Introduction to Unix commands

Following is a very brief introduction to some useful Unix commands, including examples of how to use each command. For more extensive information about any of these commands, use the man command as described below. Sources for more information appear at the end of this document.

• cal
• cat
• cd
• chmod
• cp
• date
• df
• du
• find
• ifconfig
• jobs
• kill
• less and more
• lpr and lp
• ls
• man
• mkdir
• mv
• ps
• pwd
• rm
• rmdir
• set
• vi
• w and who

---

cal

This command will print a calendar for a specified month and/or year.

To show this month's calendar, enter:

cal

To show a twelve-month calendar for 2008, enter:

cal 2008

To show a calendar for just the month of June 1970, enter:

cal 6 1970

cat

This command outputs the contents of a text file. You can use it to read brief files or to concatenate files together.

To append file1 onto the end of file2, enter:

cat file1 >> file2 

To view the contents of a file named myfile, enter:

cat myfile

Because cat displays text without pausing, its output may quickly scroll off your screen. Use the less command (described below) or an editor for reading longer text files.

For more, see Combine several text files into a single file in Unix.

cd

This command changes your current directory location. By default, your Unix login session begins in your home directory.

To switch to a subdirectory (of the current directory) named myfiles, enter:

cd myfiles

To switch to a directory named /home/dvader/empire_docs, enter:

cd /home/dvader/empire_docs 

To move to the parent directory of the current directory, enter:

cd .. 

To move to the root directory, enter:

cd /

To return to your home directory, enter:

cd

chmod

This command changes the permission information associated with a file. Every file (including directories, which Unix treats as files) on a Unix system is stored with records indicating who has permission to read, write, or execute the file, abbreviated as r, w, and x. These permissions are broken down for three categories of user: first, the owner of the file; second, a group with which both the user and the file may be associated; and third, all other users. These categories are abbreviated as u for owner (or user), g for group, and o for other.

To allow yourself to execute a file that you own named myfile, enter:

chmod u+x myfile 

To allow anyone who has access to the directory in which myfile is stored to read or execute myfile, enter:

chmod o+rx myfile 

You can view the permission settings of a file using the ls command, described below.

For more, see Manage file permissions on Unix-like systems.

cp

This command copies a file, preserving the original and creating an identical copy. If you already have a file with the new name, cp will overwrite and destroy the duplicate. For this reason, it's safest to always add -i after the cp command, to force the system to ask for your approval before it destroys any files. The general syntax for cp is:

cp -i oldfile newfile 

To copy a file named meeting1 in the directory /home/dvader/notes to your current directory, enter:

cp -i /home/dvader/notes/meeting1 . 

The . (period) indicates the current directory as destination, and the -i ensures that if there is another file named meeting1 in the current directory, you will not overwrite it by accident.

To copy a file named oldfile in the current directory to the new name newfile in the mystuff subdirectory of your home directory, enter:

cp -i oldfile ~/mystuff/newfile 

The ~ character (tilde) is interpreted as the path of your home directory.

date

The date command displays the current day, date, time, and year.

To see this information, enter:

date 

df

This command reports file system disk usage (that is, the amount of space taken up on mounted file systems). For each mounted file system, df reports the file system device, the number of blocks used, the number of blocks available, and the directory where the file system is mounted.

To find out how much disk space is used on each file system, enter the following command:

df

If the df command is not configured to show blocks in kilobytes by default, you can issue the following command:

df -k

du

This command reports disk usage (that is, the amount of space taken up by a group of files). The du command descends all subdirectories from the directory in which you enter the command, reporting the size of their contents, and finally reporting a total size for all the files it finds.

To find out how much disk space your files take up, switch to your home directory with the cd command, and enter:

du 

The numbers reported are the sizes of the files; on different systems, these sizes will be in units of either 512 byte blocks or kilobytes. To learn which is the case, use the man command, described below. On most systems, du -k will give sizes in kilobytes.

find

The find command lists all of the files within a directory and its subdirectories that match a set of conditions. This command is most commonly used to find all of the files that have a certain name.

To find all of the files named myfile.txt in your current directory and all of its subdirectories, enter:

find . -name myfile.txt -print 

To look in your current directory and its subdirectories for all of the files that end in the extension .txt, enter:

find . -name "*.txt" -print 

In these examples, the . (period) represents your current directory. It can be replaced by the full pathname of another directory to search. For instance, to search for files named myfile.txt in the directory /home/user/myusername and its subdirectories, enter:

find /home/user/myusername/ -name myfile.txt -print 

On some systems, omitting the final / (slash) after the directory name can cause find to fail to return any results.

As a shortcut for searching in your home directory, enter:

find "$HOME/" -name myfile.txt -print 

For more, see Use the Unix find command to search for files.

ifconfig

This command is used to configure network interfaces (wifi, ethernet, etc.). Generally, it is used during initial network configuration and during network troubleshooting.

To view all active network interfaces on a given machine, enter:

ifconfig 

To view all network interfaces, active and inactive, on a given machine, enter:

ifconfig -a 

jobs

This command reports any programs that you suspended and still have running or waiting in the background (if you had pressed Ctrl-z to suspend an editing session, for example). For a list of suspended jobs, enter:

 jobs 

Each job will be listed with a number; to resume a job, enter % (percent sign) followed by the number of the job. To restart job number two, for example, enter:

%2 

This command is only available in the csh, bash, tcsh, and ksh shells.

kill

Use this command as a last resort to destroy any jobs or programs that you suspended and are unable to restart. Use the jobs command to see a list of suspended jobs. To kill suspended job number three, for example, enter:

kill %3 

Now check the jobs command again. If the job has not been cancelled, harsher measures may be necessary. Enter:

kill -9 %3

less and more

Both less and more display the contents of a file one screen at a time, waiting for you to press the Spacebar between screens. This lets you read text without it scrolling quickly off your screen. The less utility is generally more flexible and powerful than more, but more is available on all Unix systems while less may not be.

To read the contents of a file named textfile in the current directory, enter:

less textfile 

The less utility is often used for reading the output of other commands. For example, to read the output of the ls command one screen at a time, enter:

ls -la | less 

In both examples, you could substitute more for less with similar results. To exit either less or more, press q. To exit less after viewing the file, press q.

lpr and lp

These commands print a file on a printer connected to the computer network. The lpr command is used on BSD systems, and the lp command is used in System V. Both commands may be used on the UITS systems.

To print a file named myfile on a printer named lp1 with lpr, enter:

lpr -Plp1 myfile

To print the same file to the same printer with lp, enter:

lp -dlp1 myfile

ls

This command will list the files stored in a directory. To see a brief, multi-column list of the files in the current directory, enter:

ls 

To also see "dot" files (configuration files that begin with a period, such as .login), enter:

ls -a 

To see the file permissions, owners, and sizes of all files, enter:

ls -la 

If the listing is long and scrolls off your screen before you can read it, combine ls with the less utility, for example:

ls -la | less 

For more, see List the files in a directory in Unix.

man

This command displays the manual page for a particular command. If you are unsure how to use a command or want to find out all its options, you might want to try using man to view the manual page.

For example, to learn more about the ls command, enter:

man ls 

To learn more about man, enter:

man man 

If you are not sure of the exact command name, you can use man with the -k option to help you find the command you need. To see one line summaries of each reference page that contains the keyword you specify, enter:

man -k keyword 

Replace keyword in the above example with the keyword which you want to reference. Also see Use the Unix man command to read manual pages.

mkdir

This command will make a new subdirectory.

To create a subdirectory named mystuff in the current directory, enter:

mkdir mystuff 

To create a subdirectory named morestuff in the existing directory named /tmp, enter:

mkdir /tmp/morestuff 

mv

This command will move a file. You can use mv not only to change the directory location of a file, but also to rename files. Unlike the cp command, mv will not preserve the original file.

To rename a file named oldname in the current directory to the new name newname, enter:

mv -i oldname newname 

To move a file named hw1 from a subdirectory named newhw to another subdirectory named oldhw (both subdirectories of the current directory), enter:

mv -i newhw/hw1 oldhw

If, in this last operation, you also wanted to give the file a new name, such as firsthw, you would enter:

mv -i newhw/hw1 oldhw/firsthw 

ps

The ps command displays information about programs (that is, processes) that are currently running. Entered without arguments, it lists basic information about interactive processes you own. However, it also has many options for determining what processes to display, as well as the amount of information about each. Like lp and lpr, the options available differ between BSD and System V implementations. For example, to view detailed information about all running processes, in a BSD system, you would use ps with the following arguments:

ps -alxww

To display similar information in System V, use the arguments:

ps -elf

For more about ps refer to the ps man page on your system. Also see About the output fields of the ps command in Unix.

pwd

This command reports the current directory path. Enter the command by itself:

 pwd

For more, see Determine your current working directory in Unix.

rm

This command will remove (destroy) a file. You should enter this command with the -i option, so that you'll be asked to confirm each file deletion. To remove a file named junk, enter:

 rm -i junk

Note: Using rm will remove a file permanently, so be sure you really want to delete a file before you use rm.

To remove a non-empty subdirectory, rm accepts the -r option. On most systems this will prompt you to confirm the removal of each file. This behavior can be prevented by adding the -f option. To remove an entire subdirectory named oldstuff and all of its contents, enter:

 rm -rf oldstuff

Note: Using this command will cause rm to descend into each subdirectory within the specified subdirectory and remove all files without prompting you. Use this command with caution, as it is very easy to accidently delete important files. As a precaution, use the ls command to list the files within the subdirectory you wish to remove. To browse through a subdirectory named oldstuff, enter:

 ls -R oldstuff | less

rmdir

This command will remove a subdirectory. To remove a subdirectory named oldstuff, enter:

 rmdir oldstuff 

Note: The directory you specify for removal must be empty. To clean it out, switch to the directory and use the ls and rm commands to inspect and delete files.

set

This command displays or changes various settings and options associated with your Unix session.

To see the status of all settings, enter the command without options:

 set 

If the output scrolls off your screen, combine set with less:

 set | less

The syntax used for changing settings is different for the various kinds of Unix shells; see the man entries for set and the references listed at the end of this document for more information.

vi

This command starts the vi text editor. To edit a file named myfile in the current directory, enter:

 vi myfile 

The vi editor works fairly differently from other text editors. If you have not used it before, you should probably look at a tutorial, such as Use the vi text editor.

The very least you need to know to start using vi is that in order to enter text, you need to switch the program from command mode to insert mode by pressing i. To navigate around the document with the cursor keys, you must switch back to command mode by pressing Esc. To execute any of the following commands, you must switch from command mode to ex mode by pressing : (the colon key): Enter w to save; wq to save and quit; q! to quit without saving.

w and who

The w and who commands are similar programs that list all users logged into the computer. If you use w, you also get a list of what they are doing. If you use who, you also get the IP numbers or computer names of the terminals they are using.