Linux study note _ 10_Shell programming _ 2_Shell programming syntax (2)

Variable test statement: used to test whether the variables are equal, empty, and file type.
Format:
Test conditions
Range: integer, string, and file
(1) integer test:
Test int1-eq int2 test whether the integer is equal
Test int1-ge int2 test whether int1> = int2
Test int1-gt int2 test whether int1> int2
Test int1-le int2 test int1 <= int2
Test int1-lt int2 test whether int1 is <int2
Test int1-ne int2 test whether the integer is not equal
(2) string test:
Test str1 = str2 test whether the string is equal
Test str1! = Str2 test whether the string is not equal
Test str1 test whether the string is not empty
Test-n str1 test string is not empty
Test-z str1 test whether the string is null
(3) file test:
Test-d file specifies whether the file is a directory
Test-f file specifies whether the file is a regular file
Test-x file specifies whether the file is executable
Test-r file specifies whether the file is readable.
Test-w file specifies whether the file is writable.
Test-a file specifies whether the file exists
Whether the size of the test-s file is not 0


Flow Control statement flow control statement: used to control the flow of shell programs
Exit statement: exit the program and return a return code. If the return code is 0, the program Exits normally. If the return code is not 0, the program exits abnormally.
Example: exit 0


1. if/else
Variable test statements are generally not used independently. They are generally used as test conditions for if statements, for example:
If test-d $1
Then
...
Fi
Variable test statements can be simplified using []. For example, test-d $1 is equivalent to [-d $1].
[Instance]
#! /Bin/sh
If [$ #-ne 2]; then
Echo "Not enough parameters"
Exit 0
Fi
If [$1-eq $2]; then
Echo "$1 equals $2"
Elif [$1-lt $2]; then
Echo "$1 littler than $2"
Elif [$1-gt $2]; then
Echo "$1 greater than $2"
Fi


More complex if statements:
If condition 1
Then
Command 1
Elif condition 2
Then
Command 2
Else
Command 3
Fi


Union of multiple conditions:
-A: logical and. The result is true only when both conditions are true.
-O: logical or. If either of the two conditions is true, the result is true.


[Example]
#! /Bin/bash
Echo "please input a file name :"
Read file_name
If [-d $ file_name]
Then
Echo "$ file_name is a directory"
Elif [-f $ file_name]
Then
Echo "$ file_name is a common file"
Elif [-c $ file_name-o-B $ file_name]
Then
Echo "$ file_name is a device file"
Else
Echo "$ file_name is an unknown file"
Fi


2. for/in
Format: for variable in name table
Do
Command list
Done
[Example]
#! /Bin/sh
For DAY in Sunday Monday Tuesday Wednesday Thursday Friday Saturday
Do
Echo "The day is: $ DAY"
Done


[Awk command application: Segment extraction]
Awk-F domain separator 'COMMAND '[single quotation marks here]
[If you do not use-F to specify a delimiter, the default Delimiter is space]
1. Check users with UID 0 in the system.
Awk-F: '$3 = 0 {print $1}'/etc/passwd
-F: Specify the delimiter as follows:
$3 indicates the third digit of the separator
2. Check users with blank passwords in the system
Awk-F: 'length ($2) = 0 {print $1} '/etc/shadow

[Important instance]
#! /Bin/bash
# A test shell for and awk
Username = $1
# Extract the pid of all usernames
/Bin/ps aux |/bin/grep $ username |/bin/awk '{print $2}'>/home/xiaofang/test/tmp.txt
Killid = $ (cat/home/xiaofang/test/tmp.txt)
For killtmp in $ killid
Do
/Bin/kill-9 $ killtmp 2>/dev/null
Done