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Use Unix online documentation (man pages) | |
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| © February 8, 1996 | B-10 | |
Unix systems have an online documentation feature called man pages. The man command displays information from the official Unix reference manuals (man is short for manual).
You can view the man page entry for a specific Unix command or program, or search all the man pages for keywords you specify.
The basic use of man is straightforward. Type man, followed by the command you're interested in. For example, to find out about the tail command, type
man tail
The man page entry for tail is shown below.
man page for the tail command |
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TAIL(1)
NAME
tail - deliver the last part of a file
SYNOPSIS
tail [ +[number][lbc[f] ] ] [ file ]
DESCRIPTION
tail copies the named file to the standard output beginning at a
designated place. If no file is named, the standard input is used.
Copying begins at distance +number from the beginning, or -number from
the end of the input (if number is null, the value 10 is assumed).
Number is counted in units of lines, blocks, or characters, according to
the appended option l, b, or c. When no units are specified, counting is
by lines.
With the -f (``follow'') option, if the input file is not a pipe, the
program will not terminate after the line of the input file has been
copied, but will enter an endless loop, wherein it sleeps for a second
and then attempts to read and copy further records from the input file.
Thus it may be used to monitor the growth of a file that is being written
by some other process. For example, the command:
tail -f fred
will print the last ten lines of the file fred, followed by any lines
that are appended to fred between the time tail is initiated and killed.
As another example, the command:
tail -15cf fred
will print the last 15 characters of the file fred, followed by any lines
that are appended to fred between the time tail is initiated and killed.
SEE ALSO
dd(1M).
BUGS
Tails relative to the end of the file are stored in a buffer, and thus
are limited in length. Various kinds of anomalous behavior may happen
with character special files.
WARNING
The tail command will only tail the last 256 Kbytes of a file regardless
of its line count.
Page 1 Release 3.3.2 May 1990
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If the man page entry is longer than can be displayed on the screen, press the spacebar to see the next screenful. If you don't want to look at the whole entry, type q to interrupt the display.
man page entries are displayed using the more command. See how-to B-7, Examine Unix Files and Redirect Output, for additional information about more, including how to search for words within a man page entry.
At first (and even second) glance, the online manual pages may seem intimidating. They are written for experienced Unix users and programmers, and contain complete information on all the options and associated files for a command.
Here is a brief description of the components of a typical man page:
The first line of a man entry shows the command or utility's name, followed by a number in parentheses. This number refers to the section of the manual where the entry is located (see the section Organization of man below).
The abundance (and often cryptic nature) of Unix commands sometimes makes it difficult to remember which command you want. You can ask Unix to search through the man pages looking for a particular keyword.
For example, suppose you want to set permissions for a file but can't remember the name of the command to do this. To search the man pages for the keyword permission, type
man -k permission
Unix performs the search and displays:
| chmod (1) | - change the permissions mode of a file or directory
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| uucheck (1M) | - check the uucp directories and permissions file
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| writem (3G) | - grants write permission to bitplanes |
The command you were trying to remember is chmod. Other commands relating to permissions are also listed. (how-to B-6, Permit Unix Files and Directories, describes chmod.)
Sometimes you can remember a command name but not exactly what it does. To display just the NAME section of a man entry, type whatis followed by the command you're interested in. For tail, type
whatis tail
tail (1) - deliver the last part of a file
The online manual is divided into several sections as follows:
| 1 | commands and basic utilities |
| 2 | system services and error numbers |
| 3 | subroutines and libraries |
| 4 | file formats used or read by various programs |
| 5 | miscellany - macro packages, character set tables, etc. |
| 6 | games and demos |
| 7 | special files - hardware peripherals and device drivers |
To find out what a particular section contains (section 4 in this case), type
man 4 introSometimes man entries for the same command are found in two different sections. For example, the command chmod has man page entries in section 1 and section 2. Using the basic form of the man command (man chmod) will display the entry in section 1 followed by the entry in section 2. To see the entry in only one section, type the section number before the name of the command. To see only the section 1 entry for chmod, type
man 1 chmodTo print a man entry (for example, the one for chmod), type
man chmod | lpr
(The command lpr begins with a letter l, not a number 1.) The "|" (vertical bar) is the Unix pipe symbol, described in how-to B-7, Examine Unix Files and Redirect Output.
The -t option on the man command invokes the troff text processor to format man entries in a "typeset" style. However, man -t can only be used to format man entries for locally-added Unix programs like mpage and msgs, not for standard Unix commands like chmod and more.
To print the man entry for mpage,
type
man -t mpage
The output will automatically be sent to the default printer queue; lpr is not needed in this case.
Man pages contain some unprintable characters, so typing
man chmod > chmod.man man chmod | col -bx > chmod.man
For more on the man command, type (you guessed it! :-)
man man
