Module 3: Basic Linux Command

 What is Linux?

Linux is an open-source and community-developed operating system for computers, servers, mainframes, mobile devices, and embedded devices. It has support on almost every major computer platform including x86, ARM etc, making it one of the most widely supported operating systems.

The design of Linux is similar to UNIX but it has evolved to run on a wide variety of hardware from phones to supercomputers. Every Linux-based OS contains the Linux Kernel—which manages hardware resources—and a set of software packages that make up the rest of the operating system.

Why Linux is popular?

Linux is different from the rest of the operating systems in many important aspects. Some of them are as follows

1. Free – First, and perhaps most importantly, Linux is free. You do not have to spend any amount to download and use it, unlike windows.

2. Open Source – Linux is open-source software. The code used to create Linux is free and available to the public to view, edit, and—for users with the appropriate skills—to contribute to.

3. Secure – Once you have Linux installed on your system, there is no need to use an antivirus! Linux is a highly secure system. Moreover, there is a global development community constantly looking at ways to enhance its security. Each upgrade makes the OS becomes more secure and robust.

4. Stability and Performance – Linux provides very high stability i.e. it does not need a reboot after a short period of time. Your Linux system rarely slows down or freezes.You can work without any disturbance on your Linux systems. Linux provides remarkably high performance on various networks and workstations.

Linux Commands in DevOps

• ls – directory listing
• ls -al – formatted listing with hidden files
• cd dir – change directory to dir
• cd – change to home
• pwd – show current directory
• mkdir dir – create a directory dir
• rm file – delete file
• rm -r dir – delete directory dir
• rm -f file – force remove file
• rm -rf dir – force remove directory dir *
• cp file1 file2 – copy file1 to file2
• cp -r dir1 dir2 – copy dir1 to dir2; create dir2 if it doesn’t exist
• mv file1 file2 – rename or move file1 to file2 if file2 is an existing directory, moves file1 into directory file2
• ln -s file link – create symbolic link link to file
• touch file – create or update file
• cat > file – places standard input into file
• more file – output the contents of file
• head file – output the first 10 lines of file
• tail file – output the last 10 lines of file
• tail -f file – output the contents of file as it grows, starting with the last 10 lines

Process Management

• ps – display your currently active processes
• top – display all running processes
• kill pid – kill process id pid
• killall proc – kill all processes named proc *
• bg – lists stopped or background jobs; resume a stopped job in the background
• fg – brings the most recent job to foreground
• fg n – brings job n to the foreground

File Permissions

• chmod octal file – change the permissions of file to octal, which can be found separately for user, group, and world by adding.

             4 – read (r)   2 – write (w)    1 – execute (x)
             Examples:
             chmod 777– read, write, execute for all
             chmod 755 – rwx for owner, rx for group and world For more options, see man chmod.

SSH

• ssh user@host – connect to host as user
• ssh -p port user@host – connect to host on port port as user
• ssh-copy-id user@host – add your key to host for user to enable a keyed or passwordless login

Searching

• grep pattern files – search for pattern in files
• grep -r pattern dir – search recursively for pattern in dir
• command | grep pattern – search for pattern in the output of command
• locate file – find all instances of file

System Info

• date – show the current date and time
• cal – show this month’s calendar
• uptime – show current uptime
• w – display who is online
• whoami – who you are logged in as
• finger user – display information about user
• uname -a – show kernel information
• cat /proc/cpuinfo – cpu information
• cat /proc/meminfo – memory information
• man command – show the manual for command
• df – show disk usage
• du – show directory space usage
• free – show memory and swap usage
• whereis app – show possible locations of app
• which app – show which app will be run by default

Compression

• tar cf file.tar files – create a tar named file.tar containing files
• tar xf file.tar – extract the files from file.tar
• tar czf file.tar.gz files – create a tar with Gzip compression
• tar xzf file.tar.gz – extract a tar using Gzip
• tar cjf file.tar.bz2 – create a tar with Bzip2 compression
• tar xjf file.tar.bz2 – extract a tar using Bzip2
• gzip file – compresses file and renames it to file.gz
• gzip -d file.gz – decompresses file.gz back to file

Network

• ping host – ping host and output results
• whois domain – get whois information for domain
• dig domain – get DNS information for domain
• dig -x host – reverse lookup host
• wget file – download file
• wget -c file – continue a stopped download

Installation

• Install from source:

              ./configure
              make
              make install

• dpkg -i pkg.deb – install a package (Debian)
• rpm -Uvh pkg.rpm – install a package (RPM)

Shortcuts

• Ctrl+C – halts the current command
• Ctrl+Z – stops the current command, resume with fg in the foreground or bg in the background
• Ctrl+D – log out of current session, similar to exit
• Ctrl+W – erases one word in the current line
• Ctrl+U – erases the whole line
• Ctrl+R – type to bring up a recent command
• !! – repeats the last command
• exit – log out of current session