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Thirty Useful Unix Commands

U5

This leaflet contains basic information on thirty of the most frequently used Unix Commands. It is intended for Unix beginners who need a guide to the names and details of commands that are likely to be of use to them.

Every effort has been made to make this leaflet as generally valid as possible, but there are many different versions of Unix available within the University, so if you should find a command option behaving differently on your local machine you should consult the on-line manual page for that command. Some commands have numerous options and there is not enough space to detail them all here, so for fuller information on these commands use the relevant on-line manual page.

The names of commands are printed in bold, and the names of objects operated on by these commands (e.g. files, directories) are printed in teletype.

Index of Commands

cat - display or concatenate files
cd - change directory
chmod - change the permissions on a file or directory
cp - copy a file
date - display the current date and time
diff - display differences between text files
file - determine the type of a file
find - find files of a specified name or type
ftp - file transfer program
grep - searches files for a specified string or expression
gzip - compress a file
help - display information about bash builtin commands
info - read online documentation
kill - kill a process
lpr - print out a file
ls - list names of files in a directory
man - display an on-line manual page
mkdir - make a directory
more - scan through a text file page by page
mv - move or rename files or directories
nice - change the priority at which a job is being run
passwd - change your password
ps - list processes
pwd - display the name of your current directory
quota - disk quota and usage
rm - remove files or directories
rmdir - remove a directory
sort - sort and collate lines
ssh - secure remote access
tar - create and use archives of files

cat - display or concatenate files

cat takes a copy of a file and sends it to the standard output (i.e. to be displayed on your terminal, unless redirected elsewhere), so it is generally used either to read files, or to string together copies of several files, writing the output to a new file.

cat ex
displays the contents of the file ex.
cat ex1 ex2 > newex
creates a new file newex containing copies of ex1 and ex2, with the contents of ex2 following the contents of ex1.

cd - change directory

cd is used to change from one directory to another.

cd dir1
changes directory so that dir1 is your new current directory. dir1 may be either the full pathname of the directory, or its pathname relative to the current directory.
cd
changes directory to your home directory.
cd ..
moves to the parent directory of your current directory.

chmod - change the permissions on a file or directory

chmod alters the permissions on files and directories using either symbolic or octal numeric codes. The symbolic codes are given here:-

     u  user      +  to add a permission                   r  read
     g  group     -  to remove a permission                w  write
     o  other     =  to assign a permission explicitly     x  execute (for files),
                                                               access (for directories)

The following examples illustrate how these codes are used.

chmod u=rw file1
sets the permissions on the file file1 to give the user read and write permission on file1. No other permissions are altered.
chmod u+x,g+w,o-r file1
alters the permissions on the file file1 to give the user execute permission on file1, to give members of the user's group write permission on the file, and prevent any users not in this group from reading it.
chmod u+w,go-x dir1
gives the user write permission in the directory dir1, and prevents all other users having access to that directory (by using cd. They can still list its contents using ls.)

cp - copy a file

The command cp is used to make copies of files and directories.

cp file1 file2
copies the contents of the file file1 into a new file called file2. cp cannot copy a file onto itself.
cp file3 file4 dir1
creates copies of file3 and file4 (with the same names), within the directory dir1. dir1 must already exist for the copying to succeed.
cp -r dir2 dir3
recursively copies the directory dir2, together with its contents and subdirectories, to the directory dir3. If dir3 does not already exist, it is created by cp, and the contents and subdirectories of dir2 are recreated within it. If dir3 does exist, a subdirectory called dir2 is created within it, containing a copy of all the contents of the original dir2.

date - display the current date and time

date returns information on the current date and time in the format shown below:-
Wed Jan 30 11:27:50 GMT 2008

It is possible to alter the format of the output from date. For example, using the command line
date '+The date is %d/%m/%y, and the time is %H:%M:%S.'

at exactly 11.30am on 30th January 2008, would produce the output
The date is 30/01/08, and the time is 11:30:00.

diff - display differences between text files

diff file1 file2 reports line-by-line differences between the text files file1 and file2. The default output will contain lines such as n1 a n2,n3 and n4,n5 c n6,n7 , (where n1 a n2,n3 means that file2 has the extra lines n2 to n3 following the line that has the number n1 in file1, and n4,n5 c n6,n7 means that lines n4 to n5 in file1 differ from lines n6 to n7 in file2). After each such line, diff prints the relevant lines from the text files, with < in front of each line from file1 and > in front of each line from file2.

There are several options to diff, including diff -i, which ignores the case of letters when comparing lines, and diff -b, which ignores all trailing blanks.

diff -cn
produces a listing of differences with n lines of context, where the default is three lines. The form of the output is different from that given by diff, with + indicating lines which have been added, - indicating lines which have been removed, and ! indicating lines which have been changed.
diff dir1 dir2
will sort the contents of directories dir1 and dir2 by name, and then run diff on the text files which differ.

file - determine the type of a file

file tests named files to determine the categories their contents belong to.

file file1
can tell if file1 is, for example, a source program, an executable program or shell script, an empty file, a directory, or a library, but (a warning!) it does sometimes make mistakes.

find - find files of a specified name or type

find searches for files in a named directory and all its subdirectories.

find . -name '*.f' -print
searches the current directory and all its subdirectories for files ending in .f, and writes their names to the standard output. In some versions of Unix the names of the files will only be written out if the -print option is used.
find /local -name core -user user1 -print
searches the directory /local and its subdirectories for files called core belonging to the user user1 and writes their full file names to the standard output.

ftp - file transfer program

ftp is an interactive file transfer program. While logged on to one system (described as the local system), ftp is used to logon to another system (described as the remote system) that files are to be transferred to or from. As well as file transfers, it allows the inspection of directory contents on the remote system. There are numerous options and commands associated with ftp, and man ftp will give details of those.

WARNING! When you use ftp the communications between the systems are not encrypted. This means that your password could be snooped if you use it make an ftp connection. If you wish to transfer files between two systems where you have accounts it is better to use the commands sftp (secure file transfer program) or scp (secure remote file copy program) if available, as they provide encrypted file transfer. See the section on ssh for examples.

Some systems offer a service called "anonymous ftp", usually to allow general access to certain archives. To use such a service, enter anonymous instead of your username when you ftp to the system. It is fairly standard practice for the remote system to ask you to give your email address in place of a password. Once you have logged on you will have read access in a limited set of directories, usually within the /pub directory tree. It is good etiquette to follow the guidelines laid down by the administrators of the remote system.

A simple example anonymous ftp session is shown below:-

ftp isccp.giss.nasa.gov
If the connection to the remote system isccp.giss.nasa.gov is established, it will respond with the prompt:-
Name (isccp.giss.nasa.gov:user1)
(supposing user1 is your username on your local system). Enter anonymous and press Return. You will then be asked to enter your email address instead of a password.
After logging in some Unix commands, such as cd and ls, will be available. Other useful commands are:
help
lists the commands available to you while using ftp
get remote1 local1
creates a copy on your local system of the file remote1 from the remote system. On your local system this new file will be called local1. If no name is specified for the file on the local system, it will be given the same name as the file on the remote system.
quit
finishes the ftp session. bye and close can also be used to do this.
See File transfer for more detailed examples of using ftp and other methods of file transfer.

grep - searches files for a specified string or expression

grep searches for lines containing a specified pattern and, by default, writes them to the standard output.

grep motif1 file1
searches the file file1 for lines containing the pattern motif1. If no file name is given, grep acts on the standard input. grep can also be used to search a string of files, so
grep motif1 file1 file2 ... filen
will search the files file1, file2, ... , filen, for the pattern motif1.
grep motif1 a*
will search all the files in the current directory with names beginning with 'a' for the pattern motif1.
grep -c motif1 file1
will give the number of lines containing motif1 instead of the lines themselves.
grep -v motif1 file1
will write out the lines of file1 that do NOT contain motif1.

gzip - compress a file

gzip reduces the size of named files, replacing them with files of the same name extended by .gz . The amount of space saved by compression varies.

gzip file1
results in a compressed file called file1.gz, and deletes file1.
gzip -v file2
compresses file2 and gives information, in the format shown below, on the percentage of the file's size that has been saved by compression:-
file2 : Compression 50.26 -- replaced with file2.gz

To restore files to their original state use the command gunzip. If you have a compressed file file2.gz, then

gunzip file2
will replace file2.gz with the uncompressed file file2.

help - display information about bash builtin commands

help gives access to information about builtin commands in the bash shell. Using help on its own will give a list of the commands it has information about. help followed by the name of one of these commands will give information about that commands. help history, for example, will give details about the bash shell history listings.

info - read online documentation

info is a hypertext information system. Using the command info on its own will enter the info system, and give a list of the major subjects it has information about. Use the command q to exit info. For example, info bash will give details about the bash shell.

kill - kill a process

To kill a process using kill requires the process id (PID). This can be found by using ps. Suppose the PID is 3429, then

kill 3429
should kill the process.

lpr - print out a file

lpr is used to send the contents of a file to a printer. If the printer is a laserwriter, and the file contains PostScript, then the PostScript will be interpreted and the results of that printed out.

lpr -Pprinter1 file1
will send the file file1 to be printed out on the printer printer1. To see the status of the job on the printer queue use
lpq -Pprinter1
for a list of the jobs queued for printing on printer1. (This may not work for remote printers.)

ls - list names of files in a directory

ls lists the contents of a directory, and can be used to obtain information on the files and directories within it.

ls dir1
lists the names of the files and directories in the directory dir1, (excluding files whose names begin with . ). If no directory is named, ls lists the contents of the current directory.
ls -R dir1
also lists the contents of any subdirectories dir1 contains.
ls -a dir1
will list the contents of dir1, (including files whose names begin with . ).
ls -l file1
gives details of the access permissions for the file file1, its size in kbytes, and the time it was last altered.
ls -l dir1
gives such information on the contents of the directory dir1. To obtain the information on dir1 itself, rather than its contents, use
ls -ld dir1

man - display an on-line manual page

man displays on-line reference manual pages.

man command1
will display the manual page for command1, e.g man cp, man man.
man -k keyword
lists the manual page subjects that have keyword in their headings. This is useful if you do not yet know the name of a command you are seeking information about.
man -Mpath command1
is used to change the set of directories that man searches for manual pages on command1

mkdir - make a directory

mkdir is used to create new directories. In order to do this you must have write permission in the parent directory of the new directory.

mkdir newdir
will make a new directory called newdir.

mkdir -p can be used to create a new directory, together with any parent directories required.

mkdir -p dir1/dir2/newdir
will create newdir and its parent directories dir1 and dir2, if these do not already exist.

more - scan through a text file page by page

more displays the contents of a file on a terminal one screenful at a time.

more file1
starts by displaying the beginning of file1. It will scroll up one line every time the return key is pressed, and one screenful every time the space bar is pressed. Type ? for details of the commands available within more. Type q if you wish to quit more before the end of file1 is reached.
more -n file1
will cause n lines of file1 to be displayed in each screenful instead of the default (which is two lines less than the number of lines that will fit into the terminal's screen).

mv - move or rename files or directories

mv is used to change the name of files or directories, or to move them into other directories.

mv file1 file2
changes the name of a file from file1 to file2 unless dir2 already exists, in which case dir1 will be moved into dir2.
mv dir1 dir2
changes the name of a directory from dir1 to dir2.
mv file1 file2 dir3
moves the files file1 and file2 into the directory dir3.

nice - change the priority at which a job is being run

nice causes a command to be run at a lower than usual priority. nice can be particularly useful when running a long program that could cause annoyance if it slowed down the execution of other users' commands. An example of the use of nice is

nice gzip file1
which will execute the compression of file1 at a lower priority
If the job you are running is likely to take a significant time, you may wish to run it in the background, i.e. in a subshell. To do this, put an ampersand & after the name of your command or script. For instance,
rm -r mydir &
is a background job that will remove the directory mydir and all its contents.
The command jobs gives details of the status of background processes, and the command fg can be used to bring such a process into the foreground.

passwd - change your password

Use passwd when you wish to change your password. You will be prompted once for your current password, and twice for your new password. Neither password will be displayed on the screen.

ps - list processes

ps displays information on processes currently running on your machine. This information includes the process id, the controlling terminal (if there is one), the cpu time used so far, and the name of the command being run.

ps
gives brief details of your own processes in your current session.
To obtain full details of all your processes, including those from previous sessions use:-
ps -fu user1
using your own user name in place of user1.

ps is a command whose options vary considerably in different versions of Unix (such as BSD and SystemV). Use man ps for details of all the options available on the machine you are using.

pwd - display the name of your current directory

The command pwd gives the full pathname of your current directory.

quota - disk quota and usage

quota gives information on a user's disk space quota and usage.

quota
will only give details of where you have exceeded your disc quota on local disks, whereas
quota -v
will display your quota and usage, whether the quota has been exceeded or not, and includes information on disks mounted from other machines, as well as the local disks.

rm - remove files or directories

rm is used to remove files. In order to remove a file you must have write permission in its directory, but it is not necessary to have read or write permission on the file itself.

rm file1
will delete the file file1. If you use
rm -i file1
instead, you will be asked if you wish to delete file1, and the file will not be deleted unless you answer y. This is a useful safety check when deleting lots of files.
rm -r dir1
recursively deletes the contents of dir1, its subdirectories, and dir1 itself, and should be used with suitable caution.

rmdir - remove a directory

rmdir removes named empty directories. If you need to delete a non-empty directory rm -r can be used instead.

rmdir exdir
will remove the empty directory exdir.

sort - sort and collate lines

The command sort sorts and collates lines in files, sending the results to the standard output. If no file names are given, sort acts on the standard input. By default, sort sorts lines using a character by character comparison, working from left to right, and using the order of the ASCII character set.

sort -d
uses "dictionary order", in which only letters, digits, and white-space characters are considered in the comparisons.
sort -r
reverses the order of the collating sequence.
sort -n
sorts lines according to the arithmetic value of leading numeric strings. Leading blanks are ignored when this option is used, (except in some System V versions of sort, which treat leading blanks as significant. To be certain of ignoring leading blanks use sort -bn instead.).

ssh - secure remote access

ssh (also known as slogin) is used for logging onto a remote system, and provides secure encrypted communications between the local and remote systems using the SSH protocol. The remote system must be running an SSH server for such connections to be possible. For example,

ssh linux.pwf.cam.ac.uk
initiates a login connection to a MCS Linux server.
You can authenticate access by using your password for the remote system, or you can set up a passphrase to avoid typing the login password directly (see the man page for ssh-keygen for information on how to create these).
If you wish to transfer files over an encrypted connection you can use sftp (secure remote file transfer program) or scp (secure remote file copy program), with authentication being handled as for ssh. For example, you could use sftp to connect to the remote system sftp.pwf.cam.ac.uk:
sftp sftp.pwf.cam.ac.uk
Once you have authenticated access to sftp.pwf.cam.ac.uk, you will be in your home directory on the MCS. You can use the command {\bf cd} to change directories on sftp.pwf.cam.ac.uk and lcd to change directories on your local system; get can be used to transfer files from the remote system, and put to transfer files to the remote system. The command quit will terminate the sftp session.
Alternatively, you could use scp to transfer files. In this example scp is used to transfer a copy of the file file1 in your home directory on the remote system linux.pwf.cam.ac.uk to the current directory on the local system, naming the file newfile1.
scp linux.pwf.cam.ac.uk:file1 newfile1
Similarly, if you wish to copy the local file file2 to the remote system, calling the copy newfile2, you can use the command
scp file2 linux.pwf.cam.ac.uk:newfile2

tar - create and use archives of files

tar can be used to create and manage an archive of a set of files.

tar cf archive1.tar
creates an archive file called archive1.tar containing the contents of the current directory (and any subdirectories it contains). The c option stands for "create" and the f for "filename".
tar cf archive2.tar mydir
creates an archive file called archive2.tar containing the contents of the directory mydir.
tar tvf archive1.tar
lists the contents of the archive file archive1.tar. The t stands for "list" and the v for "verbose listing".
tar xf archive1.tar
extracts the contents of archive1.tar and copy them into the current directory. The x stands for "extract".
tar xf archive1.tar file2
extracts file2 from archive1.tar (if file2 is in the archive).
tar uf archive1.tar file2
If file2 is not already in the archive it will be added. The u stands for "update". If there is already a file called file2 in the archive, file2 will be appended to the archive if it has a more recent timestamp than the file2 already in the archive. This means the most recent version of file2 will be obtained when file2 is extracted from the archive.

 

Last updated: February 2008 Last reviewed: October 2011