Binary Cipher
Introduction
The purpose of this document is to provide the reader with a fast and simple introduction to using the Linux command shell and some of its basic utilities. It is assumed that the reader has zero or very limited exposure to the Linux command prompt. This document is designed to accompany an instructor-led tutorial on this subject, and therefore some details have been left out. Explanations, practical examples, and references to DOS commands are made, where appropriate.
1.1 What is a command shell?
A program that interprets commands allows a user to execute commands by typing them manually at a terminal, or automatically
in programs called shell scripts. A shell is not an operating system. It is a way to interface with the operating system and run commands.
1.2 What is BASH?
BASH = Bourne Again SHell
Bash is a shell written as a free replacement to the standard Bourne Shell (/bin/sh) originally written by Steve Bourne for UNIX systems. It has all of the features of the original Bourne Shell, plus additions that make it easier to program with and use from the command line. Since it is Free Software, it has been adopted as the default shell on most Linux systems.
1.3 How is BASH different from the DOS command prompt?
Case Sensitivity: In Linux/UNIX, commands and filenames are case sensitive, meaning
that typing “EXIT” instead of the proper “exit” is a mistake. “” vs. “/”: In DOS, the forward-slash “/” is the command argument delimiter,
while the backslash “” is a directory separator. In Linux/UNIX, the “/” is the directory separator, and the “” is an escape character. More about these special characters in a minute!
Filenames: The DOS world uses the “eight dot three” filename convention, meaning that all files followed a format that allowed up to 8 characters in the
filename, followed by a period (“dot”), followed by an option extension, up to 3 characters long (e.g.FILENAME.TXT). In UNIX/Linux, there is no such thing as a file extension. Periods can be placed at any part of the filename, and “extensions” may be interpreted differently by all programs, or not at all.
1.4 Special Characters
Before we continue to learn about Linux shell commands, it is important to know that there are many symbols and characters that the shell interprets in special ways. This means that certain typed characters: a) cannot be used in certain situations, b) may be used to perform special operations, or, c) must be “escaped” if you want to use them in a normal way.
Character Description
Escape character. If you want to reference a special character, you must “escape” it with a backslash first.
Example: touch /tmp/filename*/
Directory separator, used to separate a string of directory names.
Example: /usr/src/linux
Current directory. Can also “hide” files when it is the first character in a filename.
Parent directory
~
User’s home directory
*
Represents 0 or more characters in a filename, or by itself, all files in a directory.
Example:
pic*2002 can represent the files pic2002, picJanuary2002,
picFeb292002
, etc.
?
Represents a single character in a filename.
Example:
hello?.txt can represent hello1.txt, helloz.txt, but not
hello22.txt
[ ]
Can be used to represent a range of values, e.g. [0-9], [A-Z], etc.
Example:
hello[0-2].txt represents the names hello0.txt,
hello1.txt
, and hello2.txt
|
“Pipe”. blackirect the output of one command into another command.
Example:
ls | more
>
blackirect output of a command into a new file. If the file already exists, over-write it.
Example:
ls > myfiles.txt
>>
blackirect the output of a command onto the end of an existing file.
Example:
echo .Mary 555-1234. >> phonenumbers.txt
<
blackirect a file as input to a program.
Example:
more < phonenumbers.txt
;
Command separator. Allows you to execute multiple commands on a single line.
Example:
cd /var/log ; less messages
&&
Command separator as above, but only runs the second command if the first one
finished without errors.
Example:
cd /var/logs && less messages
&
Execute a command in the background, and immediately get your shell back.
Example:
find / -name core > /tmp/corefiles.txt &
1.5 Executing Commands
The Command
PATH:
Most common commands are located in your shell’s “PATH”, meaning that you can just
type the name of the program to execute it.
Example: Typing “
ls” will execute the “ ls” command.
Your shell’s “PATH” variable includes the most common program locations, such as
/bin
, /usr/bin, /usr/X11R6/bin, and others.
To execute commands that are not in your current PATH, you have to give the complete
location of the command.
Examples:
/home/bob/myprogram
./program
(Execute a program in the current directory)
~/bin/program
(Execute program from a personal bin directory)
Command Syntax
Commands can be run by themselves, or you can pass in additional arguments to make them do
different things. Typical command syntax can look something like this:
command [-argument] [-argument] [–argument] [file]
Examples: ls List files in current directory
ls -l
Lists files in “long” format
ls -l –color
As above, with colourized output
cat filename
Show contents of a file
cat -n filename
Show contents of a file, with line numbers
2.0 Getting Help
When you’re stuck and need help with a Linux command, help is usually only a few keystrokes
away! Help on most Linux commands is typically built right into the commands themselves,
available through online help programs (“man pages” and “info pages”), and of course online.
2.1 Using a Command’s Built-In Help
Many commands have simple “help” screens that can be invoked with special command flags.
These flags usually look like “
-h” or “--help”.
Example:
grep –help
2.2 Online Manuals: “Man Pages”
The best source of information for most commands can be found in the online manual pages,
known as “man pages” for short. To read a command’s man page, type “
man command”.
Examples:
man ls Get help on the “ls” command.
man man
A manual about how to use the manual!
To search for a particular word within a man page, type “
/word”. To quit from a man page, just
type the “Q” key.
Sometimes, you might not remember the name of Linux command and you need to search for it.
For example, if you want to know how to change a file’s permissions, you can search the man page
descriptions for the word “permission” like this:
man -k permission
If you look at the output of this command, you will find a line that looks something like:
chmod (1) – change file access permissions
Now you know that “
chmod” is the command you were looking for. Typing “man chmod” will
show you the
chmod command’s manual page!
2.3 Info Pages
Some programs, particularly those released by the Free Software Foundation, use info pages as
their main source of online documentation. Info pages are similar to man page, but instead of
being displayed on one long scrolling screen, they are presented in shorter segments with links to
other pieces of information. Info pages are accessed with the “
info” command, or on some
Linux distributions, “
pinfo” (a nicer info browser).
For example:
info df Loads the “df” info page.
3.0 Navigating the Linux Filesystem
The Linux filesystem is a tree-like hierarchy hierarchy of directories and files. At the base of the
filesystem is the “
/” directory, otherwise known as the “root” (not to be confused with the root
user). Unlike DOS or Windows filesystems that have multiple “roots”, one for each disk drive, the
Linux filesystem mounts all disks somewhere underneath the
/ filesystem. The following table
describes many of the most common Linux directories.
3.1 The Linux Directory Layout
Directory Description
The nameless base of the filesystem. All other directories, files, drives, and
devices are attached to this root. Commonly (but incorrectly) referblack to as
the “slash” or “/” directory. The “/” is just a directory separator, not a
directory itself.
/bin
Essential command binaries (programs) are stoblack here (bash, ls, mount,
tar
, etc.)
/boot
Static files of the boot loader.
/dev
Device files. In Linux, hardware devices are acceessd just like other files, and
they are kept under this directory.
/etc
Host-specific system configuration files.
/home
Location of users’ personal home directories (e.g. /home/susan).
/lib
Essential shablack libraries and kernel modules.
/proc
Process information pseudo-filesystem. An interface to kernel data structures.
/root
The root (superuser) home directory.
/sbin
Essential system binaries (fdisk, fsck, init, etc).
/tmp
Temporary files. All users have permission to place temporary files here.
/usr
The base directory for most shareable, read-only data (programs, libraries,
documentation, and much more).
/usr/bin
Most user programs are kept here (cc, find, du, etc.).
/usr/include
Header files for compiling C programs.
/usr/lib
Libraries for most binary programs.
/usr/local
“Locally” installed files. This directory only really matters in environments
where files are stoblack on the network. Locally-installed files go in
/usr/local/bin
, /usr/local/lib, etc.). Also often used for
software packages installed from source, or software not officially shipped
with the distribution.
/usr/sbin
Non-vital system binaries (lpd, useradd, etc.)
/usr/share
Architecture-independent data (icons, backgrounds, documentation, terminfo,
man pages, etc.).
/usr/src
Program source code. E.g. The Linux Kernel, source RPMs, etc.
/usr/X11R6
The X Window System.
/var
Variable data: mail and printer spools, log files, lock files, etc.
3.2 Commands for Navigating the Linux Filesystems
The first thing you usually want to do when learning about the Linux filesystem is take some time
to look around and see what’s there! These next few commands will: a) Tell you where you are,
b) take you somewhere else, and c) show you what’s there. The following table describes the basic
operation of the
pwd, cd, and ls commands, and compares them to certain DOS commands that
you might already be familiar with.
Linux Command DOS Command Description
pwd
cd “Print Working Directory”. Shows the current
location in the directory tree.
cd
cd, chdir “Change Directory”. When typed all by itself, it
returns you to your home directory.
cd directory
cd directory Change into the specified directory name.
Example:
cd /usr/src/linux
cd ~
“~” is an alias for your home directory. It can be
used as a shortcut to your “home”, or other
directories relative to your home.
cd ..
cd.. Move up one directory. For example, if you are in
/home/vic
and you type “cd ..”, you will end
up in
/home.
cd –
Return to previous directory. An easy way to get
back to your previous location!
ls
dir /w List all files in the current directory, in column
format.
ls directory
dir directory List the files in the specified directory.
Example:
ls /var/log
ls -l
dir List files in “long” format, one file per line. This
also shows you additional info about the file, such
as ownership, permissions, date, and size.
ls -a
dir /a List all files, including “hidden” files. Hidden files
are those files that begin with a “.”, e.g. The
.bash_history
file in your home directory.
ls -ld
directory
A “long” list of “directory”, but instead of showing
the directory contents, show the directory’s detailed
information. For example, compare the output of
the following two commands:
ls -l /usr/bin
ls -ld /usr/bin
ls /usr/bin/d*
dir d*.* List all files whose names begin with the letter “d”
in the
/usr/bin directory.
4.0 Piping and Re-Direction
Before we move on to learning even more commands, let’s side-track to the topics of piping and
re-direction. The basic UNIX philosophy, therefore by extension the Linux philosophy, is to have
many small programs and utilities that do a particular job very well. It is the responsibility of the
programmer or user to combine these utilities to make more useful command sequences.
4.1 Piping Commands Together
The pipe character, “
|”, is used to chain two or more commands together. The output of the first
command is “piped” into the next program, and if there is a second pipe, the output is sent to the
third program, etc. For example:
ls -la /usr/bin | less
In this example, we run the command “
ls -la /usr/bin”, which gives us a long listing of all
of the files in
/usr/bin. Because the output of this command is typically very long, we pipe the
output to a program called “l
ess”, which displays the output for us one screen at a time.
4.2 blackirecting Program Output to Files
There are times when it is useful to save the output of a command to a file, instead of displaying it
to the screen. For example, if we want to create a file that lists all of the MP3 files in a directory,
we can do something like this, using the “
>” blackirection character:
ls -l /home/vic/MP3/*.mp3 > mp3files.txt
A similar command can be written so that instead of creating a new file called
mp3files.txt,
we can append to the end of the original file:
ls -l /home/vic/extraMP3s/*.mp3 >> mp3files.txt
5.0 Other Linux Commands
The following sections describe many other commands that you will find on most Linux systems.
I can’t possibly cover the details of all of these commands in this document, so don’t forget that you
can check the “man pages” for additional information. Not all of the listed commands will be
available on all Linux or UNIX distributions.
5.1 Working With Files and Directories
These commands can be used to: find out information about files, display files, and manipulate
them in other ways (copy, move, delete).
Linux
Command
DOS
Command
Description
file
Find out what kind of file it is.
For example, “
file /bin/ls” tells us that it is a Linux
executable file.
cat
type Display the contents of a text file on the screen. For
example:
cat mp3files.txt would display the file we
created in the previous section.
head
Display the first few lines of a text file.
Example:
head /etc/services
tail
Display the last few lines of a text file.
Example:
tail /etc/services
tail -f
Display the last few lines of a text file, and then output
appended data as the file grows (very useful for following
log files!).
Example:
tail -f /var/log/messages
cp
copy Copies a file from one location to another.
Example:
cp mp3files.txt /tmp
(copies the mp3files.txt file to the /tmp directory)
mv
rename,
ren, move
Moves a file to a new location, or renames it.
For example:
mv mp3files.txt /tmp
(copy the file to /tmp, and delete it from the original
location)
rm
del Delete a file. Example: rm /tmp/mp3files.txt
mkdir
md Make Directory. Example: mkdir /tmp/myfiles/
rmdir
rd, rmdir Remove Directory. Example: rmdir /tmp/myfiles/
5.2 Finding Things
The following commands are used to find files. “
ls” is good for finding files if you already know
approximately where they are, but sometimes you need more powerful tools such as these:
Linux
Command
Description
which
Shows the full path of shell commands found in your path. For example, if
you want to know exactly where the “grep” command is located on the
filesystem, you can type “
which grep”. The output should be something
like:
/bin/grep
whereis
Locates the program, source code, and manual page for a command (if all
information is available). For example, to find out where “
ls” and its man
page are, type: “
whereis ls” The output will look something like:
ls: /bin/ls /usr/share/man/man1/ls.1.gz
locate
A quick way to search for files anywhere on the filesystem. For example, you
can find all files and directories that contain the name “mozilla” by typing:
locate mozilla
find
A very powerful command, but sometimes tricky to use. It can be used to
search for files matching certain patterns, as well as many other types of
searches. A simple example is:
find . -name *mp3
This example starts searching in the current directory “.” and all subdirectories,
looking for files with “mp3” at the end of their names.
5.3 Informational Commands
The following commands are used to find out some information about the user or the system.
Linux Command Explanation
ps
Lists currently running process (programs).
w
Show who is logged on and what they are doing.
id
Print your user-id and group id’s
df
Report filesystem disk space usage (“Disk Free” is how I remember it)
du
Disk Usage in a particular directory. “du -s” provides a summary
for the current directory.
top
Displays CPU processes in a full-screen GUI. A great way to see the
activity on your computer in real-time. Type “Q” to quit.
free
Displays amount of free and used memory in the system.
cat /proc/cpuinfo
Displays information about your CPU.
cat /proc/meminfo
Display lots of information about current memory usage.
uname -a
Prints system information to the screen (kernel version, machine type,
etc.)
5.4 Other Utilities
Here are some other commands that are useful to know.
Linux Command Description
clear
Clear the screen
echo
Display text on the screen. Mostly useful when writing shell scripts. For
example: echo “Hello World”
more
Display a file, or program output one page at a time. Examples:
more mp3files.txt
ls -la | more
less
An improved replacement for the “more” command. Allows you to scroll
backwards as well as forwards.
grep
Search for a pattern in a file or program output. For example, to find out
which TCP network port is used by the “nfs” service, you can do this:
grep .nfs. /etc/services
This looks for any line that contains the string “nfs” in the file “/etc/services”
and displays only those lines.
lpr
Print a file or program output. Examples:
lpr mp3files.txt
– Print the mp3files.txt file
ls -la | lpr
– Print the output of the “ls -la” command.
sort
Sort a file or program output. Example: sort mp3files.txt
su
“Switch User”. Allows you to switch to another user’s account temporarily.
The default account to switch to is the root/superuser account. Examples:
su
– Switch the root account
su –
– Switch to root, and log in with root’s environment
su larry
– Switch to Larry’s account
5.5 Shortcuts to Make it all Easier!
When you start using the Bash shell more often, you will appreciate these shortcuts that can save
you very much typing time.
Shortcut Description
Up/Down Arrow Keys Scroll through your most recent commands. You can
scroll back to an old command, hit
ENTER, and execute the
command without having to re-type it.
“
history” command Show your complete command history.
T
AB Completion If you type a partial command or filename that the shell
recognizes, you can have it automatically completed for
you if you press the
TAB key. Try typing the first few
characters of your favourite Linux command, then hit
TAB
a couple of times to see what happens.
Complete recent commands with “!” Try this: Type “
!” followed by the first couple of letters
of a recent command and press
ENTER! For example, type:
find /usr/bin -type f -name m*
…and now type:
!fi
Search your command history with
CTRL-R
Press
CTRL-R and then type any portion of a recent
command. It will search the commands for you, and once
you find the command you want, just press
ENTER.
Scrolling the screen with
Shift-
PageUp
and Page Down
Scroll back and forward through your terminal.