CSCI 1320 (Principles of Algorithm Design I):
Unix Hints and Tips (Command Line Interface)

Caveat: Last updated in 2007. Use with caution!

Useful commands

This is a list of commands mentioned in class, plus a few other useful ones. For complete information about any command, read its man page (see the description of the man command below).

Format for command descriptions

Typewriter font (like this) denotes text you should type exactly as shown,
italics (like this) denotes something you should supply (e.g., a filename), and
square brackets (like these []) enclose optional text.

In typing a command, do not include the square brackets used to denote optional text.

Commands

man commandname
Displays the "man page" for command commandname. A command's man page describes all its options and arguments. Use this to display additional options or arguments for the commands listed here. The information is displayed using the less command; see the description below for some basics on how to work with this command.
Linux systems (like the CS lab machines) provide the same or similar information via the command info. Type info info for more information about this command.
apropos keyword
Displays a list of commands related to keyword. The list is presented using the less command.
less file
Displays file (or output of another program) in a pageable/searchable form. Some things you can do:
- Type carriage return to page forward.
- Type b to page backward.
- Type / searchstring to search for searchstring.
- Type q to quit.
- Type h for help.
ls [ -F ] [ -l ] [ filenames ]
Displays information about filenames, or all files in the current directory if this option is omitted.
If you include -F, files that are really directories (a directory in Unix is just a special kind of file) are printed with a / after their names, and executable files (programs, etc.) are printed with a * after their names.
If you include -l, more information is printed for each file, including the date of last modification.
cp [ -i ] fromfile tofile
Copies fromfile to tofile. If you add the -i flag and tofile exists, you will be asked whether you want to overwrite it.
mv [ -i ] fromfile tofile
Moves (renames) fromfile to tofile. If you add the -i flag and tofile exists, you will be asked whether you want to overwrite it.
mv [ -i ] fromfile dir
Moves fromfile to directory dir. (I.e., after execution, directory dir contains fromfile.) If you add the -i flag and dir already contains a file named fromfile exists, you will be asked whether you want to overwrite it. You can replace fromfile with an expression containing wildcards to move multiple files.
rm [ -i ] file
Removes (deletes) file. You can replace file with an expression containing wildcards to remove multiple files. If you add the -i flag, you will be asked for each file whether you really want to delete it. Warning: Once a file has been rm'd, it is basically impossible to retrieve, unless a backup has been made. Be careful!
mkdir dir
Creates a directory (folder) named dir within the current directory.
rmdir dir
Removes directory dir. Note that the directory must be empty. If it is not, you can use the following command to delete the directory and all its contents (but be sure that's what you want to do):
rm -r dir
cd dir
Changes the current directory to dir. Some useful special cases:
- cd changes to your "home directory" (the root directory for all your files).
- cd .. changes to the parent of the current directory.
pwd
Displays the current directory.
script [ filename ]
Starts a "screen-capture" session, putting output in filename. If you omit filename, the output file is called typescript. Everything you type or the computer responds, until you type exit, will be recorded in the output file (filename). This is useful as a simple way of capturing an interactive session.
lpr [-Pprintername ] filename
Prints file filename on printer printername. See Hints and Tips for the CS Lab Machines for names and locations of printers.
enscript [-Pprintername ] filename
Adds page headers (with file name, page numbers, and date and time) to file filename and prints the result on printer printername. See Hints and Tips for the CS Lab Machines for names and locations of printers.

lpq -Pprintername
Shows the queue of "print jobs" (print requests) queued for printername. Output should look something like this:

	Rank   Owner      Job  Files                                 Total Size
	1st    me         41   lamport_quote, rustan_quote ...       1534 bytes
	2nd    you        42   myfile.output                           40 bytes
	....

lprm -Pprintername jobnumber
Cancels the print request numbered jobnumber on printer printername. If you previously sent something to printer printername and now want it not to print, you can remove it from the print queue -- even if it's already printing -- with this command. You get jobnumber by executing the lpq command described above. (If user me wanted to cancel the request in the example, he/she would use the command lprm -Pportia 41.) Note that unless you are the system administrator, you can only cancel your own requests, and you must do so from the machine on which you made the request.

Other tips and useful shortcuts

Interrupting programs

Most Unix programs can be interrupted by typing control-C.
Print requests can be cancelled as described in the preceding section (lprm command).

Redirecting input and output

Many programming languages (C among them) have an idea of "standard input" and "standard output". By default, reading from standard input means reading what is typed in at the keyboard, and writing to standard output means printing to the screen. But Unix allows you to "redirect" both input and output, i.e., to read standard input from a file and/or write standard output to a file.

To have program mypgm read from file inputfile rather than from the keyboard, "redirect its input" using this command:
```
	mypgm < inputfile
	
```
To have program mypgm write to file outputfile rather than from the keyboard, "redirect its output" using this command:
```
	mypgm > outputfile
	
```
You can redirect both input and output, for example with this command:
```
	mypgm < inputfile > outputfile
	
```

Input redirection can be useful in program testing (so you don't have to type in your test data over and over). Output redirection can be useful in capturing program output for printing.

Command-line shortcuts

Note: Whether the following tips work depends to some extent on how your environment is set up (what shell you are using -- shells were discussed briefly in class). If they don't work for you, talk to your instructor or a more knowledgeable user to see what might be wrong. But try them -- you can save yourself much typing if you know some of these shortcuts.

To clear the command line (e.g., if you've started typing and completely made a mess), type control-U.
To repeat the same command you just typed, type !!. Depending on how your environment is set up, it will probably be possible to repeat other previously-typed commands as well. Try typing the up arrow or control-P at the command prompt; it should put the previous command on the command line, with the cursor at the end. Typing another up arrow will then get the command before that, and so forth. (The down arraow, or control-N, should cycle back through the commands in the other order.) At any time, you can press ENTER to repeat the command, use the arrow keys and backspace to edit it, or control-U to clear the line.
When you're typing a filename, if you've typed enough of the name to uniquely determine it, if you press the TAB key the system should fill in the rest of the name. If there are several files starting with the characters you typed in, the system will fill in as much as it can and then beep and wait for you to type some more.
Example: ls produces the following output:
```
	        program1.c
	        program2.c
		another_program.c
	
```
To edit another_program.c, you can type vi an and then press the TAB key. The system should fill in the rest of the file name another_program.c on the command line; you can then press ENTER to execute the command. To edit one of the other C files, you might type vi p and press the TAB key. The system should then fill in as much of the name as it can and beep. You will see
```
		vi program
	
```
on the command line. You can then enter the rest of the name (1.c or 2.c) and press ENTER to execute the command. (Or you can just type the 1 or 2 and press TAB again to let the system fill in the rest of the name.)

Customizing your environment

Unix permits you to customize your environment extensively. The following assumes that you have started with the default setup on the CS lab machines.

Adding command aliases

You can add aliases (short names) for commands you use frequently by modifying the file .bashrc in your home directory. You can even redefine existing commands. For example, inserting the following lines:

alias cp='cp -i'
alias mv='mv -i'
alias rmi='rm -i'
alias l='ls -alF'

redefines the cp and mv commands to always use the -i option, and defines new commands rmi (rm with the -i option) and l (ls with options -alF). (Note that ls -alF is the same as ls -a -l -F.)

File security

By default the files you create on the lab machines should be readable and writable only by you. File security and how to control it is briefly described in J. Oldham's tutorial on Unix file security. A way to check that no one else can access your files is to have a friend try; if he/she gets the message Permission denied, all is well.

CSCI 1320 (Principles of Algorithm Design I): Unix Hints and Tips (Command Line Interface)