Today you are going to learn some basic information on 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. But remember there are many different versions of Unix available so if you should find a command option behaving differently on your local machine you should consult the on-line manual for that command for that version of Unix.
Firstly we will give you all the commands in a list and after we will get in details of them one by one. Here is the large list of commands.
Common Unix commands in details with examples
split - Splits a file into several files of equal length. Usage: split [options] <filename> <outfile>.
cmp - Compare 2 files. Usage: cmp <file1> <file2>
diff - Reports the lines that differ between 2 files Usage: diff <file> <file2>
join - A database operator that joins the common lines of two sorted files. Usage: join <file1> <file2>
xemacs - Opens emacs editor. Usage: xemacs & or xemacs filename &
dclock - Displays a digital clock. Options: -bg - fg - g -date Eg: dclock -date "%d %m %y" &
cd - change directory
man - Displays the manual page for the selected command. Usage: man <command-name>
clear - Clears the terminal.
help - Opens the default web browser in the andrew unix help web site.
rtfm - Displays the man page and help files ont he terminal. Usage: rtfm <command-name>
id - Displays the user id and the group id of the invoking user.
tty - Displays users terminal name.
whatis - Displays the command description. Usage: whatis <command>
chmod - change the permissions on a file or directory
compress - compress a file
hostname - Displays the computer’s or server’s name on the terminal.
who - Displays who is on the system.
who am i - Displays the invoking user.
history - Lists the commands typed during the session. Options: -r displays the list in reverse.
cp - copy a file
date - display the current date and time
diff - display differences between text files
echo - echo arguments to the standard output
file - determine the type of a file
find - find files of a specified name or type
finger - display information about a user
ftp - file transfer program
grep - searches files for a specified string or expression
kill - kill a process
lpr - print out a file ls - list names of files in a directory
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
talk - talk to another user
wc - display a count of lines, words and characters
Now Lets see details of some important commands one by one, Here we go.
cat - display or concatenate filescat 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 directorycd 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 directorychmod 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.)
|Important Unix Command|
compress - compress a filecompress reduces the size of named files, replacing them with files of the same name extended by .Z . The amount of space saved by compression varies. If no saving of space would occur, then the file will not be altered.
compress file1 results in a compressed file called file1.Z, and deletes file1.
compress -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.ZTo restore files to their original state use the command uncompress. If you have a compressed file file2.Z, then
uncompress file2 will replace file2.Z with the uncompressed file file2.
cp - copy a fileThe 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 timedate returns information on the current date and time in the format shown below:-
Tue Mar 25 15:21:16 GMT 1997It 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 3.10pm on 14th December 1997, would produce the output The date is 14/12/97, and the time is 15:10:00.
diff - display differences between text filesdiff 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 within 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.
echo - echo arguments to the standard outputecho echoes given arguments to the standard output, and is generally used in shell programs.
echo argument1 writes argument1 to the standard output.
file - determine the type of a filefile 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 typefind 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.
finger - display information about a userfinger can be used to obtain information on users on your own and other machines. finger on its own will give information on all users currently logged onto your machine: their user names, real names, the terminal they are using and its idle time, the time they logged on, and the name of the machine from which they logged on:-
Login Name TTY Idle When Where
user1 Julian Brown p1 2 Mon 09:04 sole.cam.ac.uk
user2 Joyce Smith p3 17 Tue 08:24 carp.cam.ac.uk
finger user1 gives further information about user1. The kind of information given varies from machine to machine, but may include the name of user1’s home directory and login shell, the last time user1 received mail, and when mail was last read. It may also display the contents of user1’s .plan and .project files, if these exist. finger -l user1 may give more information than finger user1.
It is also possible to obtain this kind of information about users on other hosts, if these machines permit. So
finger email@example.com could give similar information about Julian Brown’s account on the host sole.cam.ac.uk.
ftp - file transfer programftp is an interactive file transfer program. While logged on to one machine (described as the local machine), ftp is used to logon to another machine (described as the remote machine) that files are to be transferred to or from. As well as file transfers, it allows the inspection of directory contents on the remote machine.
There are numerous options and commands associated with ftp, and man ftp will give details of those. A simple example ftp session, in which the remote machine is the Central Unix Service (CUS), is shown below:-
ftp cus.cam.ac.uk If the connection to CUS is made, it will respond with the prompt:-
Name (cus.cam.ac.uk:user1) : (supposing user1 is your username on your local machine). If you have the same username on CUS, then just press Return; if it is different, enter your username on CUS before pressing Return. You will then be prompted for your CUS password, which will not be echoed. After logging in using ftp you will be in your home directory on CUS Some Unix commands, such as cd, mkdir, and ls, will be available. Other useful commands are: help lists the commands available to you while using ftp.
send local2 remote2 copies the file local2 to the file remote2 on CUS, i.e. it is the reverse of get.
quit finishes the ftp session. bye and close can also be used to do this. Some machines 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 machine.
It is fairly standard practice for the remote machine to ask you to give your email address as 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 machine, as they are being generous in allowing such access.
grep - searches files for a specified string or expressiongrep 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 -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.
kill - kill a processTo 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 filelpr 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 directoryls 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 -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 pageman 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 directorymkdir 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 pagemore 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 cannot move directories from one file-system to another, so, if it is necessary to do that, use cp instead.
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 runnice 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 compress file1 which will execute the compression of file1 at a lower priority.
passwd - change your passwordUse 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.
ps -a gives information on other users’ processes as well as your own. 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 directoryThe command pwd gives the full pathname of your current directory.
quota - disk quota and usagequota 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 directoriesrm 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 directoryrmdir 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 linesThe 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.).
talk - "talk" to another usertalk copies lines from your terminal to another user’s terminal. Use talk user1 if you wish to talk with user user1, who is logged onto the same machine. If the user is on another machine, called, say, sole, then use talk user1@sole
Initially, talk will send a message to the other user, requesting a talk connection. A talk session will only be established if that user responds by using talk to ask to talk to you. So if your username is user2 and your host machine is carp, then user1 must reply talk user2@carp Then two windows are formed on both terminals for the users to type their messages to each other. To exit from talk use the interrupt character.
wc - display a count of lines, words and characterswc counts the number of lines, words, and characters in files. If no filename is given, wc will count the standard input instead. wc file1 will produce output of the form 3 12 184 file1 showing that file1 contains 3 lines, 12 words, and 184 characters. There are options to wc that restrict the count to lines, or words, or characters. wc -l will just count lines, wc -w will only count words, and wc -c only counts characters.
That's all in the list. Some commands have numerous options and I have not discussed in detail them all here, so for fuller information on these commands use the relevant on-line manual page.
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