Introduction to Linux CLI

**HomerJ** · 02-09-2008 03:02 PM

This is a basic introduction to the command line interface (CLI) in the GNU/Linux operating system. The default shell in Linux is bash, which stands for Bourne Again SHell. The name is a play on the older Bourne shell (sh) found on many Unix OSes.

To begin, some useful terms:

stdin - standard input. This is what is typed at the command line.

stdout - standard output. This is the output from programs to the command line.

arguments - the initial input made when executing a program from the CLI. For example, command arg1 arg2 arg3. command is the name of the program or command, and arg1, arg2, and arg3 are the arguments.

Switches - Arguments that usually turn a feature on or off. Unlike DOS or Windows, Unix programs use - or -- as switch character. For example:
in Unix: cp -Ra ~ /mnt/backup
in DOS: format /s A:
(Those two do entirely different things, I just don't know any other DOS programs' switches)

EOF - a constant representing the end of a file

Comment - a line not interpreted by the shell interpreter (bash). In bash, comments start with a # and go to the end of a line.

Hidden File - A file that is not shown or operated on by default. It's name begins with a .(dot) . Generally, programs have a switch of -a to operate on them.

These will become important later on.

Some basic commands:

echo - prints its arguments it received to stdout, followed by a new line. Example:

Code:

echo text text text 
text text text

cd - changes directories. With no arguments, changes to the user's home directory, represented by the ~ character.

Code:

user@host ~ $ cd /
user@host / $ cd /etc/
user@host /etc $ cd
user@host ~ $

cp - copies files. Has many command-line switches that can be used

Code:

cp file1 file2
#copies file1 to file2.
cp -R dir1/ dir2/
#copies dir1/ to dir2/
#All of the subfolders and files of dir1 are also present under the same names in dir2/
#the -R stands for recursive, it is needed when copying directories

rm - removes files. Has many switches similar to cp. rmdir removes directories.

Code:

rm file1
#file1 is gone
rmdir dir1/
#dir1 is gone. dir1 must be empty or else this will fail
rm -rf /dir1
#dir1 is gone, including all subfolders. This works for removing non-empty directories
#-r stands for recursive, -f for force
#the -r in rm is lowercase, for cp -R is uppercase
rm -rf /
#DO NOT attempt this as the root user. it deletes all files on all mounted filesystems

ls - lists files has many possible switches. Some commonly used ones are:
-l - long version
--color - coloured output, usually aliased to ls
-F - outputs extra characters such as a / after directory names.
-a - shows hidden files (which begin with a .)

Code:

ls 
#shows the contents of the current directory
ls ~
#shows the contents of the user's home directory
ls -lFa --color
#combines some commonly used switches

mv - moves files. Unlike rm or cp, has no -rR option for recursion. It moves entire directories and all subfolders. Is also used to rename files or directories without actually moving them anywhere else in the directory structure.

Code:

mv dir1/ dir2/
#essentially renames dir1/ to dir2/
mv ~/dir1/dir2/file1 .
#moves file1 from ~/dir1/dir2 to the present directory (represented by the '.' (dot))

alias - makes a shortcut command. These are usually put into the user's ~/.bashrc file.

Code:

alias ls="ls --color -F"
#now whenever ls is typed,it is interpreted as ls --color -F

mkdir - makes a new directory. A useful switch is the -p, creating a direcory and all its parent directories.

Code:

mkdir test
#makes a directory called test
mkdir -p a/b/c/d
#creates a directories a, b, c, and d. d/ is inside c/ is inside b/ is inside a/

pwd - outputs the path to the current directory (pwd = present working directory)

Code:

cd #remember, no arguments results in changing to ~ (home directory)
pwd
/home/user/

cat - concatenates files. commonly used to output files to stdout. Can be used as a basic text editor. Examples:

Code:

cat /path/to/a/file
file contents

cat /etc/file > ~/file
#contents of /etc/file are now in ~/file
#anything that was in ~/file was truncated (erased)

cat /etc/file >> ~/file
#contents of /etc/file were appended to ~/file
#whatever was in ~/file remains with /etc/file after it

#using cat as a text editor
 cat > text <<EOF
> this is
> a test
> EOF
#the file text now contains this is\na test\n
#\n indicates a new line or line break <br /> is the HTML equivalent

zcat, bzcat - Perform the same function as cat except they work on compressed files. Zcat is for gzip files and bzcat for bzip2 files. A common usage of this could be to output the configuration of the running kernel.

Code:

zcat /proc/config.gz
#outputs the configuration of the running linux kernel

head, tail - head outputs the beginning of a file, tail the end. The -n option allows you to specify the number of lines. The default is 10 lines

Code:

head file1
#outputs the first 10 lines of file1
tail -n 100 file1
#outputs the last 100 lines of file1

tar - Creates an archive of files. It offers no compression. Thus, it is commonly used with bzip2 or gzip. Examples:

Code:

#extracting a file
tar xvzpf file.tar.gz
#the xvzpf switches say:
#x - extract the file
#v - verbose output, not really necessary, but nice to see
#z - use gzip decompression, could be substituted for j for bzip2
#p - preserve permissions, often necessary if you are extraction important system files, 
#like in a gentoo linux install
#f - use a file, not stdin as input.

#creating a tar archive
tar cvzpf archive.tar.gz directory1/ directory2/
#c - creates an archive
#z - compresses the archive with gzip
#vpf - same as above
#it is important that the name of the archive come before the files to create it from
#tar will not create a new directory in the archive. When this is extracted, it will create
#directories directory1/ and directory2/

gzip, bzip2, gunzip, bunzip2 - common compression tools. They compress or decompress files. It is common to use them in conjunction with tar.

Code:

gzip archive.tar
#compresses archive.tar, changes file name to archive.tar.gz
gunzip archive.tar.gz
#decompressed archive.tar.gz, leaving archive.tar
tar xvpf archive.tar
#extracts archive.tar
#Notice the absence of the z option. It is not needed as the file is not compressed.

chmod - Changes the "mode" of a file. Example:

Code:

chmod +x file.sh
#adds the x (eXecutable) flag. file.sh could now be executed by typing ./file.sh

Some other important tips and tricks:

.(dot) - a symbolic link in every directory referring to itself. For example

Code:

cd . 
#does nothing, changes to the current directory

..(dotdot) - a symbolic link in every directory pointing to its parent directory

Code:

pwd
/home/user/dir1
cd ..
pwd
/home/user

~(tilde) - refers to the users home directory

Code:

cp ~/file1 ./file1
#copies file1 from users home dir (~) to the present one.

.bashrc - a file that exists in the user's home (~) directory. Executed as a shell script whenever bash is started. Often used to alias commands.

Tab completion - if you start typing the name of a file, and press the tab key, the name will be finished. If there are multiple possibilities, the PC speaker will make a beep. If you press the tab key twice quickly with multiple possibilities, all of the possibilities will be outputted.

Copy and Paste - non-graphical terminals do not support the Ctrl-C and Ctrl-v method of copying and pasting. X11 (the windowing environment) does, so this can be used with certain graphical terminals. Gnome-terminal and KDE's konsole most likely support this. However, a simple xterm (like I use) or a non-graphical tty terminal does not.

Copying and pasting with these terminals requires a three button mouse. The scroll wheel can often function as a button.

To copy from a terminal: just highlight the text you want copied. The copy action is implied and no further action is needed.

To paste to a terminal or other application: click the middle mouse button. This can be used to copy from a terminal to a web browser or any graphical application as well. In a bash shell, line breaks are interpreted as though you are entering command. If you are pasting to command line editor or other command line program, it works as expected.

To paste to a terminal from a graphical application: Do a normal copy to clipboard (highlight text + press Ctrl-C). Then, you can paste to a terminal with the middle mouse button.

Phew, that took me a while to write.