a match) Execution failed, returned non 0 (grep, no match found)
if [Expression_r_r_r] Then	The result of the expression is true, then return 0,if the value of 0 to then
if test Expression_r_r_r Then	If the expression result is false, it returns a 0,if to lead a non-0 value to then

[] &&--Quick IF

[f "/etc/shadow"] && echo "This computer uses Shadow Passwors"

&& can be understood as then Executes the statement on the right if the expression on the left is true

The functional differences between the IF and C language of the shell

Shell if	C language If
0 for Real, go then	On the contrary, not 0 go then
An integer variable is not supported directly if Must: If [I–ne 0] But supporting string variables directly if If [str] If the string is not 0	Support Variable Direct IF if (i)

================================= takes command as an if condition ===================================

With more than one command or function as if condition

echo–n "input:" Read user If Multiple instructions, which are equivalent to "and" grep $user/etc/passwd >/tmp/null& NBSP;    &NBSP Who-u | grep $user then               The instructions above are executed successfully, the return value $? is 0,0 true, run then  echo "$user has logged" else       command execution failed, $? 1, run Else                            &NBSP  echo "$user has not logged " fi

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

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

If Using a function as an if condition, Getyn Then function reture value 0 is true, go then echo "Your answer is yes" else function return value is not 0 false, go else 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 t Ing1 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 This is 222file Thisting1 are 111file $ sh testsh.sh Thisting1 is 111fi Le found	$ VI 111-tmp.txt This is 222file Thisting1 is 111file $ sh testsh.sh Thisti NG1 is 111file 0 found

======================================== the conditional expression as an if condition =============================

Traditional if from sentence--conditional expression as if condition
If [conditional expression]
Then
Command
Command
Command
Else
Command
Command
Fi

Conditional expression 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 an integer variable expression is executable

If [Int1-eq Int2] if int1 equals Int2
If [Int1-ne Int2] if not equal to
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 = $b] if string1 equals string2
string allows an equal sign to be used with an assignment number
If [$string 1!= $string 2]