Command Line (Shell) Interface

The ssh ("secure shell") protocol provides a text-based session to a remote host, with no graphics and no mouse support. Interaction with the system consists of typing commands and viewing the system's responses in a terminal window.

(Modern Unix and Linux systems also provide a graphical user interface with mouse support via the X Window protocol. It's overkill for our purposes, but if you already know how to use it and are set up for it, feel free to do so.)

Linux and other Unix-like platforms provide a rich environment with many applications and development tools. Hence there are hundreds of commands available to the user. However, for our purposes, all you need to know for Perl programming are a few commands for managing files.

Only the most common modes of usage are described here. Most of the commands have a number of options that affect their behavior. For full details, use the man command. Many commands also support a --help option that displays a synopsis of the available options.

logout
Logs you off and disconnects you from the system. On most platforms, exit and ctrl-D does the same thing.
ls
Lists the files in your current directory. With a -l argument, gives a more detailed listing of file properties. The form ls DIR, where DIR is a directory, lists the files in that directory.
rm FILE1 FILE2 ...
Deletes the named files. Warning #1: Use with caution! You might not be prompted for confirmation, and once a file is gone, it's gone. Warning #2: Do not type rm *. The * character is a "wildcard" that matches every filename (except for files whose names begin with "."), so this will delete all your non-hidden files. To be prompted for confirmation, run rm with the -i option.
more FILE
Displays the contents of a file, one screenful at a time. Hit the spacebar to advance to the next screenful. Type q to abort the listing, h or ? for additional options.

The more command can be used as a filter to paginate the output of other programs. For example, type last | more to see the output of the last command one screen at a time.

cp FILE1 FILE2
cp FILES DIR
The first form copies FILE1 to FILE2. The second form makes new copies of all files in the list in the specified directory.
mv FILE1 FILE2
mv FILES DIR
The first form renames ("moves") FILE1 to FILE2. The second form moves each file in the list to the specified directory.
cd DIR
Changes your current working directory to the one specified. The cd command without arguments returns you to your "home" directory.
mkdir DIR
Creates a directory. This will fail if the directory exists.
rmdir DIR
Removes the directory. To succeed, the directory must be empty.
pico FILE
Runs the pico text editor, which allows you to modify the specified file (and create it if it doesn't already exist). Note: Pico is a full-screen editor. No mouse support, but you should be able to use the arrow keys to move the cursor around, and the PageUp and PageDown keys to scroll. Pico displays a command menu at the bottom of the screen. The notations like ^X that you see there denote control characters, generated by holding down the Ctrl key while you type the character. One of the Pico commands will give you online help.

Note: The pico editor is easy to learn and should be just fine for the relatively short Perl programs you'll be writing, but its limited functionality make it unsuitable for medium to large scale projects. For that, you should learn to use one or both of the standard editors vi and emacs.

perl OPTIONS FILES
Runs the Perl interpreter, where the files contain Perl source code. A useful option is -w ("warnings"), which will cause the interpreter to print warning messages about what it thinks is questionable code.

Note: Another way to run Perl programs is described in a later section of these notes.

man COMMAND
Displays online documentation ("manual entry") about the given command, including a list of all possible arguments and their effects. For example, man ls gives detailed documentation on the ls command, and man man displays documentation on the man command itself.
info COMMAND
Similar to man, but presents documentation in a more structured format. Some commands have more detailed info entries than man entries; on the other hand, some don't have info entries at all. Try both. Type info info for instructions on how to use the info facility.
mail
The basic Unix mail program. You can use it to read and send email messages.
pine
A more full-featured, screen-oriented email program.
who
Shows who is currently logged in.
last
Shows who's logged in recently, and the times they logged in and out.

When in terminal mode, you are communicating with the shell and applications through a piece of software called a terminal controller. Normally, the controller simply passes on keys that you press to the application that you are running. However, certain keys are intercepted by the controller and produce special effects:

ctrl-D
Produces an EOF (end-of-file) condition on keyboard input. Warning: When typed at the shell command prompt, this might have the effect of logging you off and disconnecting you.
ctrl-C
Sends a terminate signal to the application. Normally, this causes the program to terminate. This is a good way to abort a program that is caught in an infinite loop.
ctrl-Z
Sends a suspend signal to the application. This suspends the application but does not terminate it. The application can be resumed later using the fg command or by other means. Warning: The system will resist logging you off if you have suspended tasks. (You'll see the message ""There are stopped jobs.") You should exit from any stopped jobs before logging off.
Backspace
Moves the cursor one space to the left, erasing the last character typed. (Note: Depending on the ssh client you are running, you might need to type some other key to get this effect. Possibilties include delete, ctrl-delete, and ctrl-backspace.)
ctrl-U
Erases the current line of input, moving the cursor to the beginning of the line.
ctrl-W
Erases the last word typed, moving the cursor to the beginning of the word.

Caveat: The above describes the default behavior of these keys. But programs can modify the effects of any of the above keys. Hence these keys may have different effects in different applications. In particular, some of them work differently in the pico editor.

Basically, to run a Perl program, first create the source file using a text editor such as pico. Then run the Perl interpreter with the source file as input. This second step can be done in two ways:

  1. Run the perl command with the source file as argument. This method was described earlier.
  2. Make the first line of the source file a "shebang" line that looks like this: 
                    #!/usr/bin/perl
    Change the mode of the source file to "executable". Then type ./FILENAME (replacing FILENAME with the name of your source file) to run the program. This method is described in detail in the first two sections of Nik Silver's Perl Tutorial. Note that for our platform, the correct shebang line is the one given above, not the one given in the tutorial. Also, if you want warnings always to be enabled (probably a good idea), use the shebang line 
                    #!/usr/bin/perl -w

You may find it useful to name your Perl source files to end in .pl, to distinguish them from other types of files, but this is not required. Perl will accept any filename.