Useful Linux commands

Note: All commands must be prefixed with “sudo” in case your user account doesn’t have enough privileges.

System

$ uname -a    Displays Linux system information
$ uname -r    Displays kernel release information
$ uptime      Shows how long system running + load
$ hostname    Shows system host name
$ hostname -i Displays the IP address of the host
$ last reboot Shows system reboot history
$ date        Shows the current date and time
$ cal         Shows this month calendar
$ whoami      Shows who you are logged in as

Hardware

$ dmesg                Detected hardware and boot messages
$ cat /proc/meminfo    Hardware memory information
$ cat /proc/cpuinfo    CPU model information
$ cat /proc/interrupts Lists the number of interrupts per CPU per I/O device
$ sudo lshw            Displays information on hardware configuration of the system
$ lsblk                Displays block device related information in Linux (sudo yum install util-linux-ng)
$ free -m              Displays used and free memory (-m for MB)
$ lsusb -tv            Shows USB devices
$ sudo dmidecode       Shows hardware info from the BIOS
$ sudo hdparm -i /dev/sda    # Shows info about disk sda
$ sudo hdparm -tT /dev/sda   # Do a read speed test on disk sda
$ sudo badblocks -s /dev/sda # Test for unreadable blocks on disk sda

Statistics

$ top            Displays the top CPU processes (exit with Q)
$ vmstat 2       Displays virtual memory statistics
$ sudo tcpdump -i eth0 Captures all packets flows on interface eth0
$ sudo tcpdump -i eth0 'port 80' Monitors all traffic on port 80 ( HTTP )
$ lsof           Lists all open files belonging to all active processes.
$ lsof -u myuser Lists files opened by specific user
$ watch df -h    Shows changeable data continuously

Users

$ id             Shows the active user id with login and group
$ last           Shows last logins on the system
$ who            Shows who is logged on the system
$ groupadd admin Adds group "admin" 
$ useradd -c "Sam Tomshi" -g admin -m sam Creates user "sam" and adds to group "admin"
$ userdel sam    Deletes user sam
$ adduser sam    Adds user "sam" 
$ usermod        Modifies user information

File

$ cd                    Goes to $HOME directory
$ rm -rf ./aaa          Removes -/aaa and everything below
$ cat science.txt       Display contents of a file on the screen
$ less science.txt      Displays on a different page ( type q to close the page)
$ less science.txt and then /name finds the occurences of name
$ head science.txt      displays the first ten lines of the file 
$ tail science.txt      displays the last ten lines of the file 
$ tail -20 science.txt  displays the last 20 lines of the file
$ grep 'searchedkeyword' science.txt searches and finds the keyword in the file.(case sensitive)
$ grep -i SeaRchEdKeyWoRd science.txt case insensitive search
$ grep -i 'SeaRched Sentence is this one' science.txt case insensitive search
instead of i we can use;
-n precede each matching line with the line number
-v display those lines that do not match
-c print only the total count of matching lines
$ find -name "*.txt" -print  finds the text files in the current directory
$ diff a.txt b.txt gives the different lines
$ wc -w science.txt gives the word count
$ wc -l science.txt gives the line count
$ cat > list1
   pear
   banana
   ...
   ctrl+d
creates a list and we can print this list by using:
$ cat biglist | grep p | sort gives sorted list elements which include p
$ sort < biglist > sortedlist sorts the biglist and writes it to the sortedlist
$ ls list* outputs the filenames starting with 'list'
$ ls *list outputs the filenames ending with 'list'
$ ls ?un outputs the filenames ending with 'un' but just one letter. (e.g. sun, gun, bun)
$ man ____ gives information about the command in the underlined section.
$ whatis ____ gives information about the command in the underlined section.
$ ls -l gives detailed information about the gfiles in the directory
u:user
g:group
o:other people
rwx: read write execute
rw: read write
r: read
x: execute
$ chmod u+x  TheFile adds writing permission to the user(owner) of TheFile
$ chmod go-rwx biglist to remove read write and execute permissions on the file biglist for the group and others
$ chmod 754 TheFile 7, 5, 4 represents the individual permissions for user, group, other (7:rwx, 5:rx, 4:r)
4 – stands for “read”
2 – stands for “write”
1 – stands for “execute”
0 – no permissions
$ du -s * The du command outputs the number of kilobyes used by each subdirectory. 
$ df . The df command reports on the space left on the file system. 
$ gzip science.txt Compresses into a gzip file
$ gunzip science.txt.gz De-compresses into the original file
$ tar cvf New.tar addthisfileintotar  Create a tar file called New and add this file.
$ tar xvf New.tar Extracts the tar file
$ zcat science.txt.gz reads zipped files without unzipping
$ file * Classifies the files in the current directory ( folder, text, gzip, etc.)
$ name=Berk
$ echo Hello $name  Prints 'Hello Berk'
$ sha1sum FileName | grep e509760917361307015  Compares the checksum of a downloaded file and the calculated one.
$ gpg -c file Encrypts file
$ gpg file.gpg Decrypts file 

Processes

$ ps aux | grep 'telnet' Finds all process id related to telnet process 
$ pmap Memory map of process
$ top Display all running processes
$ kill pid Kill process with mentioned pid id
$ killall proc Kill all processes named proc
$ sleep 10 & Sleeps at the background
$ kill JobNumber  Terminates the job
$ jobs Display the jobs  
$ pkill processname Send signal to a process with its name 
$ bg Resumes suspended jobs without bringing them to foreground
$ fg Brings the most recent job to foreground 
$ fg n Brings job n to the foreground 

File Permissions

$ chmod 777 /data/test.c Sets rwx permission for owner , rwx permission for group, rwx permission for world 
$ chmod 755 /data/test.c Sets rwx permission for owner,rx for group and world 
$ chown owner-user file Changes the owner of the file
$ chown owner-user:owner-group file-name Changes the owner and group owner of the file 
$ chown owner-user:owner-group directory Changes the owner and group owner of the directory

Network

$ ifconfig -a Display all network ports and ip address
$ ifconfig eth0 Display specific ethernet port ip address and details 
$ ip addr show Display all network interfaces and ip address(available in iproute2 package,powerful than ifconfig) 
$ ip address add 192.168.0.1 dev eth0 Set ip address 
$ ethtool eth0 Linux tool to show ethernet status
$ mii-tool eth0 Linux tool to show ethernet status 
$ ping host Sends echo request to test connection
$ whois domain Get who is information for domain 
$ dig domain Get DNS information for domain
$ dig -x host Reverse lookup host 
$ host google.com Lookup DNS ip address for the name
$ hostname -i Lookup local ip address 
$ wget file  Download file
$ netstat -tupl Listing all active listening ports(tcp,udp,pid)
$ ssh user@host Connects to host as user
$ ssh -p port user@host Connects to host using specific port
$ telnet host Connects to the system using telnet port

Zip

$ tar cf home.tar home Creates tar named home.tar containing home/
$ tar xf file.tar Extracts the files from file.tar 
$ tar czf file.tar.gz files Creates a tar with gzip compression 
$ gzip file Compresses the file and renames it to file.gz

File Transfer

$ scp file.txt server2:/tmp                 Secure copies file.txt to remote host /tmp folder 
$ scp nixsavy@server2:/www/*.html /www/tmp  Copies *.html files from remote host to current system /www/tmp folder 
$ scp -r nixsavy@server2:/www /www/tmp      Copies all files and folders recursively from remote server to the current system /www/tmp folder
$ rsync -a /home/apps /backup/           
    Synchronizes source to destination 
$ rsync -avz /home/apps linoxide@192.168.10.1:/backup                    
    Synchronizes files/directories between the local and remote system with compression enabled