Red Hat [Electronic resources] : The Complete Reference Enterprise Linux Fedora Edition؛ The Complete Reference نسخه متنی

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.

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
computers

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 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 .