Useful unix commands

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

cat for creating and displaying short files
chmod change file permissions
cd change directory
cp for copying files
date display date
echo echo argument
ftp connect to a remote machine to download or upload files
grep search file
head display first part of file
ls see what files you have
more use to read files
mkdir create directory
mv for moving and renaming files
ncftp especially good for downloading files via anonymous ftp.
passwd change your login password
pwd find out what directory you are in
rm remove a file
rmdir remove directory
rsh remote shell
setenv set an environment variable
sort sort file
tail display last part of file
tar create an archive, add or extract files
telnet log in to another machine
vi vi (visual) editor
wc count characters, words, lines


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 dictionary.

If we wish to add (append) text to an existing file we do this:

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

Then we can join the two files, dictionary and tmp, into one new file called dictionary2 like this:

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

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


This command is used to change the permissions (or access mode) of a file or directory. Only the owner of a file or a priveleged user may change its mode. For example to make a file essay.001 readable by everyone, we do this:

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

Now we can run mycommand as an executable command.

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


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

Our user above, 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 directory 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, then cd'd to poems. If had wanted to jump back one level, then go to poems with just one command, he could have said cd ../poems. Finally he used cd with no argument to jump back to his home directory.

Remember, cd with no argument always returns you to your home directory.


Use cp to copy files or directories.

This makes a copy of the source-file foo with the new (destination-file) name foo.2.

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 user's home directory.

Note: If the destination-file exists BEFORE you give a cp command, cp just overwrites (and destroys the contents of) it. It is a good idea to use the interactive -i option with cp, it checks with you before it overwrites a file.


Use this command to check the date and time.

Using the -u option displays the time in Greenwich Mean Time.

Note: This command also sets the current date and time if a numeric string is supplied, but only a priveleged user may do so.


The echo command copies (echoes) its arguments to standard output (the screen). Here are some examples:

Things like PRINTER are 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:

Use the command printenv to display values that have been set for your environment variables.


note: it is usually better to use either scp or sftp to copy files as these commands encrypt the communications to prevent eavesdropping on the passwords or transferred content.

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

Example 1: We'll connect to the machine, then change directory to mystuff, then download the file homework11:

Example 2: We'll connect to the machine, then change directory to mystuff, then upload the file collected-letters:

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

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 dictionary whose contents are

Then we can look up items in our file like this

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.

The symbol "|" is called "pipe." It pipes (think of it as a connector) the output of the grep command into the input of the sort command.

For more information on grep, use the command man grep


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

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

This displays the first 20 lines of the file.


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

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

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

To see all variants of ls, use the command man ls. One of the most useful is ls -trl.


Use this command to create a directory.

To get "into" this directory, do

To see what files are in essays, do this:

There shouldn't be any files there yet, since you just made it. To create files, see cat or use your favorite text editor (like vi, emacs, or pico).


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

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:
spacebaradvance one screen
return keyadvance one line
bgo back one screen

For still more information, use the command man more.


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

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.

The file jokes.txt is downloaded from the machine


Use this command to change your account password.

Jeremy typed his current password at the Enter existing login password: prompt, but the passwd command turns off the terminal echo so that what Jeremy types wont show up in the terminal history or be visible to someone looking on. After typing his new password at the New Password: prompt Jeremy is asked to re-enter the password to ensure he typed it correctly. Depending on the system a password change may fail due to insuficient complexity or password history.


Use this command to find out what directory you are working in.

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.

The first command removed a single file. The second command removed all files beginning with the string "letter". Be careful when using this command, the file(s) are automatically removed with no prompt to the user for confirmation. Using the -i option will prompt for confirmation before deleting each file.


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

A directory must be empty (no files, no sub-directories) 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

connects you to the machine solitude.

See also telnet


Assigns a value to an environment variable. By itself, with no arguments, displays the names and values of all environment variables.

In the above example, the first echo shows the value of the environment variable PRINTER, the setenv command changes the value of PRINTER to myprinter, and the final echo shows the new value of PRINTER.


Use this commmand to sort a file. For example, suppose we have a file dictionary with contents

Then we can do this:

Here the output of sort went to the screen. To store the output in file we do this:

You can check the contents of the file dictionary.sorted using cat, more, or your favorite text editor (vi, pico, emacs).


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

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

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:

displays the file names (because of the t option in particular) in the compressed archive foo.tar.gz while

extracts (the x option) 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:

You'll have to have an account (i.e., a login and password) on the machine you try to log in to. See also rsh.


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

Then we can do this

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

The word count command has a few options, as illustrated below:

wc -lprints the line count only
wc -wprints the word count only
wc -cprints the character count only

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