发信人: netiscpu (说不如做), 信区: Linux
标 题: [B] Red Hat Linux Unleashed (7)
发信站: 紫 丁 香 (Sat Jul 25 03:09:19 1998), 转信
Basic Linux Commands and Utilities
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o How Linux Commands Work
# Command Options
# Other Parameters
# Input and Output Redirection
o Notational Conventions Used to Describe Linux Commands
o Online Help Available in Linux
o The Linux Man Pages
# Finding Keywords in Man Pages
# The bash Shell help Facility
o Wildcards * and ?
o Environment Variables
o Processes and How to Terminate Them
# The Process Status Command ps
# The Process Termination Command kill
o Becoming Someone Else The su Command
o The grep Command
o Summary
_________________________________________________________________
7
Basic Linux Commands and Utilities
In this chapter, we will discover the following:
* How to modify the basic function of Linux commands by using
command options
* How to run two or more Linux commands in tandem by using input and
output re-direction
* How to use parameters, such as filenames, with Linux commands
* How to read and understand the notational shorthand used in Linux
and UNIX documentation
* How to use Linux online man pages and help facilities
* How to use wildcards that fill in for one or more filenames
* How to check your environment variables
* How to list processes running on the Linux system
* How to kill processes
* How to temporarily become another user
* How to use grep (and understand what grep means!)
How Linux Commands Work
Most Linux commands are very flexible. When you enter a Linux command,
there are several ways to tailor the basic command to your specific
needs. We will look at the two main ways used to modify the effect of
a command:
* Specifying or redirecting a command's input and output
* Using command options
A simple way to picture what a Linux command does is to imagine that
it's a black box that is part of an assembly line. Items come down a
conveyor belt, enter the black box, get processed in some way, come
out of the black box, and are taken away on another conveyor belt.
Command options let you fine-tune the basic process happening inside
the black box. Command redirection lets you specify which conveyor
belt will supply the black box with items and which conveyor belt will
take away the resulting products.
Once you understand how redirection and command options work, you will
be able to (at least in principle) use any Linux or UNIX command. This
is because UNIX was based on a few simple design principles. Commands,
therefore, should work in consistent ways. Of course, UNIX has grown
and changed over the years, and the design principles can sometimes
get buried under all the changes. But they still make up the
foundation, so that UNIX-based systems such as Linux are quite
coherent and consistent in how they work.
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NOTE: Pressing Ctrl-U at any point, right up to before you press
Enter, lets you clear everything you've typed on the command line.
You can use this whenever you spot an error at the very beginning
of your typing, or when you decide you don't want to run a
particular command after all. You can also use the Backspace key to
"back up" by erasing characters (in fact, it can be almost a reflex
action), but it's usually faster to just erase the whole command
line and start again.
Perhaps the most powerful keys to use at the command prompt are the
arrow keys. The left and right arrows move the cursor
non-destructively. If you make a typo early in the line, you can
left-arrow your way to the character and type in a correction.
Additionally, the up and down arrows enable you to jump through a
list of the last several commands used (similar to DOS's DOSKEY
utility).
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Command Options
You can use command options to fine-tune the actions of a Linux
command. Quite often, a Linux command will do almost—but not
quite—what you want it to do. Instead of making you learn a
second command, Linux lets you modify the basic, or default, actions
of the command by using options.
The ls command is an excellent, and useful, example of a command that
has a great many options. The ls command lists the files found on the
Linux system's hard drive. This sounds simple enough, doesn't it? Try
entering the command
darkstar:~$ ls
darkstar:~$
Well, nothing much seemed to happen.
Now try typing ls -a. Type it exactly as listed. The space between ls
and -a is necessary, and there must be no space between the - and the
a.
darkstar:~$ ls -a
./ .bash_history .less .term/
../ .kermrc .lessrc
What you have done is modified what ls does by adding a command
option—in this case, -a. By default, ls lists only files whose
names don't begin with a period. However, -a tells ls to list all
files, even ones that begin with a period. (These are usually special
files created for you by Linux.) At present, all the files in your
directory start with a period, so ls by itself does not list any
files; you must add -a to see the files you have at present.
The ls command has many more options. You can use more than one option
at a time. For example, try typing ls -al:
darkstar:~$ ls -al
total 10
drwxr-xr-x 3 fido users 1024 Dec 21 22:11 ./
drwxr-xr-x 6 root root 1024 Dec 14 01:39 ../
-rw-r—r— 1 fido users 333 Dec 21 22:11 .bash_history
-rw-r—r— 1 fido users 163 Dec 7 14:31 .kermrc
-rw-r—r— 1 fido users 34 Jun 6 1993 .less
-rw-r—r— 1 fido users 114 Nov 23 1993 .lessrc
drwxr-xr-x 2 fido users 1024 Dec 7 13:36 .term/
You now get a listing with many more details about the files. (These
will be explained in Chapter 8, "The Linux File System.") The l option
can be used by itself; ls -l will give detailed descriptions of files
that don't begin with a period. Sometimes filenames are so long they
don't fit on a single line. Linux simply wraps the remainder to the
next line.
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NOTE: Strictly speaking, the dash (-) is not part of the command
option. The dash simply tells Linux to understand each letter
immediately following it as a command option. There must be a space
before the dash, and there must not be a space between the dash and
the letter or letters making up the command option. There must be a
space after the command option if anything else is to be entered on
the command line after it.
You can type more than one command option after the dash, as we did
with ls -al. In this case, we are specifying both the a and the l
options. The order you specify options in usually doesn't matter;
ls -al will give the same results as ls -la. Combining options
doesn't work with all Linux commands, and then only with those that
use a single letter to specify each option.
Multiple options can also be specified individually, with each
option preceded by a dash and separated from other options by
spaces—for example, ls -a -l. This is usually done only when a
particular option requires a further parameter.
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By default, ls lists files in alphabetical order. Sometimes you might
be more interested in when a file was created or last modified. The t
option tells ls to sort files by date instead of alphabetically by
filename, showing the newest files first. Therefore, typing ls -alt
gives
darkstar:~$ ls -alt
total 10
drwxr-xr-x 3 fido users 1024 Jan 2 13:48 ./
-rw-r—r— 1 fido users 333 Dec 21 22:11 .bash_history
drwxr-xr-x 6 root root 1024 Dec 14 01:39 ../
-rw-r—r— 1 fido users 163 Dec 7 14:31 .kermrc
drwxr-xr-x 2 fido users 1024 Dec 7 13:36 .term/
-rw-r—r— 1 fido users 114 Nov 23 1993 .lessrc
-rw-r—r— 1 fido users 34 Jun 6 1993 .less
The r option tells ls to produce a reverse output. This is often used
with the t option. The following is an example of what you might get
if you entered ls -altr:
darkstar:~$ ls -altr
total 10
-rw-r—r— 1 fido users 34 Jun 6 1993 .less
-rw-r—r— 1 fido users 114 Nov 23 1993 .lessrc
drwxr-xr-x 2 fido users 1024 Dec 7 13:36 .term/
-rw-r—r— 1 fido users 163 Dec 7 14:31 .kermrc
drwxr-xr-x 6 root root 1024 Dec 14 01:39 ../
-rw-r—r— 1 fido users 333 Dec 21 22:11 .bash_history
drwxr-xr-x 3 fido users 1024 Jan 2 13:48 ./
Many other options can be used with ls, although we have now tried the
most commonly used ones. The important thing to remember is that you
can usually customize a Linux command by using one or more command
options.
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NOTE: As with basic Linux commands, case is important! For
instance, ls has an R option (recursive: show files in
subdirectories, too) that gives much different results from the r
option.
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NOTE: You can think of a as the "all files" option, l as the "long
list" option, t as the "sort by time" option, r as the "reverse
sort" option, and so on. In fact, most options in Linux are
mnemonic—the option letter stands for a word or phrase. Some
option letters mean the same thing in many different Linux
commands. For instance, v often means verbose—in other words,
"Give me lots of detail."
However, do not assume that, on any unfamiliar command, certain
options will work in the "usual" way! For instance, r is the
recursive option for many Linux commands; however, in the case of
ls, reverse sort is more commonly used, and therefore it gets the
easier-to-type lowercase r, while recursive is left with the
capital R. It might seem like not much extra effort to press the
Shift key to get the capital letter, but try typing a string of
four or five options, one of which is capitalized!
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______________________________________________________________
NOTE: You can easily find out which options are available for any
Linux command by using the man command. See the section "The Linux
Man Pages" later in this chapter.
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Other Parameters
Linux commands often use parameters that are not actual command
options. These parameters, such as filenames or directories, are not
preceded by a dash.
For instance, by default ls lists the files in your current directory.
You can, however, tell ls to list the files in any other directory
simply by adding the directory to the command line. For instance, ls
/bin will list everything in the /bin directory. This can be combined
with command options, so that ls -l /bin gives you detailed listings
of the files in /bin. Try this. You will be impressed by the number of
files in the /bin directory!
You can also specify ls to list information about any particular file
by entering its filename. For instance, ls -la .lessrc gives detailed
information only about the .lessrc file.
Input and Output Redirection
Many Linux commands let you specify which file or directory they are
to act upon, as we saw with the example ls -l /bin earlier.
You can also "pipe" the output from a command so that it becomes
another command's input. This is done by typing two or more commands
separated by the | character. (This character normally is found on the
same key as the \ character. You must hold down the Shift key or you
will get \ instead of |). The | character means "Use the output from
the previous command as the input for the next command." Therefore,
typing command_1|command_2 does both commands, one after the other,
before giving you the results.
Using our assembly-line metaphor, we are processing items through two
black boxes instead of just one. When we use piping, it's like hooking
up the first command's output conveyor belt to become the input
conveyor belt for the second command.
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NOTE: Although Linux doesn't care whether | is set off by spaces,
if command_1 | command_2 is easier for you to read and understand
than command_1|command_2, by all means use spaces around |.
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You will have noticed that the output of ls -l /bin is many lines
long, so that much of the information scrolls off the screen before
you can read it. You can pipe this output to a formatting program
called more, which displays information in screen-sized chunks. When
you enter ls -l /bin | more, you will see the following:
darkstar:~$ ls -l /bin | more
total 1611
-rwxr-xr-x 1 root bin 1248 Sep 17 04:25 arch*
-rwxr-xr-x 1 root bin 295940 Sep 5 01:45 bash*
-rwxr-xr-x 1 root bin 4840 Nov 24 1993 cat*
-rwxr-xr-x 1 root bin 9220 Jul 20 12:06 chgrp*
-rwxr-xr-x 1 root bin 13316 Jul 20 12:06 chmod*
-rwxr-xr-x 1 root bin 13316 Jul 20 12:06 chown*
lrwxrwxrwx 1 root root 17 Dec 7 13:37 compress -> /usr/bin/comp_ress*
-rwxr-xr-x 1 root bin 21508 Jul 20 12:06 cp*
-rwxr-xr-x 1 root bin 41988 May 1 1994 cpio*
lrwxrwxrwx 1 root root 4 Dec 7 13:40 csh -> tcsh*
-rwxr-xr-x 1 root bin 5192 Nov 24 1993 cut*
-rwxr-xr-x 1 root bin 19872 Mar 23 1994 date*
-rwxr-xr-x 1 root bin 17412 Jul 20 12:06 dd*
-rwxr-xr-x 1 root bin 13316 Jul 20 12:06 df*
-rwxr-xr-x 1 root bin 66564 Jun 9 1994 dialog*
-rwxr-xr-x 1 root bin 1752 Sep 17 04:25 dmesg*
lrwxrwxrwx 1 root root 8 Dec 7 13:37 dnsdomainname -> hostname*
-rwxr-xr-x 1 root bin 13316 Jul 20 12:06 du*
-rwxr-xr-x 1 root bin 3312 Mar 23 1994 echo*
-rwxr-xr-x 1 root bin 36684 May 4 1994 ed*
-rwxr-xr-x 1 root bin 326 Mar 23 1994 false*
—More—
The —More— at the bottom of the screen tells you that
there's more text to come. To go to the next screen of text, press the
spacebar. Every time you press the spacebar, more displays another
screenful of text. When the last screenful of text has been displayed,
more returns you to the Linux prompt.
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NOTE: The more command can do many other things. For instance, to
move back one screen at a time, type b for "back." Another useful
command is q for "quit." This lets you leave immediately, without
having to go through all the remaining screens of text.
While in more, type h for "help." This will list the commands
available within more.
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______________________________________________________________
NOTE: The Linux system sometimes uses the command less instead of
more. One difference you will notice is that, unlike more, less
requires you to type q to return to the command line, even if
you're at the end of the text to be displayed. This might seem
cumbersome, but it prevents you from accidentally exiting the
program by pressing the spacebar once too often.
The name less is a play on more. Originally, less was designed to
have many features that more lacked. The version of more included
in your Linux system has most of these features, however.
The Linux man program, discussed later, uses less to display text.
Most other UNIX systems use more by default. Don't get confused.
Remember to type q to exit from less!
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Another thing you can do in Linux is to send output to a file instead
of the screen. There are many different reasons why you might want to
do this. You might want to save a "snapshot" of a command's output as
it was at a certain time, or you might want to save a command's output
for further examination. You might also want to save the output from a
command that takes a very long time to run, and so on.
To send output to a file, use the > symbol (found above the period on
your keyboard). For instance, you can place the output of the ls -l
/bin command into a file called test by typing ls -l /bin > test.
Again, spaces around > are optional and not strictly necessary, but
they do make the command much more readable.
If you now do an ls or ls -l, you will see that you've created a new
file called test in your own directory.
To see the contents of a file, you can again use the more command.
Just specify the name of the file you want to look at. In this case,
you would type more test.
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NOTE: Be careful! When you use >, you completely overwrite the
previous contents of the file you specify to take the output (if
that file existed). For example, if we already had a file called
test in our directory, its old contents would be completely
replaced by the output from ls -l /bin. Linux will not warn you
that you are about to do this!
Be particularly careful if you're not in your usual directory, or
if you're logged in as root. You could, for instance, accidentally
clobber the Linux program test, which exists as a file named
test—fortunately, not in the directory where we created our
test file! It's a good idea to check if the output file already
exists before using >. In our example, we could have typed ls -l
test beforehand. If no information is displayed, the file does not
exist.
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You can specify that you want to add your output to the end of the
file, rather than replace the file's contents, by using >>. Type who
>> test to add the output of the who command to the end of the text
already in the file test.
You can examine the results by using either more or less and paging
through to the end of the file, or by using the Linux command tail,
which displays the last few lines of the specified file. In this case,
you would type tail test to see the last few lines of the file test.
Try using tail!
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NOTE: For a more detailed discussion of redirection and piping, see
Chapter 10, "Using bash."
______________________________________________________________
Notational Conventions Used to Describe Linux Commands
There is a set of accepted notational conventions used to describe, in
a concise and consistent way, the correct syntax for any given Linux
command. This specifies what options or parameters you must use, what
options or parameters you can use or not use, and so on. Sometimes
this set of conventions is used to give a complete and exhaustive
listing of a command's syntax, showing every possible command and
parameter. Sometimes it is used to make a particular example more
general and the command's basic usage clearer.
If you remember the following six basic rules, you will be able, in
principle, to understand the syntax of any Linux or UNIX command.
1. Any text standing by itself, and not within [], <>, or {}, must be
typed exactly as shown.
2. Any text within square brackets ([]) is optional. You can type it
or not type it. For instance, the syntax ls [-l] means you must
type ls (per the first rule), while adding -l is optional, but not
necessary. Do not type the square brackets themselves! In our
example, type ls or ls -l. Don't type ls [-l].
3. Angle brackets (<>) and the text within them must be replaced by
appropriate text (usually a name or value). The text within the
brackets usually indicates the nature of the replacement. For
instance, the syntax more <filename> means that you should replace
<filename> with the name of the file you wish to examine using
more. If you want to look at the file test, you would type more
test. Remember, do not use the angle brackets when you actually
type the command!
4. Curly braces ({}) indicate that you must choose one of the values
given within the braces. The values are separated by | (which in
this case means or, not pipe!). For example, the syntax command
-{a|b} means you must enter either command -a or command -b.
5. An ellipsis (...) means "and so on." It is normally used with
parameters such as filenames, as described later.
6. The sixth basic rule states that the brackets can be combined as
necessary. For instance, you don't have to type a filename with
the more command. This would be indicated as more [<filename>].
The outer set of square brackets makes the entire parameter
optional. If you do decide to use the parameter, replace the inner
set of angle brackets with the appropriate value. Because the more
command enables one or more filenames to be specified, the syntax
becomes more [<filename>...]. The ellipsis means you can have as
many <filenames> as you wish.
Online Help Available in Linux
Linux has help facilities available online. If you forget the exact
use of a command, or you're looking for the right command to use, the
answer might be available straight from Linux. The two help facilities
we will try out are the bash shell's help command, and the man
command, which is available on almost all UNIX systems, including
Linux.
______________________________________________________________
NOTE: If you have not installed the man pages package, you should
do so now. Although it is possible to get by without man pages,
they are a very valuable resource for both novice and expert Linux
users.
______________________________________________________________
The Linux Man Pages
The "man" in "man pages" stands for "manual." (As usual, the creators
of UNIX shortened a long but descriptive word to a shorter, cryptic
one!) Typing man <command> lets you view the manual pages dealing with
a particular command.
Try typing man passwd to see what the Linux manual has to say about
the passwd command.
The general layout of a man page is as follows:
COMMAND(1) Linux Programmer's Manual COMMAND(1)
NAME
command - summary of what command does
SYNOPSIS
<complete syntax of command in the standard Linux form>
DESCRIPTION
More verbose explanation of what "command" does.
OPTIONS
Lists each available option with description of what it does
FILES
lists files used by, or related to, command
SEE ALSO
command_cousin(1), command_spouse(1), etc.
BUGS
There are bugs in Linux commands??
AUTHOR
J. S. Goobly (goobly@hurdly-gurdly.boondocks)
Linux 1.0 22 June 1994 1
The man page for passwd is actually quite understandable. Be warned,
however, that man pages are often written in a very formal and
stylized way that sometimes bears little resemblance to English. This
is done not to baffle people, but to cram a great deal of information
into as short a description as possible.
For example, try man ls. Notice how many options are available for ls
and how long it takes to explain them!
Although it can take practice (and careful reading!) to understand man
pages, once you get used to them, the first thing you'll do when you
encounter a strange command is call up the man page for that command.
Finding Keywords in Man Pages
Sometimes you know what you want to do, but you don't know which
command you should use to do it. You can use the keyword option by
typing man -k <keyword>. The man program will return the name of every
command whose name entry (which includes a very brief description)
contains that keyword.
For instance, you can search on manual:
darkstar:~$ man -k manual
man (1) - Format and display the on-line manual pages
whereis (1) - Locate binary, manual, and or source for program
xman (1) - Manual page display program for the X Window System
You have to be careful to specify your keyword well, though! Using
directory as your keyword isn't too bad, but using file will give you
many more entries than you will want to wade through.
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NOTE: You might have noticed that commands seem to be followed by
numbers in brackets, usually (1). This refers to the manual
section. Back in the days when UNIX manuals came in printed, bound
volumes, normal commands were in Section 1, files used by
administrators were in Section 5, programming routines were
described in Section 3, and so on.
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______________________________________________________________
NOTE: Therefore, some man pages are not about commands at all, but
rather about files or system calls used in Linux!
If a particular entry shows up in more than one section, man will
show you the lowest-numbered entry by default. You can see
higher-numbered entries by specifying the section number. For
instance, Section 5 has a manual entry on the passwd file. To see
this rather than the manual entry for the passwd command type man 5
passwd.
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The bash Shell help Facility
When you type a command at the prompt, the shell program takes what
you've written, interprets it as necessary, and passes the result to
the Linux operating system. Linux then performs the actions requested
of it. Many Linux commands require Linux to find and start up a new
program. However, the shell itself can perform a number of functions.
These functions can be simple, often-used commands, so that the
overhead of starting up separate programs is eliminated, or they can
be facilities that make the shell environment friendlier and more
useful. One of these facilities is the help command, which provides
information on the bash shell's built-in functions.
Type help at the prompt. You will see at least some of the following:
GNU bash, version 1.14.6(1)
Shell commands that are defined internally. Type 'help' to see this list.
Type 'help name' to find out more about the function 'name'.
Use 'info bash' to find out more about the shell in general.
A star (*) next to a name means that the command is disabled.
%[DIGITS | WORD] [&] . [filename]
: [ arg... ]
alias [ name[=value] ... ] bg [job_spec]
bind [-lvd] [-m keymap] [-f filena break [n]
builtin [shell-builtin [arg ...]] case WORD in [PATTERN [| PATTERN].
cd [dir] command [-pVv] [command [arg ...]]
continue [n] declare [-[frxi]] name[=value] ...
dirs [-l] echo [-neE] [arg ...]
enable [-n] [name ...] eval [arg ...]
exec [ [-] file [redirection ...]] exit [n]
export [-n] [-f] [name ...] or exp fc [-e name] [-nlr] [first] [last
fg [job_spec] for NAME [in WORDS ... ;] do COMMA
function NAME { COMMANDS ; } or NA getopts optstring name [arg]
hash [-r] [name ...] help [pattern ...]
history [n] [ [-awrn] [filename]] if COMMANDS; then COMMANDS; [elif
jobs [-lnp] [jobspec ...] | jobs - kill [-s sigspec | -sigspec] [pid
let arg [arg ...] local name[=value] ...
logout popd [+n | -n]
pushd [dir | +n | -n] pwd
read [-r] [name ...] readonly [-n] [-f] [name ...] or r
return [n] select NAME [in WORDS ... ;] do CO
set [—abefhknotuvxldHCP] [-o opti shift [n]
source filename suspend [-f]
test [expr] times
trap [arg] [signal_spec] type [-all] [-type | -path] [name
typeset [-[frxi]] name[=value] ... ulimit [-SHacdmstfpnuv [limit]]
umask [-S] [mode] unalias [-a] [name ...]
unset [-f] [-v] [name ...] until COMMANDS; do COMMANDS; done
variables - Some variable names an wait [n]
while COMMANDS; do COMMANDS; done { COMMANDS }
You will have to pipe the output of help to more (help | more) to keep
the first part from scrolling off your screen.
Wildcards * and ?
In many a late-night card game, jokers are shuffled into the deck. The
jokers are wildcards that can become any card of your choice. This is
obviously very useful! Linux has wildcards also. They are, if
anything, more useful than jokers in a card game.
Linux has several wildcards. Wildcards are used as a convenient and
powerful shortcut when specifying files (or directories) that a
command is to operate on. We will briefly look at the two most popular
wildcards: * and ?.
The most commonly used wildcard is *, which stands in for any
combination of one or more characters. For example, c* will match all
filenames that begin with c. You can see this for yourself by typing
ls /bin/c*.
What happens if you type ls /bin/c*t? How about ls /bin/*t?
The ? wildcard is more restrictive than *. It only stands in for any
one character. You can see this by comparing ls/bin/d* with ls/bin/d?.
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NOTE: Wildcards can only be used to match filenames and directory
names. You can't, for example, type pass* at the Linux prompt and
expect Linux to run the passwd program for you.
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______________________________________________________________
NOTE: Be very careful when using wildcards with dangerous commands,
such as the ones used to permanently delete files! A good check is
to run ls with the wildcards you plan to use and examine the
resulting list of files to see if the wildcard combination did what
you expected it to do. Also double-check that you typed everything
correctly before pressing the Enter key!
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Environment Variables
When you log in, Linux keeps a number of useful data items in the
background ready for the system to use. The actual data is held in
something called an environment variable, whose name is often
descriptive or mnemonic. In fact, this is no different from the way
you and I remember things. We know that there always is a piece of
information called "day of the week" (the environment variable);
however, we change the data in this variable, from Monday to Tuesday
to Wednesday, and so on, as days go by.
To see the list of exported environment variables, type env. The
environment variable's name is on the left, and the value held by the
variable is on the right.
The most important variable to note is the PATH, whose value is your
search path. As we will see in the next chapter, when you type a
command, Linux will search every place listed in your search path for
that command.
A longer list of environment variables, consisting of several new
variables in addition to the ones you saw earlier, is displayed by the
command set. The new variables are local: they have not been marked
for export. For more information on exporting variables, see Chapter
10. You can think of local variables as items of information you need
for only a certain time or location. For instance, remembering the
variable "what-floor-am-I-on" becomes an unnecessary piece of
information once you leave the building!
Processes and How to Terminate Them
In the previous chapter, we learned about the who command, which shows
you the usernames of everyone who is logged into the system. The who
program actually gets its information from the Linux system, which
maintains and updates the list of the system's current users.
In fact, Linux keeps much more detailed records about what is
happening on the system than just who is logged in. Because Linux is a
multitasking system, in which many programs or program threads may be
running simultaneously, Linux keeps track of individual tasks or
processes.
Although these processes are usually well-behaved and well-managed by
Linux, sometimes they might go out of control. This can happen if a
program hits a bug or a flaw in its internal code or supplied data, or
if you accidentally enter the wrong command or command option.
Being able to identify these misbehaving processes, and then being
able to terminate or kill them, is an essential piece of knowledge for
all Linux/UNIX users. (Obviously the world was a less kind and gentle
place when the kill command was developed and named.) When you are
your own system administrator, as in our case, it's doubly important!
The Process Status Command ps
To find out what processes are running, we use the ps command. ps
stands for "process status," not the "post script" you would write at
the end of a letter.
Typing ps by itself gives you a concise listing of your own processes:
darkstar:~$ ps
PID TTY STAT TIME COMMAND
41 v01 S< 0:00 -bash
134 v01 R< 0:00 ps
The information in the first column, headed PID, is important. This is
the Process ID number, which is unique, and which Linux uses to
identify that particular process. You must know a process's PID to be
able to kill it.
The TTY column shows you which terminal the process was started from.
The STAT column gives the status of the process. The two most common
entries in the status column are S for sleeping and R for running. A
sleeping process is one that isn't currently active. However, don't be
misled. A sleeping process might just be taking a very brief catnap!
In fact, a process might switch between sleeping and running many
times every second.
The TIME column shows the amount of system time used by the process.
Clearly, neither of our processes are taking up any appreciable system
time!
Finally, the COMMAND column contains the name of the program you're
running. This will usually be the command you typed at the command
line. However, sometimes the command you type starts one or more child
processes, and in this case, you would see these additional processes
show up as well, without ever having typed them yourself. Your login
shell will have a - before it, as in -bash in the previous example.
This helps to distinguish this primary shell from any shells you might
enter from it. These will not have the - in front.
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NOTE: If you are logged in as root, you will see a list of all
processes on the system. This is because the root username, being
the superuser, owns everything that happens on the Linux system.
If you are an "ordinary" user, but have also logged in on another
terminal (including another virtual terminal you have selected by
pressing Alt-Fn as discussed in Chapter 6, "Getting Started with
Linux"), you will see the processes you are running on the other
terminal (or terminals) as well.
______________________________________________________________
One useful option with ps is u. Although it stands for "user," as in
"List the username as well," it actually adds quite a few more columns
of information in addition to just the username:
darkstar:~$ ps -u
USER PID %CPU %MEM SIZE RSS TTY STAT START TIME COMMAND
fido 41 0.1 6.8 364 472 v01 S< 23:19 0:01 -bash
fido 138 0.0 3.3 72 228 v01 R< 23:34 0:00 ps -u
In addition to the username in the USER column, other interesting new
items include %CPU, which shows you what percentage of your computer's
processing power is being used by the process, and %MEM, which shows
you what percentage of your computer's memory is being used by the
process.
If you want to see all processes running on the system, and not just
the processes started by your own username, you can use the a command
option. (The root login sees everyone's processes automatically and
does not have to use a, so root can get the following output by simply
typing ps.)
darkstar:~$ ps -a
PID TTY STAT TIME COMMAND
62 v03 S< 0:00 /sbin/agetty 38400 tty3
63 v04 S< 0:00 /sbin/agetty 38400 tty4
64 v05 S< 0:00 /sbin/agetty 38400 tty5
65 v06 S< 0:00 /sbin/agetty 38400 tty6
330 v02 S< 0:00 -bash
217 v01 S< 0:00 -bash
217 v01 S< 0:00 ps -a
As you can see, quite a few "other" processes are happening on the
system! In fact, most of the processes we see here will be running
whether or not anyone is actually logged into the Linux system. All
the processes listed as running on tty psf are actually system
processes, and are started every time you boot up the Linux system.
Processes of the form /sbin/agetty 38400 tty6 are login processes
running on a particular terminal waiting for your login.
It can be useful to combine the a and u options (if you're not root).
darkstar:~$ ps -au
USER PID %CPU %MEM SIZE RSS TTY STAT START TIME COMMAND
root 72 0.0 3.6 390 532 v01 S< 17:55 0:01 -bash
root 74 0.0 1.5 41 224 v03 S< 17:55 0:00 /sbin/agetty 38400 tty3
root 75 0.0 1.5 41 224 v04 S< 17:55 0:00 /sbin/agetty 38400 tty4
root 76 0.0 1.5 41 224 v05 S< 17:55 0:00 /sbin/agetty 38400 tty5
root 77 0.0 1.5 41 224 v06 S< 17:55 0:00 /sbin/agetty 38400 tty6
root 78 0.0 1.5 56 228 s00 S< 17:55 0:00 gpm -t mman
root 98 0.0 1.5 41 224 v02 S< 18:02 0:00 /sbin/agetty 38400 tty2
root 108 18.8 3.6 384 528 pp0 S< 18:27 0:01 -bash
A more technical l option can sometimes be useful:
darkstar:~$ ps -l
F UID PID PPID PRI NI SIZE RSS WCHAN STAT TTY TIME COMMAND
0 501 41 1 15 0 364 472 114d9c S< v01 0:00 -bash
0 501 121 41 29 0 64 208 0 R< v01 0:00 ps -l
The interesting information is in the PPID column. PPID stands for
"Parent Process ID"—in other words, the process that started the
particular process. Notice that the ps -l command was started by
-bash, the login shell. In other words, ps -l was started from the
command line. Notice also that the PPID for the login shell is PID 1.
If you check the output from ps -au shown previously, you will see
that the process with PID of 1 is init. The init process is the one
that spawns, or starts, all other processes. If init dies, the system
crashes!
______________________________________________________________
NOTE: The Linux ps command has some quirks when it comes to
options.
First of all, the dash before the options is not necessary. In the
earlier example, ps l would work the same as ps -l. Because most
Linux commands do require the use of dashes with their command
options, and other versions of UNIX might require dashes when using
ps, it's best to use the dash anyway.
Second, the order in which you enter the options does matter,
especially if you try to combine the l and u options! Try typing ps
-lu, and then ps -ul. This behavior is not covered in the ps man
page. The moral is twofold: First, use the minimum possible number
of command options. Second, the man pages are, alas, not always
correct and complete.
______________________________________________________________
The Process Termination Command kill
The kill command is used to terminate processes that can't be stopped
by other means.
______________________________________________________________
NOTE: Before going through the following procedure, if it's a
program you're stuck in, make sure you can't stop or exit it by
typing Ctrl-C or some other key combination.
1. Switch to another virtual console and log in as root.
2. Run ps -u and identify the offending process. You will use its PID
in the next step.
3. Use the kill program by typing kill <PID>, where PID is the
Process ID you wish to kill. Make sure that you have correctly
identified the offending process! As root, you can kill any user
process, including the wrong one if you misread or mistype the
PID.
4. Verify that the process has been killed by using ps -u again. You
can type ps -u <PID>, which shows you the status of only the
specified PID. If there's a null result and you're just given the
Linux prompt again, the PID is dead, so go to step 8. However,
it's best to look at the complete ps -u list if it's not too long.
Sometimes the offending process reappears with a new PID! If that
is the case, go to step 6.
5. If the process is still alive and has the same PID, use kill's 9
option. Type kill -9 <PID>. Check it as in step 4. If this does
not kill the process, go to step 7. If the process is now dead, go
to step 8.
6. If the offending process has reappeared with a new PID, that means
that it's being created automatically by some other process. The
only thing to do now is to kill the parent process, which is the
true offender! You might also have to kill the parent process when
kill -9 does not work.
7. Use ps -l to identify the troublesome process's PPID. This is the
PID of the parent process. You should check the parent's identity
more closely by typing ps -u <Parent PID> before going ahead and
killing it as described in step 3, using the PID of the parent in
the kill command. You should follow through with step 4 and, if
necessary, step 5, making sure the parent process has been killed.
8. The process is killed. Remember to log off. You should not leave
root logged in on virtual consoles, because you will forget that
the root logins are there!
NOTE: Sometimes processes are simply unkillable! In this case,
you're best off shutting down the Linux system and rebooting.
______________________________________________________________
Linux keeps ordinary users (as opposed to root) from killing other
users' processes (maliciously or otherwise). For instance, if you are
an ordinary user and you try to kill the init process, which always
has PID=1, you will see
darkstar:~$ kill 1
kill: (1) - Not owner
Actually, not even root can kill the init process, although there is
no error message. The init process is one of those "unkillable"
processes discussed earlier, because it's such a key process. That's
all for the best!
Becoming Someone Else The su Command
Usually, when you want to temporarily become a different user, you
will simply switch to another virtual terminal, log in as the other
user, log out when you're done, and return to your "home" virtual
terminal. However, there are times when this is impractical or
inconvenient. Perhaps all your virtual terminals are already busy, or
perhaps you're in a situation (such as logged on via a telephone and
modem) in which you don't have virtual terminals available.
In these cases, you can use the su command. "su" stands for "super
user." If you type su by itself, you will be prompted for the root
password. If you successfully enter the root password, you will see
the root # prompt, and you will have all of root's privileges.
You can also become any other user by typing su <username>. If you are
root when you type su <username>, you are not asked for that user's
password since in principle you could change the user's password or
examine all the user's files from the root login anyway. If you are an
"ordinary" user trying to change to another ordinary user, you will be
asked to enter the password of the user you are trying to become.
______________________________________________________________
NOTE: Although su grants you all the privileges you would get if
you logged on as that user, be aware that you won't inherit that
login's exact environment or run that login's startup files (if
any). This means that su is not really suited to doing extended
work, and it's quite unsuitable for troubleshooting problems with
that login.
______________________________________________________________
The grep Command
"What on earth does grep mean?" you ask.
This is a fair question. grep must be the quintessential UNIX acronym,
because it's impossible to understand even when it's spelled out in
full!
grep stands for Global Regular Expression Parser. You will understand
the use of this command right away, but when "Global Regular
Expression Parser" becomes a comfortable phrase in itself, you should
probably consider taking a vacation.
What grep does, essentially, is find and display lines that contain a
pattern that you specify. There are two basic ways to use grep.
The first use of grep is to filter the output of other commands. The
general syntax is <command> | grep <pattern>. For instance, if we
wanted to see every actively running process on the system, we would
type ps -a | grep R. In this application, grep passes on only those
lines that contain the pattern (in this case, the single letter) R.
Note that if someone were running a program called Resting, it would
show up even if its status were S for sleeping, because grep would
match the R in Resting. An easy way around this problem is to type
grep " R ", which explicitly tells grep to search for an R with a
space on each side. You must use quotes whenever you search for a
pattern that contains one or more blank spaces.
The second use of grep is to search for lines that contain a specified
pattern in a specified file. The syntax here is grep <pattern>
<filename>. Be careful. It's easy to specify the filename first and
the pattern second by mistake! Again, you should be as specific as you
can with the pattern to be matched, in order to avoid "false" matches.
Summary
By this point you should have tried enough different Linux commands to
start getting familiar (if not yet entirely comfortable) with typical
Linux usage conventions.
It is important that you be able to use the man pages provided online
by Linux. A very good exercise at this point is to pull up man pages
for all the commands we have looked at in the past two chapters:
login, passwd, who, adduser, and so on. If some of the commands listed
under "See also:" look interesting, by all means take a look at their
man pages too!
______________________________________________________________
NOTE: Some man pages, such as the one for bash, are extremely long.
Do not plan to read them all in one sitting!
______________________________________________________________
In Chapter 8, we head out from "home" and poke around in the Linux
filesystem. As system administrators, we should know what our hard
drives contain! For instance, there are special "administrator-only"
directories crammed with goodies.
Several more "essential" commands will be introduced. By the end of
the next chapter, you will have seen and tried most of the important
"user" Linux commands and will have had a taste of some of the
"administrator" commands.
--
Enjoy Linux!
-----It's FREE!-----
※ 修改:．netiscpu 于 Jul 25 03:44:09 修改本文．[FROM: mtlab.hit.edu.cn]
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