Getting started with Shell programming (version 2) (medium) and getting started with shell programming

Getting started with Shell programming (version 2) (medium) and getting started with shell programming
Variable test statement-test

Purpose: test whether the variables are equal, empty, and file type.

Format:

Test condition or [] # range: integer, String, 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

 

Note: The test statement is generally not used separately. It is generally used as the test condition of the if statement, for example;

if test -d filethen....fi

 

Abbreviated form of test variables "[]"

 

Example-apachtest. sh

#!/bin/bash# A test shell script for test Apache is running or notweb=$(/usr/bin/pgrep httpd)echo "Now let's test the Apache..."echo#if [ "$web" != "" ]if [ -n "$web" ]then    echo "Apache is running..."else    echo "Apache is NOT running..."    /etc/rc.d/init.d/httpd startfi

Process 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

 

I. if

If/then format

if test -d $1 then ... fi 

Example-if_then.sh

#!/bin/bash# A test shell script for if/thenif [ -x /etc/rc.d/init.d/httpd ]then    echo "Script: /etc/rc.d/init.d/httdp have x power!"    /etc/rc.d/init.d/httpd restartfi

If/else format

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-if_else.sh

#!/bin/bash# A test shell script for if/elif/elseecho -n "Please input a filename: "read filenameif [ -d $filename ]then    echo "$filename is a directory"elif [ -f $filename ]then    echo "$filename is a commen file"elif [ -c $filename -o -b $filename ]then    echo "$filename is a device file"else    echo "$filename is a unkown file"fi

 

Example-if_elif_exit.sh

#!/bin/bash# A test shell script for if/elifif [ $# -ne 2 ] thenecho "Not enough parameters"exit 1fiif [ $1 -gt $2 ]then    echo "$1 is great then $2"elif [ $1 -lt $2 ]then    echo "$1 is little then $2"else    echo "$1 is equal as $2"fi

2. for/in

For variable in name table do command list done

Example-for. sh

#!/bin/bash# A test shell script for "for"for DAY in Sunday Monday Tuesday Wednesday Thursday Friday Saturdaydo    echo "The day is $DAY"done

Awk Command [segment extraction]

Awk-F domain separator 'COMMAND '[single quotation marks] # 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

# Awk-F: '{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

# Ps aux | grep-v root | awk '{print $2 }'

Example-awk. sh

#!/bin/bash# A test script for desplay users infomation/bin/echo -n "Please input a username: "read username/bin/grep $username /etc/passwd > /dev/null 2> /dev/nullif [ $? -eq 0 ]then    /bin/echo "username is: $username"else    /bin/echo "user: $username is not exits."    exit 1fi/bin/echo# list /etc/passwd infouserinfo=`/bin/grep ^$username:x /etc/passwd`uid=`echo $userinfo | awk -F: '{print $3}'`gid=`echo $userinfo | awk -F: '{print $4'}`dir=`echo $userinfo | awk -F: '{print $6}'`shell=`echo $userinfo | awk -F: '{print $7}'`# get /etc/group infogroupinfo=`/bin/grep x:$gid /etc/group`gname=`/bin/echo $groupinfo | awk -F: '{print $1}'`/bin/echo "user id is: $uid"/bin/echo "default group is: $gname"/bin/echo "home directory is: $dir"/bin/echo "shell is: $shell"/bin/echo "group member info:"# get group membersgroups=`/usr/bin/groups $username`/bin/echo $groups/bin/echo# get online infoonline=`/usr/bin/who | grep $username`if [ -z "$online" ]then    echo "$username is not online"else    echo "$username is online..."fi

 

Instance-killuser. sh

# Idea: shutting down all processes of a user, including shell, is equivalent to kicking the user out of the system #! /Bin/bash # A shell srjit to kill a user in Linuxusername = $1 killpid = '/bin/ps aux | grep $ username | awk' {print $2}'' PID in $ killpiddo/bin/kill-9 $ PID 2>/dev/nulldone

Which of the following statements about $2 in shell programming is true?

This is generally the case:

<ShellScript> <option...>
Example:
Shell. sh abc bcd

Then $1 is abc $2, and bcd is the second option (parameter) of the command line output)

Shell programming $ @ $ *

Both are the parameters followed by running shellscript, and are connected into a string for calling. That is, $ @ =$ * = $1 + $2 + $3 ......

For example, the following is a simple shellscript named 01.sh.
======================================
#! /Bin/bash
# Tim: For "zhidao.baidu.com" to understand the parameter "$ @" "$ *"
# Name: 01.sh
Echo $ *
Echo $ @
======================================
After the permission is changed, run:./01.sh parameter 1 parameter 2 parameter 3
The running result is:
Parameter 1 parameter 2 parameter 3
Parameter 1 parameter 2 parameter 3
---------------------------------
====================
Question: Another special parameter is $ #. This is used to count the number of all parameters when shellscript is executed.
For example, add echo $ # in the above script as follows:
======================================
#! /Bin/bash
# Tim: For "zhidao.baidu.com" to understand the parameter "$ @" "$ *"
# Name: 01.sh
Echo $ *
Echo $ @
======================================
Continue to run as you have just run. The results are as follows:
Parameter 1 parameter 2 parameter 3
Parameter 1 parameter 2 parameter 3
3
-----------------
The "3" in the last line is the result of echo $. That is, how many parameters are counted.