Let’s talk about common commands and environment variables of Linux operating system

The above two commands have the same effect.

11. Determine whether the network is connected

ping -c Number of packets ip address or domain name

ping is used Determine whether the local host can successfully exchange data packets with another host and determine whether the network is smooth.

1) Ping five packets to the local host (127.0.0.1)

ping -c 5 127.0.0.1

2) Ping five packages to the server of Baidu www.baidu.com.cn

ping -c 5 www.baidu.com.cn

It can be seen that Baidu’s server canping.

3) Ping five packets to Google www.google.com server.

ping -c 5 www.google.com

You can see that Google’s server ispingIt doesn’t make sense.

12. Display the contents of a text file

There are three commands to display the contents of a text file:cat,moreandtail.

1) cat command

cat file name

cat command displays the contents of the entire file at one time.

cat main.cpp

2) more command

more file name

In order to facilitate reading, the more command displays the contents of the file in pages. Press the space bar to display the next page, press the b key to display the previous page, and press the q key to exit.

3) tail command

tail -f file name

tail -fis used for display The last few lines of the text file will be refreshed in real time if the content of the file increases. For programmers,tail -fis extremely important. It can dynamically display the log of the background service program and be used to debug and track the running of the program.

13. Count the number of lines, words and bytes of text files

wc(word count) function is to count the number of bytes and words in the specified file , number of lines, and output the statistical results

wc file name

14. Search the content in the file

grep "Content" file name

Note that if there are no special characters such as spaces in the content, it does not need to be enclosed in double quotes.

Example:

1) Search signal

grep signal main.cppin the main.cpp file

15. Search for files

1) find

find directory name-name file name

Parameter description:
Directory name: The directory to be searched. When searching for files, in addition to this directory name, it also includes its subdirectories at all levels.
File name: Rules matching the file name to be searched.
Example:
Start searching from the OPC directory and display all *.cpp files.

find OPC -name *.cpp

2) locate

locate file name

Example ：
Search open62541.c file

locate open62541.c

Note: When using this command for the first time, you may need to update the database. Just execute the command according to the prompts.

find is mostly used for fuzzy search in a certain directory, while locate is for accurate positioning of the whole area.

16. The chmod command

is used to change the access permissions of Linux files or directories. This command has two uses. One is a text setting method that includes letters and operator expressions; the other is a numeric setting method that includes numbers.

Each file or directory has three groups of access permissions, each group is represented by three digits, which are the read, write and execute permissions of the file owner; the read, write and execution permissions of users in the same group as the owner. Execute permissions; read, write, and execute permissions for other users in the system.

Example:ls -l example

There are 10 positions in the first column, and the first character specifies the file type. In the usual sense, a directory is also a file. If the first character is a dash, it means it is a non-directory file. If it is d, it means a directory. There are a total of 9 characters starting from the second character to the tenth. A group of 3 characters represents the permissions of 3 groups of users on the file or directory. The permission characters use a horizontal line to represent empty permission, r represents read-only, w represents write, and x represents executable.

Commonly used parameters:

• -c When changes occur, report processing information

• -R Process the specified directory and its subdirectories All files

Permission scope:

• u: The current user of the directory or file

• g: The current group of the directory or file

• o: A user or group other than the current user or group of the directory or file

• a: All users and groups

Permission code:

• r: Read permission, represented by the number 4

• w: Write permission, represented by the number 2

• x: Execution permission, represented by the number 1

• -: Delete permissions, represented by the number 0

• s: Special permissions

Example:

1) Delete files example All users have executable permissions

chmod a-x example

2) Assign read, write, and execute (7) to the owner of example Permissions, assign read and execute (5) permissions to the group where example is located, and assign execution (1) permissions to other users

chmod 751 example -c

3) Add readable permissions to all files in the aaa directory and its subdirectories

chmod u r,g r,or -R text/ -c

17. Check the system disk space

df [-h] [-T]

option-hDisplay information in an easy-to-read format:

df -h

Options-TList files System type:

df -h -T

3. Explanation of Linux environment variables

3.1 Environment variables Concept

3.1.1 What are environment variables?

The execution of programs (operating system commands and applications) requires a running environment, which is composed of multiple environment variables.

3.1.2 Classification of environment variables

1) Classify according to the effective scope.

System environment variables: public, effective for all users.

User environment variables: User-private and customized personalized settings, which only take effect for this user.

2) Classification by life cycle.

Permanent environment variables: Configured in the environment variable script file, these scripts will be automatically executed every time the user logs in, which is equivalent to permanent effect.

Temporary environment variables: temporarily defined in the Shell when used, and will become invalid after exiting the Shell.

3) Linux environment variables

Linux environment variables are also called Shell environment variables. They start with an underscore and a letter and consist of an underscore, letters (case-sensitive) and numbers. They are customarily used Capital letters, such as PATH, HOSTNAME, LANG, etc.

3.2 Commonly used environment variables

3.2.1 View environment variables

1) env command

In Shell, use the env command to view all environment variables of the current user.

The above picture only captures some of the environment variables, not all.

Note: When using the env command, many environment variables are displayed on the screen, which is inconvenient to view. You can use grep to filter them.

env|grep Environment variable name

For example, check the environment variable whose name contains PATH.

env|grep PATH

2) echo command

echo $environment variable name

For example:echo $LANG

3.2.2 Commonly used environment variables

1) PATH

Search directory for executable programs. Executable programs include Linux system commands and user applications. The specific usage of the PATH variable is described in detail in later chapters of this article.

2) LANG

Language, region, and character set of the Linux system.

echo $LANG

3) HOSTNAME

The host name of the server.

echo $HOSTNAME

4) SHELL

The Shell parser currently used by the user.

echo $SHELL

5) HISTSIZE

The number of historical commands saved.

6) USER

The username of the currently logged in user.

echo $USER

7) HOME

The home directory of the currently logged in user.

echo $HOME

8) PWD

Current working directory.

echo $PWD

9) LD_LIBRARY_PATH

C/C language dynamic link library file search directory, it is not Linux default environment variables, but they are very important for C/C programmers. The specific usage is described in detail in the later chapters of this article.

3.3 Set the environment quantity

export variable name='value'

If the value of the environment variable does not have spaces or other special Symbols can be included without single quotes.

Example:

export PATH=$PATH:.

The above command adds .directory to the current shell’s environment variable PATH (yes.)

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:.

The above command adds the .directory to the environment variable LD_LIBRARY_PATH of the current shell

Environment variables set using export will become invalid after exiting the Shell and need to be reset the next time you log in. If you want the environment variables to take effect permanently, you need to configure them in the login script file.

3.3.1 System environment variables

System environment variables are effective for all users. There are three ways to set system environment variables.

1) Set in the /etc/profile file.

When the user logs in, execute the /etc/profile file to set the system environment variables. However, Linux does not recommend setting system environment variables in the /etc/profile file.

2) Add the environment variable script file in the /etc/profile.d directory. This is the recommended method for Linux.

/etc/profile will execute all script files under /etc/profile.d every time it is started. /etc/profile.d is easier to maintain than /etc/profile. If you don’t want any variables, just delete the corresponding shell script under /etc/profile.d. There are many script files in the

/etc/profile.d directory, for example:

ls /etc/profile.d

3) Set environment variables in the /etc/bashrc file.

The environment variables configured in this file will affect the bash shell used by all users. However, Linux also does not recommend setting system environment variables in the /etc/bashrc file.

3.3.2 User environment variables

User environment variables only take effect for the current user. There are many ways to set user environment variables.

In the user's home directory, there are several special files, which are invisible withlsbut can be seen withls .bash*.

1).bash_profile (recommended)

Executed when the user logs in, each user can use this file to configure their own environment variables.

2).bashrc

This file will be read when the user logs in and every time a new Shell is opened. It is not recommended to configure user-specific environment variables in it, because every time the user opens the For each shell, the file will be read once, and the efficiency will definitely be affected.

3).bash_logout

Execute this file every time you exit the system (exit the bash shell).

4).bash_history

saves the historical commands used by the current user.

3.3.3 The execution sequence of environment variable script files

The execution sequence of environment variable script files is as follows:

/etc/profile ->/etc/profile.d->/etc/bashrc->User's .bash_profile->User's .bashrc

Environment variables with the same name, if in multiple scripts If there is a configuration, the configuration in the last executed script shall prevail.

3.4 Detailed explanation of important environment variables

1. PATH environment variable

Search directory for executable programs, executable Programs include Linux system commands and user applications. If the directory of the executable program is not in the directory specified by PATH, the directory needs to be specified when executing.

1) The PATH environment variable stores a list of directories. The directories are separated by colon:, and the final dot indicates the current directory.

export PATH=Directory 1: Directory 2: Directory 3:… Directory n:.

2) PATH defaults to the directory where the Linux system commands are located (/usr/local/bin: /usr/bin:/usr/local/sbin:/usr/sbin), if these directories are not included, common Linux commands cannot be executed (the absolute path must be entered to execute).

Example:

ls /bin/lsYou can see that thelscommand is located in the /bin directory

At this time we execute the commandexport PATH=Clear the PATH variable and then execute thelscommand. The system will prompt that thelscommand cannot be found.

You can use the absolute path,/bin/ls

After adding the /bin directory to the environment variable PATH ,lscan also be executed.

3) If the PATH variable does not contain a dot, you need to add ./ or use an absolute path to execute the program in the current directory.

Example:
When the example command is executed in the current directory, it prompts that it cannot be executed.

After adding the dot. to the environment variable PATH, you can execute it.

export PATH=$PATH:.

2. LD_LIBRARY_PATH environment variable

C/ The directory where C language dynamic link library files are searched. It is not a default environment variable in Linux, but it is very important for C/C programmers.

The LD_LIBRARY_PATH environment variable also stores a list of directories. The directories are separated by colon:, and the final dot . indicates the current directory, which has the same format as PATH.

export LD_LIBRARY_PATH=Directory 1: Directory 2: Directory 3:... Directory n:.

We will use this environment variable next time This lesson will introduce Linux static libraries and dynamic libraries in detail.

3.5 Validation of environment variables

1) Under Shell, the environment variables set with export take effect immediately on the current Shell and become invalid after the Shell exits.

2) The environment variables set in the script file will not take effect immediately. They will only take effect after exiting the Shell and logging in again, or use the source command to make it take effect immediately, for example:

source /etc/profile

3.6 Application experience

Although there are many ways to set environment variables, it is recommended that you set the system environment variables in /etc/profile Configure in the .d directory, and user environment variables are configured in the user's .bash_profile. It is not recommended to configure environment variables in other script files, as it will increase the trouble of system operation and maintenance and is prone to errors.

Related recommendations: "Linux Video Tutorial"

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