Listing, Displaying, and Printing Files: ls, cat, more, less, and lpr

One of the primary functions of an operating system is the management of files. You may need to perform certain basic output operations on your files, such as displaying them on your screen or printing them. The Linux system provides a set of commands that perform basic file-management operations, such as listing, displaying, and printing files, as well as copying, renaming, and erasing files. These commands are usually made up of abbreviated versions of words. For example, the ls command is a shortened form of "list" and lists the files in your directory. The lpr command is an abbreviated form of "line print" and will print a file. The cat, less, and more commands display the contents of a file on the screen. Table 10-2 lists these commands with their different options. When you log in to your Linux system, you may want a list of the files in your home directory. The ls command, which outputs a list of your file and directory names, is useful for this. The ls command has many possible options for displaying filenames according to specific features.

Table 10-2: Listing, Displaying, and Printing Files
Command or Option	Execution
ls	This command lists file and directory names.
cat filenames	This filter can be used to display a file. It can take filenames for its arguments. It outputs the contents of those files directly to the standard output, which, by default, is directed to the screen.
more filenames	This utility displays a file screen by screen. Press the SPACEBAR to continue to the next screen and q to quit.
less filenames	This utility also displays a file screen by screen. Press the SPACEBAR to continue to the next screen and q to quit.
lpr filenames	Sends a file to the line printer to be printed; a list of files may be used as arguments. Use the -P option to specify a printer.
lpq	Lists the print queue for printing jobs.
lprm	Removes a printing job from the print queue.

Displaying Files: cat, less, and more

You may also need to look at the contents of a file. The cat and more commands display the contents of a file on the screen. cat stands for concatenate.

$ cat mydata
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The cat command outputs the entire text of a file to the screen at once. This presents a problem when the file is large because its text quickly speeds past on the screen. The more and less commands are designed to overcome this limitation by displaying one screen of text at a time. You can then move forward or backward in the text at your leisure. You invoke the more or less command by entering the command name followed by the name of the file you want to view (less is a more powerful and configurable display utility).

$ less mydata

When more or less invoke a file, the first screen of text is displayed. To continue to the next screen, you press the F key or the SPACEBAR. To move back in the text, you press the B key. You can quit at any time by pressing the Q key.

Printing Files: lpr, lpq, and lprm

With the printer commands such as lpr and lprm, you can perform printing operations such as printing files or canceling print jobs (see Table 10-2). When you need to print files, use the lpr command to send files to the printer connected to your system. See Chapter 25 to learn more about printing. In the next example, the user prints the mydata file:

$ lpr mydata

If you want to print several files at once, you can specify more than one file on the command line after the lpr command. In the next example, the user prints out both the mydata and preface files:

$ lpr mydata preface

Printing jobs are placed in a queue and printed one at a time in the background. You can continue with other work as your files print. You can see the position of a particular printing job at any given time with the lpq command. lpq gives the owner of the printing job (the login name of the user who sent the job), the print job ID, the size in bytes, and the temporary file in which it is currently held.

If you need to cancel an unwanted printing job, you can do so with the lprm command. lprm takes as its argument either the ID number of the printing job or the owner's name. lprm then removes the print job from the print queue. For this task, lpq is helpful, for it provides you with the ID number and owner of the printing job you need to use with lprm.