What is a command in Linux, and how does it work?

Key takeaways

• A command is a program or shell built-in that you run by typing its name.
• The format is command, then options, then arguments.
• The shell finds programs through the directories listed in PATH.
• Pipes send one command's output into another; redirection sends it to files.
• Every command returns an exit status; 0 means success.

What is a Linux command?

A command is an instruction you give to the operating system through the shell, the program that reads what you type. The shell works out what you asked for, runs it and prints the result. Some commands are separate programs, such as ls. Others are built into the shell itself, such as cd.

For more on shells, read what CLI shells are in Linux.

What is the anatomy of a command?

command [options] [arguments]
ls -l /etc

• Command: what to do.
• Options (flags): how to do it. Short options start with one dash, long ones with two, as in -l or --all.
• Arguments: what to do it on, such as a file or directory.

Sample output of ls -l /etc/hostname:

-rw-r--r--. 1 root root 12 Mar 10 09:15 /etc/hostname

The columns are permissions, link count, owner, group, size, modified time and name. Your output will differ.

How does the shell find a command?

When you type a name, the shell checks whether it is an alias or built-in, and if not, searches each directory in the PATH variable in order.

$ echo $PATH
/usr/local/bin:/usr/bin:/bin
$ type ls
ls is /usr/bin/ls
$ type cd
cd is a shell builtin

If a command is not found, either it is not installed or its directory is not in PATH. This is why you sometimes run a script as ./script.sh: the current directory is not in PATH.

How do you get help?

• man ls opens the manual page. Press q to quit. The Linux man-pages project hosts them online.
• ls --help prints a short summary.
• apropos keyword searches manual page descriptions.

What are pipes and redirection?

A pipe | sends one command's output into another:

$ ls /etc | wc -l

Redirection saves output into a file:

$ echo "hello" > notes.txt # overwrite
$ echo "again" >> notes.txt # append
$ cat notes.txt
hello
again

Be careful: > overwrites without asking.

What is an exit status?

Every command returns a number when it finishes. Zero means success and anything else means an error.

$ ls /nonexistent
ls: cannot access '/nonexistent': No such file or directory
$ echo $?
2

Scripts use this to decide what to do next.

Which commands should you learn on day one?

What mistakes do beginners make?

• Running rm -rf with a wrong path. Check with ls first.
• Forgetting that Linux is case-sensitive.
• Using sudo for everything.
• Not reading error messages. They usually say what is wrong.

What next?

Practise in a virtual machine. Then see how Linux works underneath.

Next steps

Practise commands with an instructor in our Linux course. For interview preparation read Linux command interview questions.

Related reading

• How to Become a Linux Administrator | Skills, Tools, and Certifications
• How to Delete a Directory in Linux?
• How To Learn Kali Linux For Beginners