Working with GPU Files and Directories

Marianne Aldridge (marianne.aldridge@ualberta.ca)
CNS Help Desk (492-9400)

Unix Command Syntax

The Unix command format specifies that the command -- the action you want to perform -- must be the first item typed on the command line. In some cases the command by itself is all that is required to perform a certain task, but in many other cases we want to use conditions (operands) to customize the command.

If you want to specify a command operand, type in the command, then hit the spacebar to put a space between the command and whatever else is to follow. Next type a hyphen (-), which tells Unix that there are command operands coming. Then type the operand(s) you want to use and press Enter or Return. For example, to list all files in a directory, including hidden files, you would type the ls command and the ­a operand, then press Enter or Return:

ls -a 

It is important to remember that Unix commands and operands are case-sensitive. If you don't type the commands or operands in the correct case, you'll get error messages like "command not found."

Directory Navigation

The files you store in your GPU account are organized in a hierarchical tree system of directories and subdirectories. Your "home" directory is where all the files and directories in your account space are stored. Some programs like Pine (electronic mail) and Tin (newsreader) create their own directories in which they store files. Keeping files organized and grouped into directories and subdirectories makes it much easier to find them later when you want to use them again.

To see where you currently are in your account's directory structure, use the "print working directory" command, as shown below, and then press Enter or Return:

pwd 

When you first log in to your GPU account, by default your working directory is your home directory. In the University of Alberta environment, a pwd command should return something like this for a home directory path:

/afs/ualberta.ca/home/j/d/jdoe 

If your GPU computing account (i.e., your CNS Computing ID) appears at the end of the string returned by pwd, you are in your home directory.

To change your current working directory, use the cd command followed by the name or pathname of the directory you want to change to, as in:

cd directory-name 

For instance, to change to a subdirectory named Mail, you would use the command:

cd Mail 

To move back one directory level, use the command:

cd .. 

To return to your home directory from anywhere on the GPU system, type the cd command without operands:

cd

Listing Files and Directories

To list the names of directories, files and other file information on your screen, use the list commands and operands shown below.

To see a simple list of the files in your current working directory, enter:

ls 

To see a list of all files, including hidden files, in your current working directory, type:

ls -a 

To see a list of all files in the current working directory, including detailed file information (permissions, owner, time and date stamps, etc.), enter:

ls -la 

Note the following command, which is useful for displaying long lists more than one screen long:

ls ­la | more 

This will cause your command output to be displayed one screen at a time. Hitting the spacebar will advance the display.

Renaming and Copying Files

In Unix, renaming a file is accomplished with the move command (mv), which moves the file contents from one filename to another. The old filename ceases to exist, and the file contents are found in the new filename. Here's an example:

mv old-filename new-filename 

When you copy a file in Unix, the two files will have identical contents, but different names. Then you can modify the contents of the new file, without disturbing the contents of the original. To make a copy of a file, use:

cp filename1 filename2

Removing Files and Directories

To remove a file, use the remove command (rm), as in:

rm filename 

When typing filenames, it's important to remember that you must type the filename exactly as it appears in the file list. If there are spaces or special characters in the name of the file that you want to remove, you will need to enclose the filename within double quotation marks. For instance, suppose you have a file named Everything I Need. To remove this file, the command would be:

rm "Everything I Need" 

Sometimes you might want to remove an entire directory. To remove an empty directory, use the remove directory (rmdir) command, as in:

rmdir directory-name 

If there are files in a directory that you want to remove, you must first change to the directory where the files reside. You can then remove all files in the directory one at a time using the rm command or you can remove the files all at once using the rm command and an asterisk wildcard character (more on wildcards below), as in:

rm * 

Both of the rm commands above will stop and prompt you for each filename, asking if you really want to delete that file. You'll have to type y to respond yes to each query, so if there are a large number of files in the directory, removing them can be tedious and time-consuming. There's a much easier way to remove all the files in a directory, but it's also much more dangerous. The following command will remove not only the directory you specify, but also all files and subdirectories in that directory:

rm -rf directory-name 

To use the command above, you must be one directory level above the directory that you want to delete -- that is, you must be able to see the directory name in the file list when you do a list command. Before using this command, be certain that you know exactly what directory you are in, and make sure that you can see the name of the directory you want to remove in the file list.

Warning: Never use wildcards with the rm -rf command!

Wildcard Characters

Unix allows the use of wildcard characters in its command strings. You can use a wildcard to represent a single character or a string of characters.

The most commonly used wildcard character is the asterisk (*). For example, suppose you use the ls command to list the files in your current working directory, and get a list like the following:

pic1.gif    pic5.gif

pic2.gif    puppy.gif

pic3.gif    hill.jpg

pic4.gif    dart.gif

Now suppose you want to delete all the files that begin with "pic." You can either use the rm (remove) command multiple times to remove the files one at a time, or you can use the rm command once by including an asterisk wildcard character so that the command is applied to all of the files at the same time, as in:

rm pic*.gif 

The above command will remove all five of the files you want to delete, and leave the rest of your directory intact.

Note that the command below will remove the files also, but will do it by removing all files in the directory which have the suffix .gif. Since you have other files with that suffix, this probably isn't something you want to do:

rm *.gif 

Whenever you use wildcards in Unix commands, it's a good idea to specify as much of the filename as possible, to better tailor the number of files the command will be applied to.

Warning: Never use the command rm *.* unless your intent is to remove every file in the current working directory!

Viewing File Contents

The Unix more command will display the contents of the specified filename, as in:

more filename 

Now press Enter or Return to advance the display of the file by one line. Press the Spacebar to advance the display by one screen at a time. Press the forward slash (/) followed by a pattern to search for a matching pattern. Press b to display the previous screen. Press h to display a help screen and press q to quit the display.

Another neat trick to know about is Tab completion of filenames. Most Unix systems support this feature, and it's really helpful when you have to type long filenames or filenames with spaces or special characters. To use this feature with the more command, type part of the filename and then press the Tab key to complete the filename. For example, suppose you want to list the contents of a file named This is a test. There are spaces in the filename, so you know you must enclose the filename in double quotes. Type:

more "Th 

Press the Tab key, and the following will display:

more "This is a test" 

To display the contents of This is a test, just hit Enter or Return.

One function of the Unix cat command is to display the contents of a file. To show the file contents one screen at a time, use the cat command with the | more suffix, as in:

cat filename | more 

Now press the Spacebar to advance the display by one screen at a time. Press Enter or Return to advance the display of the file by one line at a time. Press b to display the previous screen and press q to quit the display.

By default, the Unix head command displays the first 10 lines of the specified file(s), as in:

head filename filename2 

The head command can also display a different number of initial lines of a file (such as 25 lines), as in:

head ­25 filename filename2 

By default, the Unix tail command displays the last 10 lines of the specified file(s), as in:

tail filename filename2 

The tail command can also display a different number of ending lines of a file (such as 25 lines), as in:

tail ­25 filename filename2 

The Unix page (pg) command displays file contents one screen at a time, and allows you to go forward and backwards in the file. Here's an example:

pg filename filename2 

Now, at the colon prompt (:), use any of the following:

• Press Enter or Return to display the next screen.
• Type +n and press Enter or Return to move forward n screens.
• Type ­n and press Enter or Return to move backward n screens.
• Type h and press Enter or Return to display a help screen.

Editing a File

The following command opens the specified file in a Pico editor window:

pico filename 

Pico commands are executed by holding down the CTRL key and pressing the letter key associated with the function you want to perform. The table below shows a number of Pico's keystroke combination commands.

Command History File

On GPU, a hidden file named .history is automatically created and stored in your home directory. Its function is to hold your command history -- the last 40 commands you entered at the GPU system prompt. The contents of the .history file are saved from one GPU session to the next.

If you know that a command you want to use is in your command history file, you can save yourself typing it out (useful for long commands). Just go to the GPU Unix prompt and use your up and down arrow keys to navigate through the commands in the .history file. Each time you press an arrow key another command from the history list will appear at the GPU prompt. When you see the command you want, simply press Enter or Return to execute it. You can also edit a command retrieved from the history list before executing it.

Online Help with Unix Commands

All Unix systems have online manual pages, known to Unix geeks as "man pages." To find the associated man page information for any given command, use the man command as shown below:

man command-name