Unix Command Summary

See the Unix tutorial for a leisurely, self-paced introduction on how to use the commands listed below. For more documentation on a command, consult a good book, or use the man pages. For example, for more information on grep, use the command man grep.



This is one of the most flexible Unix commands. We can use to create, view and concatenate files. For our first example we create a three-item English-Spanish dictionary in a file called "dict."

   % cat >dict
     red rojo
     green verde
     blue azul

<control-D> stands for "hold the control key down, then tap 'd'". The symbol > tells the computer that what is typed is to be put into the file dict. To view a file we use cat in a different way:

   % cat dict
     red rojo
     green verde
     blue azul
If we wish to add text to an existing file we do this:
   % cat >>dict
     white blanco
     black negro

Now suppose that we have another file tmp that looks like this:

   % cat tmp
     cat gato
     dog perro
Then we can join dict and tmp like this:
   % cat dict tmp >dict2

We could check the number of lines in the new file like this:

   % wc -l dict2

The command wc counts things --- the number of characters, words, and line in a file.


This command is used to change the permissions of a file or directory. For example to make a file essay.001 readable by everyone, we do this:

   % chmod a+r essay.001

To make a file, e.g., a shell script mycommand executable, we do this

   % chmod +x mycommand
Now we can run mycommand as a command.

To check the permissions of a file, use ls -l . For more information on chmod, use man chmod.


Use cd to change directory. Use pwd to see what directory you are in.

   % cd english
   % pwd
   % /u/ma/jeremy/english
   % ls
novel poems
   % cd novel
   % pwd
   % /u/ma/jeremy/english/novel
   % ls
ch1 ch2 ch3 journal scrapbook
   % cd ..
   % pwd
   % /u/ma/jeremy/english
   % cd poems
   % cd
   % /u/ma/jeremy

Jeremy began in his home directory, then went to his english subdirectory. He listed this directory using ls , found that it contained two entries, both of which happen to be diretories. He cd'd to the diretory novel, and found that he had gotten only as far as chapter 3 in his writing. Then he used cd .. to jump back one level. If had wanted to jump back one level, then go to poems he could have said cd ../poems. Finally he used cd with no argument to jump back to his home directory.


Use cp to copy files or directories.
   % cp foo foo.2
This makes a copy of the file foo.
   % cp ~/poems/jabber .

This copies the file jabber in the directory poems to the current directory. The symbol "." stands for the current directory. The symbol "~" stands for the home directory.


Use this command to check the date and time.
   % date
Fri Jan  6 08:52:42 MST 1995


Use this command to compare two files and display their differences.
   % diff foo1 foo2
   < lines in foo1 that are not in foo2
   > lines in foo2 that are not in foo1
will show you the differences between the files foo1 and foo2. For example:
   % cat foo1          shows you what is in foo1
   Hello, world.
   I am foo1.
   % cat foo2          shows you what is in foo2
   Hello, world.
   I am foo2.
   %diff foo1 foo2     shows you the differences between foo1 and foo2
   < I am foo1.
   > I am foo2.
For more information about diff, consult:
   % man diff


The echo command echoes its arguments. Here are some examples:

   % echo this
   % echo $EDITOR
   % echo $PRINTER

Things like PRINTER are so-called environment variables. This one stores the name of the default printer --- the one that print jobs will go to unless you take some action to change things. The dollar sign before an environment variable is needed to get the value in the variable. Try the following to verify this:

   % echo PRINTER


Use ftp to connect to a remote machine, then upload or download files. See also: ncftp

Example 1: We'll connect to the machine fubar.net, then change director to mystuff, then download the file homework11:

   % ftp solitude
     Connected to fubar.net.
     220 fubar.net FTP server (Version wu-2.4(11) Mon Apr 18 17:26:33 MDT 1994) ready.
   Name (solitude:carlson): jeremy
     331 Password required for jeremy.
     230 User jeremy logged in.
   ftp> cd mystuff
     250 CWD command successful.
   ftp> get homework11
   ftp> quit

Example 2: We'll connect to the machine fubar.net, then change director to mystuff, then upload the file collected-letters:

   % ftp solitude
     Connected to fubar.net.
     220 fubar.net FTP server (Version wu-2.4(11) Mon Apr 18 17:26:33 MDT 1994) ready.
   Name (solitude:carlson): jeremy
     331 Password required for jeremy.
     230 User jeremy logged in.
   ftp> cd mystuff
     250 CWD command successful.
   ftp> put collected-letters
   ftp> quit

The ftp program sends files in ascii (text) format unless you specify binary mode:

   ftp> binary
   ftp> put foo
   ftp> ascii
   ftp> get bar
The file foo was transferred in binary mode, the file bar was transferred in ascii mode.


Use this command to search for information in a file or files. For example, suppose that we have a file dict whose contents are

   red rojo
   green verde
   blue azul
   white blanco
   black negro
Then we can look up items in our file like this;
   % grep red dict
     red rojo
   % grep blanco dict
     white blanco
   % grep brown dict

Notice that no output was returned by grep brown. This is because "brown" is not in our dictionary file.

Grep can also be combined with other commands. For example, if one had a file of phone numbers named "ph", one entry per line, then the following command would give an alphabetical list of all persons whose name contains the string "Fred".

   % grep Fred ph | sort
     Alpha, Fred: 333-6565
     Beta, Freddie: 656-0099
     Frederickson, Molly: 444-0981
     Gamma, Fred-George: 111-7676
     Zeta, Frederick: 431-0987
The symbol "|" is called "pipe." It pipes the output of the grep command into the input of the sort command.

For more information on grep, consult

   % man grep


The will create a compressed version of the file leaving the original unchanged. The compressed file will have the same name as the original with extension ".gz" attached. For example:

   % gzip foo

creates a new compressed file (takes up less memory) with the name "foo.gz". To uncompress a file:

   % gzip -d foo.gz
uncompresses the file, automatically stripping off the ".gz" extension. The compressed version of the file disappears! So foo.gz becomes foo again.


Use this command to look at the head of a file. For example,

   % head essay.001

displays the first 10 lines of the file essay.001 To see a specific number of lines, do this:

   % head -20 essay.001
This displays the first 20 lines of the file.


Less, like more, is a command used to read text files, but less allows backward movement in the file as well as forward movement. Another benefit is that you can use emacs commands from inside less.

   % less poems

The effect of this to let you read the file "poems ". It probably will not fit in one screen, so you need to know how to "turn pages". Here are the basic commands:

For still more information, use the command

   %man less.


Use ls to see what files you have. Your files are kept in something called a directory.

   % ls
     foo       letter2
     foobar    letter3
     letter1   maple-assignment1

Note that you have six files. There are some useful variants of the ls command:

   % ls l*
     letter1 letter2 letter3

Note what happened: all the files whose name begins with "l" are listed. The asterisk (*) is the " wildcard" character. It matches any string.


This is the standard Unix command for printing a file. It stands for the ancient "line printer." See

   % man lpr

for information on how it works.


Use man (short for manual page) to obtain on-line information about UNIX commands, application programs, system calls, utilities, and more. For example:

   % man split
will give you information about the UNIX command "split." Sometimes there are man pages (generally more complicated extended descriptions) that explain a whole set of related concepts. These will not be named after a command. For example:
   % man tgrepdoc
explains a set of complex concepts connected with the command "tgrep" ("grep" for trees). There is also a man page for plain tgrep that just explains the syntax of the command "tgrep". This would be readv with the usual man kind of request:
   % man tgrep


Typically these are in either:

These are usually broken up into subdirectori4es called man1, man2, etc. Most user commands are in a man1 directory. So for example, the man page for the copmmand "split" (the page that is used when you type "man split") is in:
The file that's used is:
Otherwise consult the environment variable MANPATH. Anatomy of this name:
   .gz    This is a file copmpressed using a compression program called "gzip" popular in Unix circles.  See gzip.

   .1     The extension for man pages stored in a man1 subdirectory

   split  The name of the command discussed in this man page.
Printing man pages out to stare at. First uncompress them:
   % gzip -d tgrepdoc.1.gz
creates the uncompressed file "tgrepdoc.1". If you view this in an editor like emacs it's readable but it will formatted for a program called "troff". You can get a nice reable version with the following command.
   % groff -mandoc -Tps tgrepdoc.1 > tgrepdoc.ps
creates a printable postscript file (the "Tps" option") tgrepdoc.ps corresponding to the manpage for tgrepdoc.
   % lpr tgrepdoc.ps
prints the postscript file. No muss, no fuss.


Use this command to create a directory.
   % mkdir essays
To get "into" this directory, do
   % cd essays
To see what files are in essays, do this:
   % ls

There shouldn't be any files there yet, since you just made it. To create files, see cat or emacs.


More is a command used to read text files. For example, we could do this:

   % more poems

The effect of this to let you read the file "poems ". It probably will not fit in one screen, so you need to know how to "turn pages". Here are the basic commands:

For still more information, use the command

   %man more.

Also see less.


Use this command to change the name of file and directories.

   % mv foo foobar

The file that was named foo is now named foobar


Use ncftp for anonymous ftp --- that means you don't have to have a password.

   % ncftp ftp.fubar.net
     Connected to ftp.fubar.net
   > get jokes.txt

The file jokes.txt is downloaded from the machine ftp.fubar.net.


Use this command to find out what directory you are working in.
   % pwd
   % cd homework
   % pwd
   % ls
assign-1 assign-2 assign-3
   % cd
   % pwd

Jeremy began by working in his "home" directory. Then he cd 'd into his homework subdirectory. Cd means " change directory". He used pwd to check to make sure he was in the right place, then used ls to see if all his homework files were there. (They were). Then he cd'd back to his home directory.


Use rm to remove files from your directory.
   % rm foo
     remove foo? y
   % rm letter*
     remove letter1? y
     remove letter2? y
     remove letter3? n

The first command removed a single file. The second command was intended to remove all files beginning with the string "letter." However, our user (Jeremy?) decided not to remove letter3.


Use this command to remove a directory. For example, to remove a directory called "essays", do this:

   % rmdir essays

A directory must be empty before it can be removed. To empty a directory, use rm.


Use this command if you want to work on a computer different from the one you are currently working on. One reason to do this is that the remote machine might be faster. For example, the command

   % rsh solitude

connects you to the machine solitude. This is one of our public workstations and is fairly fast.

See also: telnet and ssh .


   % echo $PRINTER
   % setenv PRINTER myprinter
   % echo $PRINTER


Use this commmand to sort a file. For example, suppose we have a file dict with contents
red rojo
green verde
blue azul
white blanco
black negro
Then we can do this:
   % sort dict
     black negro
     blue azul
     green verde
     red rojo
     white blanco
Here the output of sort went to the screen. To store the output in file we do this:
   % sort dict >dict.sorted 
You can check the contents of the file dict.sorted using cat , more , or emacs .


Ssh (Secure Shell) a program for logging into a remote machine and for executing commands in a remote machine. It is intended to replace rlogin and rsh, and provide secure encrypted communications between two untrusted hosts over an insecure network. X11 connections and arbitrary TCP/IP ports can also be forwarded over the secure channel.

Ssh connects and logs into the specified hostname. The user must prove his/her identity to the remote machine by providing a username and password. The password is sent to the remote host for checking; however, since all communications are encrypted, the password cannot be seen by someone listening on the network.

In order to login in to a remote machine using ssh, you must type the following:

   %ssh -l login_name hostname
For example, if your username is "rsmith" and you want to securely log on to rohan, you would type the following.
   %ssh -l rsmith rohan.sdsu.edu
The first time that you log in to a particular site, you will get the following message:
   Host key not found from the list of known hosts.
   !! If host key is new or changed, ssh1 protocol is vulnerable to an
   !! attack known as false-split, which makes it relativily easy to
   !! hijack the connection without the attack being detected. It is
   !! highly advisable to turn StrictHostKeyChecking to "yes" and
   !! manually copy host keys to known_hosts.
   Are you sure you want to continue connecting (yes/no)?     
If you trust the machine you are working on, you may answer yes and then it will ask you for your password:
   rsmith@rohan.sdsu.edu's password:
Enter your password and then you will be logged into the machine. Telnet is an alternate way to log into remote machines, but telnet is not as secure.

For more information about ssh, consult:

   %man ssh


Use this command to look at the tail of a file. For example,

   % tail essay.001

displays the last 10 lines of the file essay.001 To see a specific number of lines, do this:

   % tail -20 essay.001
This displays the last 20 lines of the file.


Use create compressed archives of directories and files, and also to extract directories and files from an archive. Example:

   % tar -tvzf foo.tar.gz

displays the file names in the compressed archive foo.tar.gz while

   % tar -xvzf foo.tar.gz
extracts the files.


Use this command to log in to another machine from the machine you are currently working on. For example, to log in to the machine "solitude", do this:

   % telnet solitude

See also: rsh and ssh .


Use this command to display a file with all of its identical adjacent lines replaced by a single occurence of the repeated line. For example:

   % cat dogs               shows what's in the file dogs
   Maltese   Lhasa Apso
   Maltese   Lhasa Apso
   Border Collie   Maltese
   Border Collie   Maltese
   Border Collie   German Shepard
   % uniq dogs              filters out duplicate adjacent lines     
   Maltese   Lhasa Apso
   Border Collie   Maltese
   Border Collie   German Shepard
You can also use the option -c which gives you the number of occurences of each line. For example:
 % uniq -c dogs   
   2  Maltese   Lhasa Apso
   2  Border Collie   Maltese
   1  Border Collie   German Shepard
For more information about uniq, see
   %man uniq


Use this command to count the number of characters, words, and lines in a file. Suppose, for example, that we have a file dict with contents

red rojo
green verde
blue azul
white blanco
black negro
Then we can do this
   % wc dict
     5      10      56 dict

This shows that dict has 5 lines, 10 words, and 56 characters.

The word count command has several options, as illustrated below:

   % wc -l dict
     5 dict
   % wc -w dict
     10 dict
   % wc -c dict
     56 dict     


The xhost command allows or denies access for X clients on other systems to display to your system.
   %xhost +
allows access to all hosts and
   %xhost -
denies access to all hosts. To specify a particular client, add the name after the +/-. For example:
   %xhost +bluenote
allows access to all X clients running on the computer "bluenote" to write to the display on the system on which the xhost command was run. You may need to use xhost + when you logged in as one user and then switched to another user without logging out. This is an example of an error message given when trying to use emacs after switching to be another user:
   Xlib: Client is not authorized to connect to Server
   emacs: Cannot connect to X server :0.
   Check the DISPLAY environment variable or use `-d'.
   Also use the `xhost' program to verify that it is set to permit
   connections from your machine.
This problem is easily solved by typing xhost +. For more information, consult:
   %man xhost


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