UNIX Tutorials for Beginners

Introduction

These tutorials are derived from the excellent tutorials from the University of Surrey, UK, with some minor modifications for our site. The originals can be found here.

Typographical Conventions

In what follows, we shall use the following typographical conventions:

So, for example:

% ls anydirectory [Enter]

means “at the UNIX prompt %, type ls followed by the name of some directory, then press the key marked [Enter].” Don’t forget to press the [Enter] key: commands are not sent to the computer until this is done.

Note: UNIX is case-sensitive, so “LS” is not the same as ls. The same applies to filenames, so myfile.txt, MyFile.txt and MYFILE.TXT are three separate files. Beware if copying files to a PC, since DOS and Windows do not make this distinction.

Introduction to the UNIX Operating System

This session concerns UNIX, which is a common operating system. By operating system, we mean the suite of programs which make the computer work. UNIX is used by the workstations and multi-user servers within the school. On X terminals and the workstations, X Windows provide a graphical interface between the user and UNIX. However, knowledge of UNIX is required for operations which aren’t covered by a graphical program, or for when there is no X windows system, for example, in a telnet session.

The UNIX operating system

The UNIX operating system is made up of three parts; the kernel, the shell and the programs.

The Kernel

The kernel of UNIX is the hub of the operating system: it allocates time and memory to programs and handles the filestore and communications in response to system calls. As an illustration of the way that the shell and the kernel work together, suppose a user types rm myfile (which has the effect of removing the file myfile). The shell searches the filestore for the file containing the program rm, and then requests the kernel, through system calls, to execute the program rm on myfile. When the process rm myfile has finished running, the shell then returns the UNIX prompt % to the user, indicating that it is waiting for further commands.

The Shell

The shell acts as an interface between the user and the kernel. When a user logs in, the login program checks the username and password, and then starts another program called the shell. The shell is a command line interpreter (CLI). It interprets the commands the user types in and arranges for them to be carried out. The commands are themselves programs: when they terminate, the shell gives the user another prompt (% on our systems). The adept user can customise his/her own shell, and users can use different shells on the same machine. The bash shell is now the default. The bash shell has certain features to help the user inputting commands, for example:

Files and Processes

Everything in UNIX is either a file or a process. A process is an executing program identified by a unique PID (process identifier). A file is a collection of data. They are created by users using text editors, running compilers etc. Examples of files:

The Directory Structure

All the files are grouped together in the directory structure. The file-system is arranged in a hierarchical structure, like an inverted tree. The top of the hierarchy is traditionally called root.

UNIX Tutorial 1

1.1 Listing Files and Directories

ls (list)

When you first login, your current working directory is your home directory.Your home directory has the same name as your user-name, for example, ee91ab, and it is where your personal files and subdirectories are saved. To find out what is in your home directory, type

% ls (short for list)

The ls command lists the contents of your current working directory. There may be no files visible in your home directory, in which case, the UNIX prompt will be returned. Alternatively, there may already be some files inserted by the System Administrator when your account was created. ls does not, in fact, cause all the files in your home directory to be listed, but only those ones whose name does not begin with a dot (.). Files beginning with a dot (.) are known as hidden files and usually contain important program configuration information. They are hidden because you should not change them unless you are very familiar with UNIX! To list all files in your home directory including those whose names begin with a dot, type

% ls -a

ls is an example of a command which can take options: -a is an example of an option. The options change the behaviour of the command. There are online manual pages that tell you which options a particular command can take, and how each option modifies the behaviour of the command. (See later in this tutorial for more on manual pages.)

1.2 Making Directories

mkdir (make directory)

We will now make a subdirectory in your home directory to hold the files you will be creating and using in the course of this tutorial. To make a subdirectory called unixstuff in your current working directory type

% mkdir unixstuff

To see the directory you have just created, type

% ls

1.3 Changing to a Different Directory

cd (change directory)

The command cd directory means change the current working directory to ‘directory’. The current working directory may be thought of as the directory you are in, i.e. your current position in the file-system tree. To change to the directory you have just made, type

% cd unixstuff

Type ls to see the directory’s contents (it should be empty).

Exercise 1A

Make another directory inside the unixstuff directory called backups.

1.4 The Directories . and ..

While still in the unixstuff directory, type

% ls -a

As you can see, in the unixstuff directory (and in all other directories), there are two special directories called (.) and (..). In UNIX, (.) means the current directory, so typing

% cd .

means stay where you are (the unixstuff directory). This may not seem very useful at first, but using (.) as the name of the current directory will save a lot of typing, as we shall see later in the tutorial. (..) means the parent of the current directory, so typing

% cd ..

will take you one directory up the hierarchy (back to your home directory). Try it now.

Note: there is a space between cd and the dot or double dot. Also note: typing cd with no argument always returns you to your home directory. This is very useful if you are lost in the file system.

1.5 Pathnames

pwd (print working directory)

Pathnames enable you to work out where you are in relation to the whole filesystem. For example, to find out the absolute pathname of your home-directory, type cd to get back to your home-directory and then type

% pwd

The full pathname will look something like this: /a/fservb/fservb/fservb22/eebeng99/ee91ab which means that ee91ab (your home directory) is in the directory eebeng99 (the group directory),which is located on the fservb fileserver. Note: /a/fservb/fservb/fservb22/eebeng99/ee91ab can be shortened to /user/eebeng99/ee91ab

Exercise 1B

Use the commands ls, pwd and cd to explore the file system. (Remember, if you get lost, type cd by itself to return to your home directory.)

1.6 More About Home Directories and Pathnames

Understanding Pathnames

First type cd to get back to your home directory, then type

% ls unixstuff

to list the conents of your unixstuff directory. Now type

% ls backups

You will get a message like this:

backups: No such file or directory

The reason is, backups is not in your current working directory. To use a command on a file (or directory) not in the current working directory (the directory you are currently in), you must either cd to the correct directory, or specify its full pathname. To list the contents of your backups directory, you must type

% ls unixstuff/backups

~ (your home directory)

Home directories can also be referred to by the tilde ~ character. It can be used to specify paths starting at your home directory. So typing

% ls ~/unixstuff

will list the contents of your unixstuff directory, no matter where you currently are in the file system. What do you think

% ls ~

would list? What do you think

% ls ~/..

would list?

Summary

Command Operation
ls list files and directories
ls -a list all files and directories
mkdir make a directory
cd directory change to directory
cd change to home directory
cd ~ change to home directory
cd .. change to parent directory
pwd display the path of the current directory

UNIX Tutorial 2

2.1 Copying Files

cp (copy)

cp file1 file2 is the command which makes a copy of file1 in the current working directory and calls it file2. What we are going to do now, is to take a file stored in an open access area of the file system, and use the cp command to copy it to your unixstuff directory. First, cd to your unixstuff directory.

% cd ~/unixstuff

Then at the UNIX prompt, type,

% cp /share/resources/tutorials/unix/science.txt .

Note: Don’t forget the dot (.) at the end. Remember, in UNIX, the dot means the current directory. Also note: The directory /share/resources/tutorials/unix/ is an area to which all users of the research clusters have read and copy access. If you are from outside the University or do not have a research-enabled account,you can download a copy of the file here. Use ‘File/Save As…’ from the menu bar to save it into your unixstuff directory.

The above command means copy the file science.txt to the current directory, keeping the name the same.

Exercise 2A

Create a backup of your science.txt file by copying it to a file called science.bak.

2.2 Moving Files

mv (move)

mv file1 file2 moves (or renames) file1 to file2. To move a file from one place to another, use the mv command. This has the effect of moving rather than copying the file, so you end up with only one file rather than two. It can also be used to rename a file, by moving the file to the same directory, but giving it a different name. We are now going to move the file science.bak to your backup directory. First, change directories to your unixstuff directory (can you remember how?). Then, inside the unixstuff directory, type

% mv science.bak backups/.

Type ls and ls backups to see if it has worked.

2.3 Removing Files and Directories

rm (remove), rmdir (remove directory)

To delete (remove) a file, use the rm command. As an example, we are going to create a copy of the science.txt file then delete it. Inside your unixstuff directory, type

% cp science.txt tempfile.txt

% ls (to check if it has created the file)

% rm tempfile.txt

% ls (to check if it has deleted the file)

You can use the rmdir command to remove a directory (make sure it is empty first). Try to remove the backups directory. You will not be able to since UNIX will not let you remove a non-empty directory.

Exercise 2B

Create a directory called tempstuff using mkdir, then remove it using the rmdir command.

2.4 Displaying the Contents of a File on the Screen

clear (clear screen)

Before you start the next section, you may like to clear the terminal window of the previous commands so the output of the following commands can be clearly understood. At the prompt, type

% clear

This will clear all text and leave you with the % prompt at the top of the window.

cat (concatenate)

The command cat can be used to display the contents of a file on the screen. Type:

% cat science.txt

As you can see, the file is longer than than the size of the window, so it scrolls past making it unreadable.

less

The command less writes the contents of a file onto the screen a page at a time. Type

% less science.txt

Press the [spacebar] if you want to see another page. Type [q] if you want to quit reading. As you can see, less is used in preference to cat for long files.

The head command writes the first ten lines of a file to the screen. First clear the screen then type

% head science.txt

Then type

% head -5 science.txt

What difference did the -5 make to the head command?

tail

The tail command writes the last ten lines of a file to the screen. Clear the screen and type

% tail science.txt

How can you view the last 15 lines of the file?

2.5 Searching the Contents of a File

Simple Searching Using less

Using less, you can search through a text file for a keyword (pattern). For example, to search through science.txt for the word ‘science’, type

% less science.txt

then, still in less (i.e. don’t press [q] to quit), type a forward slash [/] followed by the word to search

/science

As you can see, less finds and highlights the keyword. Type [n] to search for the next occurrence of the word.

grep (don’t ask why it is called grep)

grep is one of many standard UNIX utilities. It searches files for specified words or patterns. First clear the screen, then type

% grep science science.txt

As you can see, grep has printed out each line containg the word science… or has it? Try typing

% grep Science science.txt

The grep command is case sensitive; it distinguishes between Science and science. To ignore upper/lower case distinctions, use the -i option, i.e. type

% grep -i science science.txt

To search for a phrase or pattern, you must enclose it in single quotes (the apostrophe symbol). For example to search for spinning top, type

% grep -i 'spinning top' science.txt

Some of the other options of grep are: -v (display those lines that do NOT match); -n (precede each maching line with the line number); and -c (print only the total count of matched lines). Try some of them and see the different results. Don’t forget, you can use more than one option at a time, for example, the number of lines without the words science or Science is

% grep -ivc science science.txt

wc (word count)

A handy little utility is the wc command, short for word count. To do a word count on science.txt, type

% wc -w science.txt

To find out how many lines the file has, type

% wc -l science.txt

Summary

Command Operation
`cp file1 file2 copy file1 and call it file2
`mv file1 file2 move or rename file1 to file2
`rm file remove file
`rmdir directory remove directory
`cat file display file
`more file display file a page at a time
`head file display the first few lines of a file
`tail file display the last few lines of file
`grep ‘keyword’ file search file for keywords
wc file count number of lines/words/characters in file

UNIX Tutorial 3

3.1 Redirection

Most processes initiated by UNIX commands write to the standard output (that is, they write to the terminal screen), and many take their input from the standard input (that is, they read it from the keyboard). There is also the standard error, where processes write their error messages, by default, to the terminal screen. We have already seen one use of the cat command to write the contents of a file to the screen. Now type cat without specifing a file to read

% cat

Then type a few words on the keyboard and press the [Return] key. Finally hold the [CTRL] key down and press [d] (written as ^D for short) to end the input. What has happened? If you run the cat command without specifing a file to read, it reads the standard input (the keyboard), and on receiving the ‘end of file’ (^D), copies it to the standard output (the screen). In UNIX, we can redirect both the input and the output of commands.

3.2 Redirecting the Output

We use the > symbol to redirect the output of a command. For example, to create a file called list1 containing a list of fruit, type

% cat > list1

Then type in the names of some fruit as follows. Press [Return] after each one. Terminate with the end-of-file marker control-d (^D).

pear banana apple ^D

What happens is the cat command reads the standard input (the keyboard) and the > redirects the output, which normally goes to the screen, into a file called list1. To read the contents of the file, type

% cat list1

Exercise 3A

Using the above method, create another file called list2 containing the following fruit: orange, plum, mango, grapefruit. Read the contents of list2 The form >> appends standard output to a file. So to add more items to the file list1, type

% cat >> list1

Then type in the names of more fruit

peach grape orange ^D

To read the contents of the file, type

% cat list1

You should now have two files. One contains six fruit names, the other contains four fruits. We will now use the cat command to join (concatenate) list1 and list2 into a new file called biglist. Type

% cat list1 list2 > biglist

What this is doing is reading the contents of list1 and list2 in turn, then outputing the text to the file biglist. To read the contents of the new file, type

% cat biglist

3.3 Redirecting the Input

We use the < symbol to redirect the input of a command. The command sort alphabetically or numerically sorts a list. Type

% sort

Then type in the names of some vegetables. Press [Return] after each one and terminate with ^D.

carrot beetroot artichoke ^D

The output will be

artichoke beetroot carrot

Using < you can redirect the input to come from a file rather than the keyboard. For example, to sort the list of fruit, type

% sort < biglist

and the sorted list will be output to the screen. To output the sorted list to a file, type,

% sort < biglist > slist

Use cat to read the contents of the file slist.

3.4 Pipes

To see who is on the system with you, type

% who

One method to get a sorted list of names is to type,

% who > names.txt

% sort < names.txt

This is a bit slow and you have to remember to remove the temporary file called names when you have finished. What you really want to do is connect the output of the who command directly to the input of the sort command. This is exactly what pipes do. The symbol for a pipe is the vertical bar | which, on a US keyboard, is above Enter on the right, with the backslash. For example, typing

% who | sort

will give the same result as above, but quicker and cleaner. To find out how many users are logged on, use wc (word count) with the option -l (ell) for number of lines only:

% who | wc -l

Exercise 3B

enscript -p myfile.ps myfile.txt is the command to format file myfile.txt to a PostScript file myfile.ps. Using pipes, print all lines of list1 and list2 containing the letter ‘p’, sort the result, and print to a file sorted_plist.ps. Hint: check the manpage for grep to find an option to output only the line values. Answer available here.

Summary

Command Operation
command > file redirect standard output to a file
command >> file append standard output to a file
command < file redirect standard input from a file
command1 command2 pipe the output of command1 to the input of command2
cat file1 file2 > file0 concatenate file1 and file2 to file0
sort sort data
who list users currently logged in
enscript -p psfile textfile print text file to named file

UNIX Tutorial 4

4.1 Wildcards

The Characters * and ?

The character * is called a wildcard, and will match against none or more character(s) in a file (or directory) name. For example, in your unixstuff directory, type

% ls list*

This will list all files in the current directory starting with list. Try typing

% ls *list

This will list all files in the current directory ending with list. The character ? will match exactly one character. So ls ?ouse will match files like house and mouse, but not grouse. Try typing

% ls ?list

4.2 Filename Conventions

We should note here that a directory is merely a special type of file. So the rules and conventions for naming files apply also to directories. In naming files, characters with special meanings, such as / * & %, should be avoided. Also avoid using spaces within names. The safest way to name a file is to use only alphanumeric characters, that is, letters and numbers, together with _ (underscore) and . (dot). File names conventionally start with a lower-case letter, and may end with a dot followed by a group of letters indicating the contents of the file. For example, all files consisting of C code may be named with the ending .c, for example, prog1.c. Then in order to list all files containing C code in your home directory, you need only type ls *.c in that directory.

Beware: some applications give the same name to all the output files they generate. For example, some compilers, unless given the appropriate option, produce compiled files named a.out. Should you forget to use that option, you are advised to rename the compiled file immediately, otherwise the next such file will overwrite it and it will be lost.

4.3 Getting Help

Online Manuals

There are online manuals which give information about most commands. The manual pages tell you which options a particular command can take, and how each option modifies the behaviour of the command. Type man command to read the manual page for a particular command. For example, to find out more about the wc (word count) command, type

% man wc

Alternatively

% whatis wc

gives a one-line description of the command, but omits any information about options, etc.

apropos

When you are not sure of the exact name of a command,

% apropos keyword

will give you the commands with keyword in their manual page header. For example, try typing

% apropos copy

Summary

Command Operation
* match any number of characters
? match one character
man command read the online manual page for command
whatis command brief description of command
apropos keyword match commands with keyword in their man pages

UNIX Tutorial 5

5.1 Filesystem Security (Access Rights)

In your unixstuff directory, type

% ls -l (l for long listing!)

You will see that you now get lots of details about the contents of your directory, similar to the example below.

Each file (and directory) has associated access rights, which may be found by typing ls -l. Also, ls -lg gives additional information as to which group owns the file (beng95 in the following example):

-rwxrw-r-- 1 ee51ab beng95 2450 Sept29 11:52 file1

In the left-hand column is a 10 symbol string consisting of the symbols d, r, w, x, -, and, occasionally, s or S. If d is present, it will be at the left hand end of the string, and indicates a directory; otherwise - will be the starting symbol of the string. The nine remaining symbols indicate the permissions, or access rights, and are taken as three groups of three.

Access Rights on Files

Access Rights on Directories

So, in order to read a file, you must have execute permission on the directory containing that file, and hence on any directory containing that directory as a subdirectory, and so on, up the tree.

Some Examples

Permisisons Meaning
-rwxrwxrwx a file that everyone can read, write and execute (and delete)
-rw------- a file that only the owner can read and write: no one else can read or write and no one has execution rights (e.g., yourmailbox file)

5.2 Changing Access Rights

chmod (changing a file mode)

Only the owner of a file can use chmod to change the permissions of a file. The options of chmod are as follows

Symbol Meaning
u user
g group
o other
a all
r read
w write (and delete)
x execute (and access directory)
+ add permission
- take away permission

For example, to remove read write and execute permissions on the file biglist for the group and others, type

% chmod go-rwx biglist

This will leave the other permissions unaffected. To give read and write permissions on the file biglist to all,

% chmod a+rw biglist

Exercise 5A

Try changing access permissions on the file science.txt and on the directory backups. Use ls -l to check that the permissions have changed.

5.3 Processes and Jobs

A process is an executing program identified by a unique PID (process identifier). To see information about your processes, with their associated PID and status, type

% ps

A process may be in the foreground, in the background, or be suspended. In general the shell does not return the UNIX prompt until the current process has finished executing. Some processes take a long time to run and hold up the terminal. Backgrounding a long process has the effect that the UNIX prompt is returned immediately, and other tasks can be carried out while the original process continues executing.

Running Background Processes

To background a process, type an & at the end of the command line. For example, the command sleep waits a given number of seconds before continuing. Type

% sleep 10

This will wait 10 seconds before returning the command prompt %. Until the command prompt is returned, you can do nothing except wait. To run sleep in the background, type

% sleep 10 &

[1] 6259

The & runs the job in the background and returns the prompt straight away, allowing you do run other programs while waiting for that one to finish. The first line in the above example is typed in by the user; the next line, indicating job number and PID, is returned by the machine. The user is be notified of a job number (numbered from 1) enclosed in square brackets, together with a PID and is notified when a background process is finished. Backgrounding is useful for jobs which will take a long time to complete.

Backgrounding a Current Foreground Process

At the prompt, type

% sleep 100

You can suspend the process running in the foreground by holding down the [CTRL] key and typing z Then to put it in the background, type

% bg

Note: do not background programs that require user interaction e.g. pine.

5.4 Listing Suspended and Background Processes

When a process is running, backgrounded or suspended, it will be entered onto a list along with a job number. To examine this list, type

% jobs

An example of a job list could be

  1. Suspended sleep 100

  2. Running netscape

  3. Running nedit

To restart (foreground) a suspended processes, type

% fg %jobnumber

For example, to restart sleep 100, type

% fg %1

Typing fg with no job number foregrounds the last suspended process.

5.5 Killing a Process

kill (terminate or signal a process)

It is sometimes necessary to kill a process (for example, when an executing program is in an infinite loop). To kill a job running in the foreground, type ^C ([CTRL] + [c]). For example, run

% sleep 100 ^C

To kill a suspended or background process, type

% kill %jobnumber

For example, run

% sleep 100 &

% jobs

If it is job number 4, type

% kill %4

To check whether this has worked, examine the job list again to see if the process has been removed.

ps (process status)

Alternatively, processes can be killed by finding their process numbers (PIDs) and using kill PID_number:

% sleep 100 &

% ps

PID   TTY	TIME CMD
20077 pts/5 S	0:05 sleep 100
21563 pts/5 T	0:00 netscape
21873 pts/5 S	0:25 nedit

To kill off the process sleep 100, type

% kill 20077

and then type ps again to see if it has been removed from the list. If a process refuses to be killed, uses the -9 option, i.e., type

% kill -9 20077

Note: It is not possible to kill off other users’ processes!

Summary

Command Operation
ls -lag list access rights for all files
chmod [options] file change access rights for named file
command & run command in background
^C kill the job running in the foreground
^Z suspend the job running in the foreground
bg background the suspended job
jobs list current jobs
fg %1 foreground job number 1
kill %1 kill job number 1
ps list current processes
kill 26152 kill process number 26152

UNIX Tutorial 6

Other Useful UNIX Commands

exit

Exit the current shell. If it is the login shell, this command logs the user off.

which

The which command indicates the path to the executable specified.

% which myexec

The which command returns the location of the executable according to the rules used to search paths. The shell always searches from left to right in the list contained in the PATH environment variable; the first executable of the specified name is the one that is used.

wc

The wc command returns the number of lines, words, and characters in an ASCII file. A word is defined as a non-zero length string surrounded by whitespace.

% wc myfile.txt

To print only the number of lines, use

% wc –l &lt myfile.txt &gt

diff

diff shows the differences between two ASCII files on a per-line basis.

% diff file1 file2

A few systems offer xxdiff. This is a graphical, and for human use far more useful, version of diff. It does require the ability to display X11 windows. It is available on the ITS cluster frontend.

find

find is an extremely powerful command with many options. The simplest and most common use of it is to search for a file of a given name or with a name containing a pattern.

% find . -name myscript.sh

This starts from current directory and searches for myscript.sh. The period is optional under Linux (but not under Mac OSX). To search for a name with a pattern it must typically be enclosed in quotes

% find . -name "*.sh"

See the manpage or examples online for more usage patterns of this command.

du

The du command outputs the number of kilobyes used by each subdirectory. Useful if you have gone over quota and you want to find out which directory has the most files. Some options make it more useful; in particular, -s summarizes directories and -h prints it in human-readable format. In your home directory, type

% du -s -h *

gzip

This reduces the size of a file, thus freeing valuable disk space. For example, type

% ls -l science.txt

and note the size of the file. Then to compress science.txt, type

% gzip science.txt

This will compress the file and place it in a file called science.txt.gz. To see the change in size, type ls -l again. To uncompress the file, use the gunzip command.

% gunzip science.txt.gz

file

file classifies the named files according to the type of data they contain, for example ASCII (text), pictures, compressed data, etc. To report on all files in your home directory, type

% file *

cut

The cut command extracts selected portions of a line, based on fields separated by a delimiter

% cut ­?d delim ­?fC1,C2,C3

Example:

% cut -d ' ' ?f1 /etc/resolve.conf

% cat myfile | cut -c 80

sort

This command sorts lines of a text file, based on command-­line options. The default is to sort alphabetically, based on lexigraphical ordering (in which e.g. 100 comes before 2).

% sort mylist.txt

uniq

Removes duplicate lines (file must be sorted first since it only compares lines pairwise).

% uniq mylist.txt

A frequent pattern is to pipe the output of sort into uniq

% sort animals | uniq

history

The bash shell keeps an ordered list of all the commands that you have entered. Each command is given a number according to the order it was entered.

% history (show command history list)

If you are using the bash or tcsh shell, you can use the exclamation character (!) to recall commands easily.

% !! (recall last command)

% !-3 (recall third most recent command)

% !5 (recall 5th command in list)

% !grep (recall last command starting with grep)

You can increase the size of the history buffer by typing

% set history=100

UNIX Tutorial 7

7.1 UNIX Variables

Variables are a way of passing information from the shell to programs when you run them. Programs look “in the environment” for particular variables and if they are found will use the values stored. Some are set by the system, others by you, yet others by the shell, or any program that loads another program. Standard UNIX variables are split into two categories, environment variables and shell variables. In broad terms, shell variables apply only to the current instance of the shell and are used to set short-term working conditions; environment variables are exported and have a farther reaching significance; those set at login are valid for the duration of the session. By convention, environment variables are written in UPPERCASE while shell variables usually have lowercase names.

7.2 Environment Variables

An example of an environment variable is the $SHELL variable. The value of this is the current shell you are using. Type

% echo $SHELL

More examples of environment variables are

$USER (your login name)
$HOME (the path name of your home directory)
$PWD (current working directory)
$DISPLAY (the name of the computer screen to display X windows; only set if X is enabled)
$PATH (the directories the shell should search to find a command)

Finding Out the Current Values of These Variables

Environment variables are set using the export command (bash or ksh) or the setenv command (tcsh or csh), displayed using the printenv (bash, tcsh) or env (bash, ksh) commands, and unset using the unsetenv command. To show all values of these variables, type

% printenv | less

To set a value of an environment variable, type (for bash)

% export VAR=value

or (for tcsh)

% setenv VAR value

7.3 Using and Setting Variables

Dotfiles

Each time you login to a UNIX host, the system looks in your home directory for initialisation files. Information in these files is used to set up your working environment. The bash shell uses files called .profile, .bash_profile, and .bashrc, whereas ksh uses .profile and .kshrc, and the the C and TC shells use files called .login and .cshrc or .tcshrc (note that all these file names begin with periods or dots; hence they are called dotfiles). At login the bash shell first sources .bash_profile or .profile (if .bash_profile exists .profile will be ignored). Child shells source .bashrc. If you wish for your login shell to source the .bashrc also, add the lines

if [ -f ~/.bashrc ];
  then . ~/.bashrc
fi

to the .bash_profile script. The ksh reads .profile and child shells source .kshrc. The C shell reads .login for login shells and .tcshrc for all other shells. The .bash_profile, .profile, or .login is to set conditions which will apply to the whole session and/or to perform actions that are relevant only at login. The .bashrc, .kshrc, or .cshrc file is used to set conditions and perform actions specific to the shell and to each invocation of it. The rc stands for resource; many UNIX dotfiles use this convention to set resources.

Warning: NEVER put commands that run graphical displays (e.g. web browsers) in your dotfiles. If you change your .bashrc you can force the shell to reread it by using the shell source command.

% source ~/.bashrc

7.4 Setting the Path

When you type a command, your path (or $PATH) variable defines in which directories the shell will look to find the command you typed. If the system returns a message saying “command: Command not found”, this indicates that either the command doesn’t exist at all on the system or it is simply not in your path. For example, to run units, you either need to directly specify the units path (~/units174/bin/units), or you need to have the directory ~/units174/bin in your path. You can add it to the end of your existing path (the $PATH represents this) by issuing the command:

% export PATH=$PATH:$HOME/units174/bin

If you have units installed you can test that this worked by trying to run units in any directory other than where units is actually located.

% cd; units

Hint: You can run multiple commands on one line by separating them with a semicolon. To add this path permanently, add the line to your .bashrc file after the list of other commands. Make sure that you include the $PATH when you reset it, or you will lose access to basic system commands!