Training/Linux - command line

Introduction

This represents a series of teaching pages to allow you to learn more about Viper's Linux command line interface

Command Line

Like other operating systems Linux does have a window’s type environment too called X and hopefully in the future its successor Wayland. (Linux refers to X and Wayland with the term Display Server). However, with VIPER the vast majority of work will be carried out on the command line.

The command line in Linux is referred to as a shell. The shell is a program that allows the user to interact with Linux at the command line. In true Linux style there are a few different ones to choose from, however the one used predominantly is BASH. The name BASH is an acronym for “Bourne Again SHell”, a reference to BASH is an enhanced replacement for sh, the original Unix shell program written by Steve Bourne.

Man pages

This will explain the use of man pages (also called manual pages) on Linux. Most Linux files and commands have pretty good man pages to explain their use. Type man followed by a command (for which you want help) and start reading. Press q to quit the man page. See below:

username@viper:~$ man whois'  (shows manual page for the command whois)
username@viper:~$ man syslog.config'  (shows the manual page for a configuration file)
username@viper:~$ man syslogd (show the manual for a daemon (background program))
username@viper:~$ man –k syslog  (an apropos which shows a list of available man pages with this string contained within it)

Working with directories

As with other operating systems such as Microsoft Windows the filesystem is based around files and directories. Linux is no exception to this and uses a number of commands for the user to navigate around its own filesystem.

This module is a brief overview of the most common commands to work with directories: pwd, cd, ls, mkdir and rmdir. These commands are available on any Linux system.

This section will also discuss absolute and relative paths and path completion in the bash shell.

pwd

On the command line pwd (or print working directory) basically displays the current directory you are in. This would appear as:

username@viper:~$ pwd
/home/dbird

cd

On the command line cd (or change directory) changes your current directory to the one specified:

username@viper:~$ cd /var
username@viper:~$ pwd
/var
username@viper:~$ cd /home/user
username@viper:~$ pwd
/home/user

There is also a shortcut back to your home directory by typing the character ~ (tilda) which has the same effect as typing (for example) /home/user.

username@viper:~$ cd ~
username@viper:~$ pwd
/home/user

To go to the directory above (or parent directory), we use the characters ..

username@viper:~$ pwd
/home/user

username@viper:~$ cd ..
username@viper:~$ pwd
/home

There is also the character . which means the current directory. This is not useful for the cd command but is very useful for copying files to your current directory.

Absolute and relative paths

Although an important concept to understand the rule here is very simple. When you type a path starting with a slash (/), then the root of the file tree is assumed. If you don't start your path with a slash, then the current directory is the assumed starting point.

Here are two examples, firstly of absolute path

username@viper:~$ pwd
/home/user

user@viper:~$ cd /var
user@viper:~$ pwd
/var

And relative path

username@viper:~$ pwd
/home/user

username@viper:~$ cd myfiles
username@viper:~$ pwd
/home/user/myfiles

The command line will help you in typing a path without errors. If you type a partial command such as:

username@viper:~$ cd /home/user/my

Pressing the TAB key will fill in the rest of the directory name, if that directory exists and it is unique). So the following would appear

username@viper:~$ cd /home/user/myfiles

ls

This command lists the contents of a directory:

username@viper:~$ ls
myfile1.txt myfile2.txt mydirectory1

ls has a number of useful different options

username@viper:~$ ls –l  (show a long listing with more information)
username@viper:~$ ls –a  (show all files including those that are hidden)
username@viper:~$ ls –la  (combines both of the options above)

mkdir

This commands makes a directory from the specified directory:

username@viper:~$ mkdir mydirectory1
username@viper:~$ cd mydirectory1
username@viper:~$ pwd
/home/user/mydirectory1

rmdir

This command removes the specified directory, note the directory must by empty and must not be the directory you are currently in:

username@viper:~$ rmdir mydirectory1

Working with files

In this section we learn how to recognise, create, remove, copy and move files using commands like file, touch, rm, cp, mv and rename.

Just before we start looking at these commands we need to cover two important areas:

• Files on Linux are case sensitive. This means that FILE1 is different from file1, and /etc/hosts is different from /etc/Hosts (the latter one does not exist on a typical Linux computer).
• Everything on Linux is a file. A directory is a special kind of file, but it is still a (case sensitive!) file. Each terminal window (for example /dev/pts/4), any hard disk or partition (for example /dev/sdb1) and any process are all represented somewhere in the file system as a file.

file

This command determines the file type. Unlike Windows, Linux does not determine the file type from the extension but from examining the file header/contents itself.

username@viper:~$ file mypicture.png
pic33.png: PNG image data, 3840 x 1200, 8-bit/color RGBA, non-interlaced

username@viper:~$ file parallel.c
parallel.c: ASCII C program text

touch

This creates an empty file, which can be useful for various uses.

username@viper:~$ touch newfile.c
username@viper:~$ ls
newfile.c

rm

Remove a file, as always be very careful with this command and without a backup this file will be lost forever.

username@viper:~$ ls
newfile.c backup.c
username@viper:~$ rm newfile.c
username@viper:~$ ls
backup.c

username@viper:~$ rm –i backup.c   (this is interactive delete and will ask you to delete the file before it occurs)
username@viper:~$ rm –r mydirectory   (works recursively down the specified directory but not removing any non-empty directories)
username@viper:~$ rm –rf mydirectory   (works recursively down the specified directory however the –f option means force. This option is very powerful but also needs to be used with extreme care!)

As with many Linux commands there are a few options with can be used with rm (these can be view by typing man rm).

cp

Copy files or directories from a source to a destination:

username@viper:~$ cp parallel.c mybackup.c  (copies parallel.c to mybackup.c)
username@viper:~$ cp parallel.c mydirectory1   (copies parallel.c to mydirectory1)
username@viper:~$ cp *c backupdirectory/  (copies all *.c files to backupdirectory)
username@viper:~$ cp –r mydirectory1 mydirectory2  (copies one directory to another, note the option –r for recursive copying)

As with many Linux commands there are a few options with can be used with cp (these can be view by typing man cp).

mv

Move files from a source to a destination. A versatile command that can rename a file too:

username@viper:~$ mv file1.c testfile.c  (rename file1.c to testfile.c)
username@viper:~$ mv directory1 directory2  (rename directory)
username@viper:~$ mv file1.c /home/user/myrepo  (mv file1.c to /home/user/myrepo/file1.c)

rename

Although preferably to use the mv command, this command does exist to rename files

username@viper:~$ touch file1.c file2.c file3.c
username@viper:~$ ls
file1.c file2.c file3.c

username@viper:~$ rename .c .backup *.c
username@viper:~$ ls
file1.backup file2.backup file3.backup

Working with file contents

This section will look at working with file contents themselves, such commands are head, tail, cat, tac, more and less.

head

By default head will show the first ten lines of a file.

username@viper:~$ head /etc/passwd

root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
lp:x:7:7:lp:/var/spool/lpd:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh

tail

Similar to head but this time it will show the last 10 lines of file by default.

username@viper:~$ tail /etc/services
vboxd 20012/udp
binkp 24554/tcp 		# binkp fidonet protocol
asp 27374/tcp 		# Address Search Protocol
asp 27374/udp
csync2 30865/tcp 		# cluster synchronization tool
dircproxy 57000/tcp 	# Detachable IRC Proxy
tfido 60177/tcp 		# fidonet EMSI over telnet
fido 60179/tcp 		# fidonet EMSI over TCP