This article will introduce some practical and common Linux or Unix command , which is the Linux system administrators normally use the command . This article is not a complete list, but a brief list of commands that can be useful when needed. The following will introduce how to use these commands one by one with examples.
1. uptime command
In Linux, the uptime command shows how long your system has been running and how many users are currently logged in. It also shows the load averages at intervals of 1 minute, 5 minutes, and 15 minutes.
$ uptime 08:16:26 up 22 min, 1 user, load average: 0.00, 0.03, 0.22
Check uptime version
Except for uptime (uptime) and version (version), the uptime command has no other options. If the time is less than 1 day, it only gives information in the form of hours:mins.
$ uptime -V procps version 3.2.8
2. w command
This command will display the currently logged in users and their processes, as well as the average load. In addition, it also displays the login name, tty name, remote host, login time, idle time, JCPU, PCPU, commands and processes.
# w 08:27:44 up 34 min, 1 user, load average: 0.00, 0.00, 0.08 USER TTY FROM [email protected] IDLE JCPU PCPU WHAT tecmint pts/0 192.168.50.1 07:59 0.00s 0.29s 0.09s w Available options -h: Do not display the title. -s: Do not display JCPU and PCPU. -f: Do not display field information. -V: (uppercase V)-display version.
3. users command
The users command displays the currently logged-in users. Except for help and version, this command has no other parameters.
# users Linuxconcept
4. who command
The who command only returns the user name, date, time, and host information; the who command is similar to the w command, unlike the w command, who does not output the information about the operation performed by the user, let’s take a look at the two commands who and w The difference.
# who linuxconcept pts / 0 2020-09-18 07:59 (192.168.50.1)
# w 08:43:58 up 50 min, 1 user, load average: 0.64, 0.18, 0.06 USER TTY FROM [email protected] IDLE JCPU PCPU WHAT linuxconcept pts/0 192.168.50.1 07:59 0.00s 0.43s 0.10s w options for who command -b: Display the date and time of the last system restart. -r: Display the current run level. -a, -all: Display all accumulated information.
5. whoami command
The whoami command outputs the name of the current user; you can also use the “who am i” command to display the current user. If you log in as the root user with the sudo command, the “whoami” command returns that the root user is the current user, if you want to know who is logged in Which user is specific, use the “who am i” command.
# whoami linuxconcept
6. ls command
The ls command displays a list of files in human readable format.
# ls -l total 114 dr-xr-xr-x. 2 root root 4096 Sep 18 08:46 bin dr-xr-xr-x. 5 root root 1024 Sep 8 15:49 boot
Sort files according to the last modification time.
# ls -ltr total 40 -rw-r--r--. 1 root root 6546 Sep 17 18:42 install.log.syslog -rw-r--r--. 1 root root 22435 Sep 17 18:45 install.log -rw-------. 1 root root 1003 Sep 17 18:45 anaconda-ks.cfg
7. crontab command
You can use the crontab command and the -l option to list the scheduled tasks of the current user.
# crontab -l 00 10 * * * /bin/ls >/ls.txt
You can use the -e option to edit crontab. In the following example, you will use the VI editing tool to open the scheduled task, make the necessary changes, and press the :wq key to exit, which will automatically save the settings.
# crontab -e
8. less command
The less command allows you to quickly view files; you can page up and down and press “q” to exit the less window.
# less install.log Installing setup-2.8.14-10.el6.noarch warning: setup-2.8.14-10.el6.noarch: Header V3 RSA/SHA256 Signature, key ID c105b9de: NOKEY Installing filesystem-2.4.30-2.1.el6.i686 Installing ca-certificates-2010.63-3.el6.noarch Installing xml-common-0.6.3-32.el6.noarch nstalling tzdata-2010l-1.el6.noarch Installing iso-codes-3.16-2.el6.noarch
9. more command
The more command allows you to quickly view files and display detailed information as a percentage. You can page up and down and press “q” to exit the more window.
# more install.log Installing setup-2.8.14-10.el6.noarch warning: setup-2.8.14-10.el6.noarch: Header V3 RSA/SHA256 Signature, key ID c105b9de: NOKEY Installing filesystem-2.4.30-2.1.el6.i686 Installing ca-certificates-2010.63-3.el6.noarch Installing xml-common-0.6.3-32.el6.noarch Installing tzdata-2010l-1.el6.noarch Installing iso-codes-3.16-2.el6.noarch --More--(10%)
10. cp command
Copy files from source to destination, keeping the same pattern.
# cp -p fileA fileB
The system will prompt you before overwriting the file.
# cp -i fileA fileB
11. mv command
Rename fileA to fileB; -i option will prompt before overwriting; if the file already exists, it will ask for confirmation.
# mv -i fileA fileB
12. cat command
The cat command is used to view multiple files at the same time.
# cat fileA fileB
If a file cannot be displayed on a screen/page, you can use the cat command to combine the more and less commands to view the contents of the file.
# cat install.log | less or # cat install.log | more
13. cd command (change directory)
With the help of the cd command (change directory), it will enter the fileA directory.
# cd / fileA
14. pwd command (output working directory)
The pwd command will return the current working directory.
# pwd /root
15. sort command
Sort text files line by line in ascending order. If you use the -r option, it will be sorted in descending order.
#sort fileA.txt #sort -r fileA.txt
16. vi command
For most UNIX-like operating systems, vi is the most popular text editor. The following example uses the -R option to open a file in read-only mode and press “:q” to exit the vi window.
# vi -R /etc/shadows
17. ssh command (secure shell)
The ssh command is used to log in to the remote host; for example, the following ssh example will use the user as narad to connect to the host (192.168.50.2).
# ssh [email protected]
To check the version of ssh, use the option -V (uppercase) to display the version of ssh.
# ssh -V OpenSSH_5.3p1, OpenSSL 1.0.0-fips 29 Mar 2010
18. ftp or sftp command
The ftp or sftp command is used to connect to a remote ftp host; ftp refers to a file transfer protocol, and sftp is a secure file transfer protocol. For example, the following command will connect to the ftp host (192.168.50.2).
# ftp 192.168.50.2 # sftp 192.168.50.2
Just as you can use mput to upload multiple files to a remote host, we can also use mget to download multiple files from a remote host.
# ftp > mput *.txt # ftp > mget *.txt
19. service command
The service command calls the script located in the /etc/init.d/ directory and executes the script . There are two ways to start any service; for example, we use the service command to start a service named httpd.
# service httpd start # /etc/init.d/httpd start
20. free command
The free command displays information about free memory, total memory, and exchanges, in bytes.
# free total used free shared buffers cached Mem: 1030800 735944 294856 0 51648 547696 -/+ buffers/cache: 136600 894200 Swap: 2064376 0 2064376
The free command with the -t option displays the total memory used and the memory that can be used, in bytes.
# free -t total used free shared buffers cached Mem: 1030800 736096 294704 0 51720 547704 -/+ buffers/cache: 136672 894128 Swap: 2064376 0 2064376 Total: 3095176 736096 2359080
21. top command
The top command displays the processor activity of the system, and also displays the tasks managed by the kernel in real time. It displays the processor and memory used; if you use the top command with the “u” option, it will display the specific user process details The information is as follows: press “O” (uppercase) to sort the way you want, press “q” to exit the top screen.
# top -u linuxconcept top - 11:13:11 up 3:19, 2 users, load average: 0.00, 0.00, 0.00 Tasks: 116 total, 1 running, 115 sleeping, 0 stopped, 0 zombie Cpu (s): 0.0% us, 0.3% sy, 0.0% ni, 99.7% id, 0.0% wa, 0.0% hi, 0.0% si, 0.0% st Mem: 1030800k total, 736188k used, 294612k free, 51760k buffers Swap: 2064376k total, 0k used, 2064376k free, 547704k cached PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1889 linuxconcept 20 0 11468 1648 920 S 0.0 0.2 0:00.59 sshd 1890 linuxconcept 20 0 5124 1668 1416 S 0.0 0.2 0:00.44 bash 6698 linuxconcept 20 0 11600 1668 924 S 0.0 0.2 0:01.19 sshd 6699 linuxconcept 20 0 5124 1596 1352 S 0.0 0.2 0:00.11 bash
22. tar command
The tar command is used to compress files and folders under Linux; for example, the following command will create a compressed archive for the /home directory with the file name archive-name.tar.
# tar -cvf archive-name.tar /home
To decompress the tar compressed archive file, please use the options shown below.
# tar -xvf archive-name.tar
23. grep command
grep can search for specific strings in files; only the tecmint users in the /etc/passwd file are displayed. We can use the -i option to ignore case.
# grep linuxconcept /etc/passwd linuxconcept:x:500:500::/home/linuxconcept:/bin/bash
24. find command
The find command is used to search for files, strings and directories; in the following example, the find command searches for the word tecmint in the “/” partition and returns the output.
# find / -name linuxconcept / var / spool / mail / linuxconcept /home/linuxconcept /root/home/linuxconcept
25. lsof command
lsof means “list all open files”; below, the lsof command lists all files opened by the tecmint user.
# lsof -u linuxconcept COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME sshd 1889 linuxconcept pwd DIR 253,0 4096 2 / sshd 1889 linuxconcept txt REG 253,0 532336 298069 /usr/sbin/sshd sshd 1889 linuxconcept DEL REG 253,0 412940 /lib/libcom_err.so.2.1 sshd 1889 linuxconcept DEL REG 253,0 393156 /lib/ld-2.12.so sshd 1889 linuxconcept DEL REG 253,0 298643 /usr/lib/libcrypto.so.1.0.0 sshd 1889 linuxconcept DEL REG 253,0 393173 /lib/libnsl-2.12.so sshd 1889 linuxconcept DEL REG 253,0 412937 /lib/libkrb5support.so.0.1 sshd 1889 linuxconcept DEL REG 253,0 412961 /lib/libplc4.so
26. last command
With the last command, we can observe the user’s activities in the system; this command can also be executed as a normal user, it will display complete user information, such as terminal, time, date, system restart or startup, and kernel version, This is a useful command for troubleshooting.
# last linuxconcept pts/1 192.168.50.1 Tue Sep 18 08:50 still logged in linuxconcept pts/0 192.168.50.1 Tue Sep 18 07:59 still logged in reboot system boot 2.6.32-279.el6.i Tue Sep 18 07:54 - 11:38 (03:43) root pts/1 192.168.50.1 Sun Sep 16 10:40 - down (03:53) root pts/0 :0.0 Sun Sep 16 10:36 - 13:09 (02:32) root tty1 :0 Sun Sep 16 10:07 - down (04:26) reboot system boot 2.6.32-279.el6.i Sun Sep 16 09:57 - 14:33 (04:35) narad pts / 2 192.168.50.1 Thu Sep 13 08:07 - down (01:15)
You can use last with username to learn about the activities of a specific user, as shown below.
# last linuxconcept linuxconcept pts/1 192.168.50.1 Tue Sep 18 08:50 still logged in linuxconcpet pts/0 192.168.50.1 Tue Sep 18 07:59 still logged in linuxconcept pts / 1 192.168.50.1 Thu Sep 13 08:07 - down (01:15) linuxconcept pts/4 192.168.50.1 Wed Sep 12 10:12 - 12:29 (02:17)
27. ps command
The ps command displays information about the processes running in the system; the following example only shows the init process.
# ps -ef | grep init root 1 0 0 07:53 ? 00:00:04 /sbin/init root 7508 6825 0 11:48 pts/1 00:00:00 grep init
28. kill command
Use the kill command to terminate the process; first use the ps command to find the process id, as shown below, and then use the kill -9 command to terminate the process.
# ps -ef | grep init root 1 0 0 07:53 ? 00:00:04 /sbin/init root 7508 6825 0 11:48 pts/1 00:00:00 grep init # kill- 9 7508
29. rm command
The rm command is used to clear or delete files without prompting for confirmation.
# rm filename
Use the -i option to confirm before deleting; using the two options “-r” and “-f” will forcefully delete the file without confirming it.
# rm -i test.txt rm: remove regular file `test.txt'?
30. mkdir command
The mkdir command is used to create a directory under Linux.
# mkdir directoryname