Computer And Technologies

Computer And Technologies: Basic Unix commands for beginners!!

Tuesday, 21 July 2009

Basic Unix commands for beginners!!

This command allows you to change your login password. You are prompted to enter your current password, and then prompted (twice) to enter your new password. On Linux systems (like haribol) passwords should exceed 6 characters in length, and contain at least one non-alphanumeric character (such as #, %, *, ^, [, or @ etc.)

This command, as in DOS, changes directories. You can use .. to represent the directory above the current directory. You can use ~ to represent your root directory (also called your home or top directory). Example: cd maindir to move into the maindir directory, cd .. to move to the directory above, or cd ~ to move to your root directory.

This command tells you which directory you are currently working in. Your home directory is represented by the tilde ~ symbol. To go to your home directory from anywhere, type cd ~, however typing cd without the ~ also works on Linux systems.

This gives you a listing of all files in a directory. You can't tell which are files and which are directories.

ls -F
This shows which files are normal files (they have no special symbols at the end), which are directories (they end in a / character), which are links (they end in a @ symbol) and which are executables (they end in a * character). These special symbols are NOT part of the file name.

ls -l
"Long" format. Gives more details about files and directories in the current directory.

ls -a
Lists "hidden" files in current directory (those starting with a . character).

ls -la
Options may usually be combined. This particular combination would list both hidden and unhidden files in the long format

The "move" command is how you rename files. Example: mv oldfile.txt newfile.txt

Allows you to copy one or more files. Example: cp myfile.c backup.c

Deletes a file. BE CAREFUL!! There's no "undelete" command. Example: rm janfiles.*

Sends the contents of a file to stdout (usually the display screen). The name comes from "concatenate." Example: cat index.html

Like cat but displays a file one page at a time. Example: more long_file.txt

Counts the number of lines, words, and characters in a file. Example: wc essay.rtf

tail -n
Displays the last n lines of a file. Example: tail -5 myfile

head -n
Displays the first n lines of a file. Example: head -5 myfile

Creates a new directory, located below the present directory. (Use pwd first to check where you are!) Example: mkdir new_dir

Deletes a directory. Example: rmdir old_dir

The most important Unix command! It displays the manual pages for a chosen Unix command. Press [Enter] to advance one line, [Spacebar] to advance one page, and the [Q] key to quit and return to the Unix prompt. Example: man ls

man -k
Displays all Unix commands related to a given keyword. Example: man -k date will list all Unix commands whose man pages contain a reference to the word date.

Shows the current time and date.

Terminates the current login session, (and returns you to your telnet client, if that is how you established the session originally).

I/O Redirection

Input redirection. This allows you to take input from a file rather than stdin. Example: tr a z

Output redirection. This allows you to send output to a file rather than stdout. Example: ls -l >listing

Pipe. This allows you to connect stdout from one command with stdin of another. Example: ls -la | more
File Compression:-
    • gzip filename --- compresses files, so that they take up much less space. Usually text files compress to about half their original size, but it depends very much on the size of the file and the nature of the contents. There are other tools for this purpose, too (e.g. compress), but gzip usually gives the highest compression rate. Gzip produces files with the ending '.gz' appended to the original filename.
    • gunzip filename --- uncompresses files compressed by gzip.
    • gzcat filename --- lets you look at a gzipped file without actually having to gunzip it (same as gunzip -c). You can even print it directly, using gzcat filename | lpr
  • printing
    • lpr filename --- print. Use the -P option to specify the printer name if you want to use a printer other than your default printer. For example, if you want to print double-sided, use 'lpr -Pvalkyr-d', or if you're at CSLI, you may want to use 'lpr -Pcord115-d'. See 'help printers' for more information about printers and their locations.
    • lpq --- check out the printer queue, e.g. to get the number needed for removal, or to see how many other files will be printed before yours will come out
    • lprm jobnumber --- remove something from the printer queue. You can find the job number by using lpq. Theoretically you also have to specify a printer name, but this isn't necessary as long as you use your default printer in the department.
    • genscript --- converts plain text files into postscript for printing, and gives you some options for formatting. Consider making an alias like alias ecop 'genscript -2 -r \!* | lpr -h -Pvalkyr' to print two pages on one piece of paper.
    • dvips filename --- print .dvi files (i.e. files produced by LaTeX). You can use dviselect to print only selected pages.

Finding things

  • ff --- find files anywhere on the system. This can be extremely useful if you've forgotten in which directory you put a file, but do remember the name. In fact, if you use ff -p you don't even need the full name, just the beginning. This can also be useful for finding other things on the system, e.g. documentation.
  • grep string filename(s) --- looks for the string in the files. This can be useful a lot of purposes, e.g. finding the right file among many, figuring out which is the right version of something, and even doing serious corpus work. grep comes in several varieties (grep, egrep, and fgrep) and has a lot of very flexible options. Check out the man pages if this sounds good to you.

About other people

  • w --- tells you who's logged in, and what they're doing. Especially useful: the 'idle' part. This allows you to see whether they're actually sitting there typing away at their keyboards right at the moment.
  • who --- tells you who's logged on, and where they're coming from. Useful if you're looking for someone who's actually physically in the same building as you, or in some other particular location.
  • finger username --- gives you lots of information about that user, e.g. when they last read their mail and whether they're logged in. Often people put other practical information, such as phone numbers and addresses, in a file called .plan. This information is also displayed by 'finger'.
  • last -1 username --- tells you when the user last logged on and off and from where. Without any options, last will give you a list of everyone's logins.
  • talk username --- lets you have a (typed) conversation with another user
  • write username --- lets you exchange one-line messages with another user
  • elm --- lets you send e-mail messages to people around the world (and, of course, read them). It's not the only mailer you can use, but the one we recommend.

About your (electronic) self

  • whoami --- returns your username. Sounds useless, but isn't. You may need to find out who it is who forgot to log out somewhere, and make sure *you* have logged out.
  • finger & .plan files
    of course you can finger yourself, too. That can be useful e.g. as a quick check whether you got new mail. Try to create a useful .plan file soon. Look at other people's .plan files for ideas. The file needs to be readable for everyone in order to be visible through 'finger'. Do 'chmod a+r .plan' if necessary. You should realize that this information is accessible from anywhere in the world, not just to other people on turing.
  • passwd --- lets you change your password, which you should do regularly (at least once a year).
  • ps -u yourusername --- lists your processes. Contains lots of information about them, including the process ID, which you need if you have to kill a process. Normally, when you have been kicked out of a dialin session or have otherwise managed to get yourself disconnected abruptly, this list will contain the processes you need to kill. Those may include the shell (tcsh or whatever you're using), and anything you were running, for example emacs or elm. Be careful not to kill your current shell - the one with the number closer to the one of the ps command you're currently running. But if it happens, don't panic. Just try again :) If you're using an X-display you may have to kill some X processes before you can start them again. These will show only when you use ps -efl, because they're root processes.
  • kill PID --- kills (ends) the processes with the ID you gave. This works only for your own processes, of course. Get the ID by using ps. If the process doesn't 'die' properly, use the option -9. But attempt without that option first, because it doesn't give the process a chance to finish possibly important business before dying. You may need to kill processes for example if your modem connection was interrupted and you didn't get logged out properly, which sometimes happens.
  • quota -v --- show what your disk quota is (i.e. how much space you have to store files), how much you're actually using, and in case you've exceeded your quota (which you'll be given an automatic warning about by the system) how much time you have left to sort them out (by deleting or gzipping some, or moving them to your own computer).
  • du filename --- shows the disk usage of the files and directories in filename (without argument the current directory is used). du -s gives only a total.
  • last yourusername --- lists your last logins. Can be a useful memory aid for when you were where, how long you've been working for, and keeping track of your phonebill if you're making a non-local phonecall for dialling in.

Connecting to the outside world

  • nn --- allows you to read news. It will first let you read the news local to turing, and then the remote news. If you want to read only the local or remote news, you can use nnl or nnr, respectively. To learn more about nn type nn, then \tty{:man}, then \tty{=.*}, then \tty{Z}, then hit the space bar to step through the manual. Or look at the man page.
  • rlogin hostname --- lets you connect to a remote host
  • telnet hostname --- also lets you connect to a remote host. Use rlogin whenever possible.
  • ftp hostname --- lets you download files from a remote host which is set up as an ftp-server. This is a common method for exchanging academic papers and drafts. If you need to make a paper of yours available in this way, you can (temporarily) put a copy in /user/ftp/pub/TMP. For more permanent solutions, ask Emma. The most important commands within ftp are get for getting files from the remote machine, and put for putting them there (mget and mput let you specify more than one file at once). Sounds straightforward, but be sure not to confuse the two, especially when your physical location doesn't correspond to the direction of the ftp connection you're making. ftp just overwrites files with the same filename. If you're transferring anything other than ASCII text, use binary mode.
  • lynx --- lets you browse the web from an ordinary terminal. Of course you can see only the text, not the pictures. You can type any URL as an argument to the G command. When you're doing this from any Stanford host you can leave out the part of the URL when connecting to Stanford URLs. Type H at any time to learn more about lynx, and Q to exit.

Unix tutorials

There are a number of excellent Unix tutorials on the Web, including:

1 comment:

  1. * jobs --- lists your currently active jobs (those that you put in the background) and their job numbers. Useful to determine which one you want to foreground if you have lots of them.
    * bg --- background a job after suspending it.
    * fg %jobnumber --- foreground a job
    * !! --- repeat the previous command (but CTRL-p, is safer, because you have hit return in addition)
    * !pattern --- repeat the last command that starts with pattern
    * echo $VARIABLE --- shows the value of an environment variable
    * setenv --- lets you set environment variables. For example, if you typed a wrong value for the TERM variable when logging in, you don't have to log out and start over, but you can just do setenv TERM vt100 (or whatever). To see what all your environment variables are set to, type env. The one that you're most likely to have to set is the DISPLAY variable, when using an X-display.
    * unset VAR --- lets you un-set environment variables. Useful, for example, if you've usually set autologout but want to stay logged on for a while without typing for some reason, or if you set the DISPLAY variable automatically but want to avoid opening windows for some reason.
    * source filename --- you need to source your dotfiles after making changes for them to take effect (or log off and in again)
    * load --- will show you the load average graphically
    * ispell filename --- will check the spelling in your file. If you're running it on a LaTeX file use the -T option to tell it to ignore the LaTeX commands. You can create and use your own dictionary to avoid having it tell you that your own name, those of fellow linguists, and linguistics terminology are a typos in every paper you write.
    * weblint --- checks the syntax of html files
    * latex2html --- translates LaTeX files into HTML
    * wn word option --- lets you access the WordNet database and display, for example, synonyms, hypernyms, or hyponyms, depending on the option you select