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Next: 5. Regular Expressions Up: rute Previous: 3. PC Hardware   Contents

Subsections

4. Basic Commands

All of UNIX is case sensitive. A command with even a single letter's capitalization altered is considered to be a completely different command. The same goes for files, directories, configuration file formats, and the syntax of all native programming languages.

4.1 The ls Command, Hidden Files,
Command-Line Options

In addition to directories and ordinary text files, there are other types of files, although all files contain the same kind of data (i.e., a list of bytes). The hidden file is a file that will not ordinarily appear when you type the command ls to list the contents of a directory. To see a hidden file you must use the command ls -a. The -a option means to list all files as well as hidden files. Another variant is ls -l, which lists the contents in long format. The  -  is used in this way to indicate variations on a command. These are called command-line options or command-line arguments, and most UNIX commands can take a number of them. They can be strung together in any way that is convenient [Commands under the GNU free software license are superior in this way: they have a greater number of options than traditional UNIX commands and are therefore more flexible.], for example, ls -a -l, ls -l -a, or ls -al  --any of these will list all files in long format.

All GNU commands take the additional arguments -h and --help. You can type a command with just this on the command-line and get a usage summary. This is some brief help that will summarize options that you may have forgotten if you are already familiar with the command--it will never be an exhaustive description of the usage. See the later explanation about man pages.

The difference between a hidden file and an ordinary file is merely that the file name of a hidden file starts with a period. Hiding files in this way is not for security, but for convenience.

The option ls -l is somewhat cryptic for the novice. Its more explanatory version is ls --format=long. Similarly, the all option can be given as ls --all, and means the same thing as ls -a.

4.2 Error Messages

Although commands usually do not display a message when they execute [The computer accepted and processed the command. ] successfully, commands do report errors in a consistent format. The format varies from one command to another but often appears as follows: command-name : what was attempted : error message. For example, the command ls -l qwerty gives an error ls: qwerty: No such file or directory. What actually happened was that the command ls attempted to read the file qwerty. Since this file does not exist, an error code 2 arose. This error code corresponds to a situation where a file or directory is not being found. The error code is automatically translated into the sentence No such file or directory. It is important to understand the distinction between an explanatory message that a command gives (such as the messages reported by the passwd command in the previous chapter) and an error code that was just translated into a sentence. The reason is that a lot of different kinds of problems can result in an identical error code (there are only about a hundred different error codes). Experience will teach you that error messages do not tell you what to do, only what went wrong, and should not be taken as gospel.

The file /usr/include/asm/errno.h contains a complete list of basic error codes. In addition to these, several other header files [Files ending in .h] might define their own error codes. Under UNIX, however, these are 99% of all the errors you are ever likely to get. Most of them will be meaningless to you at the moment but are included in Table 4.1 as a reference.

Table 4.1: LINUX error codes
Number  C define Message
0   Success
1 EPERM Operation not permitted
2 ENOENT No such file or directory
3 ESRCH No such process
4 EINTR Interrupted system call
5 EIO Input/output error
6 ENXIO Device not configured
7 E2BIG Argument list too long
8 ENOEXEC Exec format error
9 EBADF Bad file descriptor
10 ECHILD No child processes
11 EAGAIN Resource temporarily unavailable
11 EWOULDBLOCK Resource temporarily unavailable
12 ENOMEM Cannot allocate memory
13 EACCES Permission denied
14 EFAULT Bad address
15 ENOTBLK Block device required
16 EBUSY Device or resource busy
17 EEXIST File exists
18 EXDEV Invalid cross-device link
19 ENODEV No such device
20 ENOTDIR Not a directory
21 EISDIR Is a directory
22 EINVAL Invalid argument
23 ENFILE Too many open files in system
24 EMFILE Too many open files
25 ENOTTY Inappropriate ioctl for device
26 ETXTBSY Text file busy
27 EFBIG File too large
28 ENOSPC No space left on device
29 ESPIPE Illegal seek
30 EROFS Read-only file system
31 EMLINK Too many links
32 EPIPE Broken pipe
33 EDOM Numerical argument out of domain
34 ERANGE Numerical result out of range
35 EDEADLK Resource deadlock avoided
35 EDEADLOCK Resource deadlock avoided
36 ENAMETOOLONG File name too long
37 ENOLCK No locks available
38 ENOSYS Function not implemented
39 ENOTEMPTY Directory not empty
40 ELOOP Too many levels of symbolic links
EWOULDBLOCK (same as EAGAIN)
42 ENOMSG No message of desired type
43 EIDRM Identifier removed
44 ECHRNG Channel number out of range
45 EL2NSYNC Level 2 not synchronized
46 EL3HLT Level 3 halted
47 EL3RST Level 3 reset
48 ELNRNG Link number out of range
49 EUNATCH Protocol driver not attached
50 ENOCSI No CSI structure available
51 EL2HLT Level 2 halted
52 EBADE Invalid exchange
53 EBADR Invalid request descriptor
54 EXFULL Exchange full
55 ENOANO No anode
56 EBADRQC Invalid request code
57 EBADSLT Invalid slot
EDEADLOCK (same as EDEADLK)
59 EBFONT Bad font file format
60 ENOSTR Device not a stream
61 ENODATA No data available
62 ETIME Timer expired
63 ENOSR Out of streams resources
64 ENONET Machine is not on the network
65 ENOPKG Package not installed
66 EREMOTE Object is remote
67 ENOLINK Link has been severed
68 EADV Advertise error
69 ESRMNT Srmount error
70 ECOMM Communication error on send
71 EPROTO Protocol error
72 EMULTIHOP Multihop attempted
73 EDOTDOT RFS specific error
74 EBADMSG Bad message
75 EOVERFLOW Value too large for defined data type
76 ENOTUNIQ Name not unique on network
77 EBADFD File descriptor in bad state
78 EREMCHG Remote address changed
79 ELIBACC Can not access a needed shared library
80 ELIBBAD Accessing a corrupted shared library
81 ELIBSCN .lib section in a.out corrupted
82 ELIBMAX Attempting to link in too many shared libraries
83 ELIBEXEC Cannot exec a shared library directly
84 EILSEQ Invalid or incomplete multibyte or wide character
85 ERESTART Interrupted system call should be restarted
86 ESTRPIPE Streams pipe error
87 EUSERS Too many users
88 ENOTSOCK Socket operation on non-socket
89 EDESTADDRREQ Destination address required
90 EMSGSIZE Message too long
91 EPROTOTYPE Protocol wrong type for socket
92 ENOPROTOOPT Protocol not available
93 EPROTONOSUPPORT Protocol not supported
94 ESOCKTNOSUPPORT Socket type not supported
95 EOPNOTSUPP Operation not supported
96 EPFNOSUPPORT Protocol family not supported
97 EAFNOSUPPORT Address family not supported by protocol
98 EADDRINUSE Address already in use
99 EADDRNOTAVAIL Cannot assign requested address
100 ENETDOWN Network is down
101 ENETUNREACH Network is unreachable
102 ENETRESET Network dropped connection on reset
103 ECONNABORTED Software caused connection abort
104 ECONNRESET Connection reset by peer
105 ENOBUFS No buffer space available
106 EISCONN Transport endpoint is already connected
107 ENOTCONN Transport endpoint is not connected
108 ESHUTDOWN Cannot send after transport endpoint shutdown
109 ETOOMANYREFS Too many references: cannot splice
110 ETIMEDOUT Connection timed out
111 ECONNREFUSED Connection refused
112 EHOSTDOWN Host is down
113 EHOSTUNREACH No route to host
114 EALREADY Operation already in progress
115 EINPROGRESS Operation now in progress
116 ESTALE Stale NFS file handle
117 EUCLEAN Structure needs cleaning
118 ENOTNAM Not a XENIX named type file
119 ENAVAIL No XENIX semaphores available
120 EISNAM Is a named type file
121 EREMOTEIO Remote I/O error
122 EDQUOT Disk quota exceeded
123 ENOMEDIUM No medium found
124 EMEDIUMTYPE Wrong medium type

4.3 Wildcards, Names, Extensions, and glob Expressions

ls can produce a lot of output if there are a large number of files in a directory. Now say that we are only interested in files that ended with the letters tter. To list only these files, you can use ls *tter. The * matches any number of any other characters. So, for example, the files Tina.letter, Mary_Jones.letter and the file splatter, would all be listed if they were present, whereas a file Harlette would not be listed. While the * matches any length of characters, the ? matches only one character. For example, the command ls ?ar* would list the files Mary_Jones.letter and Harlette.

4.3.1 File naming

When naming files, it is a good idea to choose names that group files of the same type together. You do this by adding an extension to the file name that describes the type of file it is. We have already demonstrated this by calling a file Mary_Jones.letter instead of just Mary_Jones. If you keep this convention, you will be able to easily list all the files that are letters by entering ls *.letter. The file name Mary_Jones.letter is then said to be composed of two parts: the name, Mary_Jones, and the extension, letter.

Some common UNIX extensions you may see are:

.a
Archive. lib*.a is a static library.
.alias
X Window System font alias catalog.
.avi
Video format.
.au
Audio format (original Sun Microsystems generic sound file).
.awk
awk program source file.
.bib
bibtex LATEX bibliography source file.
.bmp
Microsoft Bitmap file image format.
.bz2
File compressed with the bzip2 compression program.
.cc, .cxx, .C, .cpp
C++ program source code.
.cf, .cfg
Configuration file or script.
.cgi
Executable script that produces web page output.
.conf, .config
Configuration file.
.csh
csh shell script.
.c
 C program source code.
.db
Database file.
.dir
X Window System font/other database directory.
.deb
Debian package for the Debian distribution.
.diff
Output of the diff program indicating the difference between files or source trees.
.dvi
Device-independent file. Formatted output of .tex LATEX file.
.el
Lisp program source.
.g3
G3 fax format image file.
.gif, .giff
GIF image file.
.gz
File compressed with the gzip compression program.
.htm, .html, .shtm, .html
Hypertext Markup Language. A web page of some sort.
.h
 C/C++ program header file.
.i
SWIG source, or C preprocessor output.
.in
configure input file.
.info
Info pages read with the info command.
.jpg, .jpeg
JPEG image file.
.lj
LaserJet file. Suitable input to a HP LaserJet printer.
.log
Log file of a system service. This file grows with status messages of some system program.
.lsm
LINUX Software Map entry.
.lyx
LyX word processor document.
.man
Man page.
.mf
Meta-Font font program source file.
.pbm
PBM image file format.
.pcf
PCF image file--intermediate representation for fonts. X Window System font.
.pcx
PCX image file.
.pfb
X Window System font file.
.pdf
Formatted document similar to PostScript or dvi.
.php
PHP program source code (used for web page design).
.pl
Perl program source code.
.ps
PostScript file, for printing or viewing.
.py
Python program source code.
.rpm
RedHat Package Manager rpm file.
.sgml
Standard Generalized Markup Language. Used to create documents to be converted to many different formats.
.sh
sh shell script.
.so
Shared object file. lib*.so is a Dynamically Linked Library. [Executable program code shared by more than one program to save disk space and memory.]
.spd
Speedo X Window System font file.
.tar
tarred directory tree.
.tcl
Tcl/Tk source code (programming language).
.texi, .texinfo
Texinfo source. Info pages are compiled from these.
.tex
TEX or LATEX document. LATEX is for document processing and typesetting.
.tga
TARGA image file.
.tgz
Directory tree that has been archived with tar, and then compressed with gzip. Also a package for the Slackware distribution.
.tiff
TIFF image file.
.tfm
LATEX font metric file.
.ttf
Truetype font.
.txt
Plain English text file.
.voc
Audio format (Soundblaster's own format).
.wav
Audio format (sound files common to Microsoft Windows).
.xpm
XPM image file.
.y
yacc source file.
.Z
File compressed with the compress compression program.
.zip
File compressed with the pkzip (or PKZIP.EXE for DOS) compression program.
.1, .2 ...
Man page.

In addition, files that have no extension and a capitalized descriptive name are usually plain English text and meant for your reading. They come bundled with packages and are for documentation purposes. You will see them hanging around all over the place.

Some full file names you may see are:

AUTHORS
List of people who contributed to or wrote a package.
ChangeLog
List of developer changes made to a package.
COPYING
Copyright (usually GPL) for a package.
INSTALL
Installation instructions.
README
Help information to be read first, pertaining to the directory the README is in.
TODO
List of future desired work to be done to package.
BUGS
List of errata.
NEWS
Info about new features and changes for the layman about this package.
THANKS
List of contributors to a package.
VERSION
Version information of the package.

4.3.2 Glob expressions

There is a way to restrict file listings to within the ranges of certain characters. If you only want to list the files that begin with A through M, you can run ls [A-M]*. Here the brackets have a special meaning--they match a single character like a ?, but only those given by the range. You can use this feature in a variety of ways, for example, [a-dJW-Y]* matches all files beginning with a, b, c, d, J, W, X or Y; and *[a-d]id matches all files ending with aid, bid, cid or did; and *.{cpp,c,cxx} matches all files ending in .cpp, .c or .cxx. This way of specifying a file name is called a glob expression. Glob expressions are used in many different contexts, as you will see later.

4.4 Usage Summaries and the Copy Command

The command cp stands for copy. It duplicates one or more files. The format is

cp <file> <newfile>
cp <file> [<file> ...] <dir>
or
cp file  newfile
cp file  [file  ...]  dir
The above lines are called a usage summary. The < and > signs mean that you don't actually type out these characters but replace <file> with a file name of your own. These are also sometimes written in italics like, cp file newfile. In rare cases they are written in capitals like, cp FILE NEWFILE. <file> and <dir> are called parameters. Sometimes they are obviously numeric, like a command that takes <ioport>. [Anyone emailing me to ask why typing in literal, <, i, o, p, o, r, t and > characters did not work will get a rude reply.] These are common conventions used to specify the usage of a command. The [ and ] brackets are also not actually typed but mean that the contents between them are optional. The ellipses ... mean that <file> can be given repeatedly, and these also are never actually typed. From now on you will be expected to substitute your own parameters by interpreting the usage summary. You can see that the second of the above lines is actually just saying that one or more file names can be listed with a directory name last.

From the above usage summary it is obvious that there are two ways to use the cp command. If the last name is not a directory, then cp copies that file and renames it to the file name given. If the last name is a directory, then cp copies all the files listed into that directory.

The usage summary of the ls command is as follows:

 
 
ls [-l, --format=long] [-a, --all] <file> <file> ...
ls -al

where the comma indicates that either option is valid. Similarly, with the passwd command:

 
passwd [<username>]

You should practice using the cp command now by moving some of your files from place to place.

4.5 Directory Manipulation

The cd command is used to take you to different directories. Create a directory new with mkdir new. You could create a directory one by doing cd new and then mkdir one, but there is a more direct way of doing this with mkdir new/one. You can then change directly to the one directory with cd new/one. And similarly you can get back to where you were with cd ../... In this way, the / is used to represent directories within directories. The directory one is called a subdirectory of new.

The command pwd stands for present working directory (also called the current directory) and tells what directory you are currently in. Entering pwd gives some output like /home/<username>. Experiment by changing to the root directory (with cd /) and then back into the directory /home/<username> (with cd /home/<username>). The directory /home/<username> is called your home directory, and is where all your personal files are kept. It can be used at any time with the abbreviation ~. In other words, entering cd /home/<username> is the same as entering cd ~. The process whereby a ~ is substituted for your home directory is called tilde expansion.

To remove (i.e., erase or delete) a file, use the command rm <filename>. To remove a directory, use the command rmdir <dir>. Practice using these two commands. Note that you cannot remove a directory unless it is empty. To remove a directory as well as any contents it might contain, use the command rm -R <dir>. The -R option specifies to dive into any subdirectories of <dir> and delete their contents. The process whereby a command dives into subdirectories of subdirectories of ... is called recursion. -R stands for recursively. This is a very dangerous command. Although you may be used to ``undeleting'' files on other systems, on UNIX a deleted file is, at best, extremely difficult to recover.

The cp command also takes the -R option, allowing it to copy whole directories. The mv command is used to move files and directories. It really just renames a file to a different directory. Note that with cp you should use the option -p and -d with -R to preserve all attributes of a file and properly reproduce symlinks (discussed later). Hence, always use cp -dpR <dir> <newdir> instead of cp -R <dir> <newdir>.

4.6 Relative vs. Absolute Pathnames

Commands can be given file name arguments in two ways. If you are in the same directory as the file (i.e., the file is in the current directory), then you can just enter the file name on its own (e.g., cp my_file new_file). Otherwise, you can enter the full path name, like cp /home/jack/my_file /home/jack/new_file. Very often administrators use the notation ./my_file to be clear about the distinction, for instance, cp ./my_file ./new_file. The leading ./ makes it clear that both files are relative to the current directory. File names not starting with a / are called relative path names, and otherwise, absolute path names.

4.7 System Manual Pages

(See Chapter 16 for a complete overview of all documentation on the system, and also how to print manual pages in a properly typeset format.)

The command man [<section>|-a] <command> displays help on a particular topic and stands for manual. Every command on the entire system is documented in so-named man pages. In the past few years a new format of documentation, called info, has evolved. This is considered the modern way to document commands, but most system documentation is still available only through man. Very few packages are not documented in man however.

Man pages are the authoritative reference on how a command works because they are usually written by the very programmer who created the command. Under UNIX, any printed documentation should be considered as being second-hand information. Man pages, however, will often not contain the underlying concepts needed for understanding the context in which a command is used. Hence, it is not possible for a person to learn about UNIX purely from man pages. However, once you have the necessary background for a command, then its man page becomes an indispensable source of information and you can discard other introductory material.

Now, man pages are divided into sections, numbered 1 through 9. Section 1 contains all man pages for system commands like the ones you have been using. Sections 2-7 contain information for programmers and the like, which you will probably not have to refer to just yet. Section 8 contains pages specifically for system administration commands. There are some additional sections labeled with letters; other than these, there are no manual pages besides the sections 1 through 9. The sections are

... /man1 User programs
... /man2 System calls
... /man3 Library calls
... /man4 Special files
... /man5 File formats
... /man6 Games
... /man7 Miscellaneous
... /man8 System administration
... /man9 Kernel documentation

You should now use the man command to look up the manual pages for all the commands that you have learned. Type man cp, man mv, man rm, man mkdir, man rmdir, man passwd, man cd, man pwd, and of course man man. Much of the information might be incomprehensible to you at this stage. Skim through the pages to get an idea of how they are structured and what headings they usually contain. Man pages are referenced with notation like cp(1), for the cp command in Section 1, which can be read with man 1 cp. This notation will be used from here on.

4.8 System info Pages

info pages contain some excellent reference and tutorial information in hypertext linked format. Type info on its own to go to the top-level menu of the entire info hierarchy. You can also type info <command> for help on many basic commands. Some packages will, however, not have info pages, and other UNIX systems do not support info at all.

info is an interactive program with keys to navigate and search documentation. Inside info, typing H will invoke the help screen from where you can learn more commands.

4.9 Some Basic Commands

You should practice using each of these commands.

bc
A calculator program that handles arbitrary precision (very large) numbers. It is useful for doing any kind of calculation on the command-line. Its use is left as an exercise.
cal [[0-12] 1-9999]
Prints out a nicely formatted calender of the current month, a specified month, or a specified whole year. Try cal 1 for fun, and cal 9 1752, when the pope had a few days scrapped to compensate for round-off error.
cat <filename> [<filename> ...]
Writes the contents of all the files listed to the screen. cat can join a lot of files together with cat <filename> <filename> ... > <newfile>. The file <newfile> will be an end-on-end concatenation of all the files specified.
clear
Erases all the text in the current terminal.
date
Prints out the current date and time. (The command time, though, does something entirely different.)
df
Stands for disk free and tells you how much free space is left on your system. The available space usually has the units of kilobytes (1024 bytes) (although on some other UNIX systems this will be 512 bytes or 2048 bytes). The right-most column tells the directory (in combination with any directories below that) under which that much space is available.
dircmp
Directory compare. This command compares directories to see if changes have been made between them. You will often want to see where two trees differ (e.g., check for missing files), possibly on different computers. Run man dircmp (that is, dircmp(1)). (This is a System 5 command and is not present on LINUX. You can, however, compare directories with the Midnight Commander, mc).
du <directory>
Stands for disk usage and prints out the amount of space occupied by a directory. It recurses into any subdirectories and can print only a summary with du -s <directory>. Also try du --max-depth=1 /var and du -x / on a system with /usr and /home on separate partitions. [See page [*].]
dmesg
Prints a complete log of all messages printed to the screen during the bootup process. This is useful if you blinked when your machine was initializing. These messages might not yet be meaningful, however.
echo
Prints a message to the terminal. Try echo 'hello there', echo $[10*3+2], echo `$[10*3+2]'. The command echo -e allows interpretation of certain backslash sequences, for example echo -e "\a", which prints a bell, or in other words, beeps the terminal. echo -n does the same without printing the trailing newline. In other words, it does not cause a wrap to the next line after the text is printed. echo -e -n "\b", prints a back-space character only, which will erase the last character printed.
exit
Logs you out.
expr <expression>
Calculates the numerical expression expression. Most arithmetic operations that you are accustomed to will work. Try expr 5 + 10 '*' 2. Observe how mathematical precedence is obeyed (i.e., the * is worked out before the +).
file <filename>
Prints out the type of data contained in a file. file portrait.jpg will tell you that portrait.jpg is a JPEG image data, JFIF standard. The command file detects an enormous amount of file types, across every platform. file works by checking whether the first few bytes of a file match certain tell-tale byte sequences. The byte sequences are called magic numbers. Their complete list is stored in /usr/share/magic. [The word ``magic'' under UNIX normally refers to byte sequences or numbers that have a specific meaning or implication. So-called magic numbers are invented for source code, file formats, and file systems.]
free
Prints out available free memory. You will notice two listings: swap space and physical memory. These are contiguous as far as the user is concerned. The swap space is a continuation of your installed memory that exists on disk. It is obviously slow to access but provides the illusion of much more available RAM and avoids the possibility of ever running out of memory (which can be quite fatal).
head [-n <lines>] <filename>
Prints the first <lines> lines of a file or 10 lines if the -n option is not given. (See also tail below).
hostname [<new-name>]
With no options, hostname prints the name of your machine, otherwise it sets the name to <new-name>.
kbdrate -r <chars-per-second> -d <repeat-delay>
Changes the repeat rate of your keys. Most users will like this rate set to kbdrate -r 32 -d 250 which unfortunately is the fastest the PC can go.
more
Displays a long file by stopping at the end of each page. Run the following: ls -l /bin > bin-ls, and then try more bin-ls. The first command creates a file with the contents of the output of ls. This will be a long file because the directory /bin has a great many entries. The second command views the file. Use the space bar to page through the file. When you get bored, just press Q. You can also try ls -l /bin | more which will do the same thing in one go.
less
The GNU version of more, but with extra features. On your system, the two commands may be the same. With less, you can use the arrow keys to page up and down through the file. You can do searches by pressing ?, and then typing in a word to search for and then pressing Enter. Found words will be highlighted, and the text will be scrolled to the first found word. The important commands are:
Shift-G
Go to the end of a file.
Shift-?ssss
Search backward through a file for the text ssss.
?ssss
Search forward through a file for the text ssss. [Actually ssss is a regular expression. See Chapter 5 for more info.]
Shift-F
Scroll forward and keep trying to read more of the file in case some other program is appending to it--useful for log files.
nnn-G
Go to line nnn of the file.
Q
Quit. Used by many UNIX text-based applications (sometimes Q-Enter).
(You can make less stop beeping in the irritating way that it does by editing the file /etc/profile and adding the lines

 
 
LESS=-Q
export LESS

and then logging out and logging in again. But this is an aside that will make more sense later.)
lynx <url>
Opens a URL [URL stands for Uniform Resource Locator--a web address.]at the console. Try lynx http://lwn.net/.
links <url>
Another text-based web browser.
nohup <command> &
Runs a command in the background, appending any output the command may produce to the file nohup.out in your home directory. nohup has the useful feature that the command will continue to run even after you have logged out. Uses for nohup will become obvious later.
sleep <seconds>
Pauses for <seconds> seconds. See also usleep.
sort <filename>
Prints a file with lines sorted in alphabetical order. Create a file called telephone with each line containing a short telephone book entry. Then type sort telephone, or sort telephone | less and see what happens. sort takes many interesting options to sort in reverse ( sort -r), to eliminate duplicate entries ( sort -u), to ignore leading whitespace ( sort -b), and so on. See the sort(1) for details.
strings [-n <len>] <filename>
Writes out a binary file, but strips any unreadable characters. Readable groups of characters are placed on separate lines. If you have a binary file that you think may contain something interesting but looks completely garbled when viewed normally, use strings to sift out the interesting stuff: try less /bin/cp and then try strings /bin/cp. By default strings does not print sequences smaller than 4. The -n option can alter this limit.
split ...
Splits a file into many separate files. This might have been used when a file was too big to be copied onto a floppy disk and needed to be split into, say, 360-KB pieces. Its sister, csplit, can split files along specified lines of text within the file. The commands are seldom used on their own but are very useful within programs that manipulate text.
tac <filename> [<filename> ...]
Writes the contents of all the files listed to the screen, reversing the order of the lines--that is, printing the last line of the file first. tac is cat backwards and behaves similarly.
tail [-f] [-n <lines>] <filename>
Prints the last <lines> lines of a file or 10 lines if the -n option is not given. The -f option means to watch the file for lines being appended to the end of it. (See also head above.)
uname
Prints the name of the UNIX operating system you are currently using. In this case, LINUX.
uniq <filename>
Prints a file with duplicate lines deleted. The file must first be sorted.
usleep <microseconds>
Pauses for <microseconds> microseconds (1/1,000,000 of a second).
wc [-c] [-w] [-l] <filename>
Counts the number of bytes (with -c for character), or words (with -w), or lines (with -l) in a file.
whatis <command>
Gives the first line of the man page corresponding to <command>, unless no such page exists, in which case it prints nothing appropriate.
whoami
Prints your login name.

4.10 The mc File Manager

Those who come from the DOS world may remember the famous Norton Commander file manager. The GNU project has a Free clone called the Midnight Commander, mc. It is essential to at least try out this package--it allows you to move around files and directories extremely rapidly, giving a wide-angle picture of the file system. This will drastically reduce the number of tedious commands you will have to type by hand.

4.11 Multimedia Commands for Fun

You should practice using each of these commands if you have your sound card configured. [I don't want to give the impression that LINUX does not have graphical applications to do all the functions in this section, but you should be aware that for every graphical application, there is a text-mode one that works better and consumes fewer resources.] You may also find that some of these packages are not installed, in which case you can come back to this later.

play [-v <volume>] <filename>
Plays linear audio formats out through your sound card. These formats are .8svx, .aiff, .au, .cdr, .cvs, .dat, .gsm, .hcom, .maud, .sf, .smp, .txw, .vms, .voc, .wav, .wve, .raw, .ub, .sb, .uw, .sw, or .ul files. In other words, it plays almost every type of ``basic'' sound file there is: most often this will be a simple Windows .wav file. Specify <volume> in percent.
rec <filename>
Records from your microphone into a file. ( play and rec are from the same package.)
mpg123 <filename>
Plays audio from MPEG files level 1, 2, or 3. Useful options are -b 1024 (for increasing the buffer size to prevent jumping) and --2to1 (down-samples by a factor of 2 for reducing CPU load). MPEG files contain sound and/or video, stored very compactly using digital signal processing techniques that the commercial software industry seems to think are very sophisticated.
cdplay
Plays a regular music CD. cdp is the interactive version.
aumix
Sets your sound card's volume, gain, recording volume, etc. You can use it interactively or just enter aumix -v <volume> to immediately set the volume in percent. Note that this is a dedicated mixer program and is considered to be an application separate from any that play music. Preferably do not set the volume from within a sound-playing application, even if it claims this feature--you have much better control with aumix.
mikmod --interpolate -hq --renice Y <filename>
Plays Mod files. Mod files are a special type of audio format that stores only the duration and pitch of the notes that constitute a song, along with samples of each musical instrument needed to play the song. This makes for high-quality audio with phenomenally small file size. mikmod supports 669, AMF, DSM, FAR, GDM, IMF, IT, MED, MOD, MTM, S3M, STM, STX, ULT, UNI, and XM audio formats--that is, probably every type in existence. Actually, a lot of excellent listening music is available on the Internet in Mod file format. The most common formats are .it, .mod, .s3m, and .xm. [Original .mod files are the product of Commodore-Amiga computers and had only four tracks. Today's 16 (and more) track Mod files are comparable to any recorded music.]

4.12 Terminating Commands

You usually use Ctrl-C to stop an application or command that runs continuously. You must type this at the same prompt where you entered the command. If this doesn't work, the section on processes (Section 9.5) will explain about signalling a running application to quit.

4.13 Compressed Files

Files typically contain a lot of data that one can imagine might be represented with a smaller number of bytes. Take for example the letter you typed out. The word ``the'' was probably repeated many times. You were probably also using lowercase letters most of the time. The file was by far not a completely random set of bytes, and it repeatedly used spaces as well as using some letters more than others. [English text in fact contains, on average, only about 1.3 useful bits (there are eight bits in a byte) of data per byte.]Because of this the file can be compressed to take up less space. Compression involves representing the same data by using a smaller number of bytes, in such a way that the original data can be reconstructed exactly. Such usually involves finding patterns in the data. The command to compress a file is gzip <filename>, which stands for GNU zip. Run gzip on a file in your home directory and then run ls to see what happened. Now, use more to view the compressed file. To uncompress the file use gzip -d <filename>. Now, use more to view the file again. Many files on the system are stored in compressed format. For example, man pages are often stored compressed and are uncompressed automatically when you read them.

You previously used the command cat to view a file. You can use the command zcat to do the same thing with a compressed file. Gzip a file and then type zcat <filename>. You will see that the contents of the file are written to the screen. Generally, when commands and files have a z in them they have something to do with compression--the letter z stands for zip. You can use zcat <filename> | less to view a compressed file proper. You can also use the command zless <filename>, which does the same as zcat <filename> | less. (Note that your less may actually have the functionality of zless combined.)

A new addition to the arsenal is bzip2. This is a compression program very much like gzip, except that it is slower and compresses 20%-30% better. It is useful for compressing files that will be downloaded from the Internet (to reduce the transfer volume). Files that are compressed with bzip2 have an extension .bz2. Note that the improvement in compression depends very much on the type of data being compressed. Sometimes there will be negligible size reduction at the expense of a huge speed penalty, while occasionally it is well worth it. Files that are frequently compressed and uncompressed should never use bzip2.

4.14 Searching for Files

You can use the command find to search for files. Change to the root directory, and enter find. It will spew out all the files it can see by recursively descending [Goes into each subdirectory and all its subdirectories, and repeats the command find. ] into all subdirectories. In other words, find, when executed from the root directory, prints all the files on the system. find will work for a long time if you enter it as you have--press Ctrl-C to stop it.

Now change back to your home directory and type find again. You will see all your personal files. You can specify a number of options to find to look for specific files.

find -type d
Shows only directories and not the files they contain.
find -type f
Shows only files and not the directories that contain them, even though it will still descend into all directories.
find -name <filename>
Finds only files that have the name <filename>. For instance, find -name '*.c' will find all files that end in a .c extension ( find -name *.c without the quote characters will not work. You will see why later). find -name Mary_Jones.letter will find the file with the name Mary_Jones.letter.
find -size [[+|-]]<size>
Finds only files that have a size larger (for +) or smaller (for -) than <size> kilobytes, or the same as <size> kilobytes if the sign is not specified.
find <directory> [<directory> ...]
Starts find in each of the specified directories.
There are many more options for doing just about any type of search for a file. See find(1) for more details (that is, run man 1 find). Look also at the -exec option which causes find to execute a command for each file it finds, for example:

 
find /usr -type f -exec ls '-al' '{}' ';'

find has the deficiency of actively reading directories to find files. This process is slow, especially when you start from the root directory. An alternative command is locate <filename>. This searches through a previously created database of all the files on the system and hence finds files instantaneously. Its counterpart updatedb updates the database of files used by locate. On some systems, updatedb runs automatically every day at 04h00.

Try these ( updatedb will take several minutes):

 
 
 
 
5 
 
updatedb
locate rpm
locate deb
locate passwd
locate HOWTO
locate README

4.15 Searching Within Files

Very often you will want to search through a number of files to find a particular word or phrase, for example, when a number of files contain lists of telephone numbers with people's names and addresses. The command grep does a line-by-line search through a file and prints only those lines that contain a word that you have specified. grep has the command summary:

 
grep [options] <pattern> <filename> [<filename> ...]

[The words word, string, or pattern are used synonymously in this context, basically meaning a short length of letters and-or numbers that you are trying to find matches for. A pattern can also be a string with kinds of wildcards in it that match different characters, as we shall see later.]

Run grep for the word ``the'' to display all lines containing it: grep 'the' Mary_Jones.letter. Now try grep 'the' *.letter.

grep -n <pattern> <filename>
shows the line number in the file where the word was found.
grep -<num> <pattern> <filename>
prints out <num> of the lines that came before and after each of the lines in which the word was found.
grep -A <num> <pattern> <filename>
prints out <num> of the lines that came After each of the lines in which the word was found.
grep -B <num> <pattern> <filename>
prints out <num> of the lines that came Before each of the lines in which the word was found.
grep -v <pattern> <filename>
prints out only those lines that do not contain the word you are searching for. [ You may think that the -v option is no longer doing the same kind of thing that grep is advertised to do: i.e., searching for strings. In fact, UNIX commands often suffer from this--they have such versatility that their functionality often overlaps with that of other commands. One actually never stops learning new and nifty ways of doing things hidden in the dark corners of man pages.]
grep -i <pattern> <filename>
does the same as an ordinary grep but is case insensitive.

4.16 Copying to MS-DOS and Windows Formatted Floppy Disks

A package, called the mtools package, enables reading and writing to MS-DOS/Windows floppy disks. These are not standard UNIX commands but are packaged with most LINUX distributions. The commands support Windows ``long file name'' floppy disks. Put an MS-DOS disk in your A: drive. Try

 
 
 
 
mdir A:
touch myfile
mcopy myfile A:
mdir A:

Note that there is no such thing as an A: disk under LINUX. Only the mtools package understands A: in order to retain familiarity for MS-DOS users. The complete list of commands is

 
 
 
 
5 
floppyd     mcopy     mformat     mmount      mshowfat
mattrib     mdel      minfo       mmove       mtoolstest
mbadblocks  mdeltree  mkmanifest  mpartition  mtype
mcat        mdir      mlabel      mrd         mzip
mcd         mdu       mmd         mren        xcopy

Entering info mtools will give detailed help. In general, any MS-DOS command, put into lower case with an m prefixed to it, gives the corresponding LINUX command.

4.17 Archives and Backups

Never begin any work before you have a fail-safe method of backing it up.

One of the primary activities of a system administrator is to make backups. It is essential never to underestimate the volatility [Ability to evaporate or become chaotic. ] of information in a computer. Backups of data are therefore continually made. A backup is a duplicate of your files that can be used as a replacement should any or all of the computer be destroyed. The idea is that all of the data in a directory [As usual, meaning a directory and all its subdirectories and all the files in those subdirectories, etc. ] are stored in a separate place--often compressed--and can be retrieved in case of an emergency. When we want to store a number of files in this way, it is useful to be able to pack many files into one file so that we can perform operations on that single file only. When many files are packed together into one, this packed file is called an archive. Usually archives have the extension .tar, which stands for tape archive.

To create an archive of a directory, use the tar command:

 
tar -c -f <filename> <directory>

Create a directory with a few files in it, and run the tar command to back it up. A file of <filename> will be created. Take careful note of any error messages that tar reports. List the file and check that its size is appropriate for the size of the directory you are archiving. You can also use the verify option (see the man page) of the tar command to check the integrity of <filename>. Now remove the directory, and then restore it with the extract option of the tar command:

 
tar -x -f <filename>

You should see your directory recreated with all its files intact. A nice option to give to tar is -v. This option lists all the files that are being added to or extracted from the archive as they are processed, and is useful for monitoring the progress of archiving. It is obvious that you can call your archive anything you like, however; the common practice is to call it <directory>.tar, which makes it clear to all exactly what it is. Another important option is -p which preserves detailed attribute information of files.

Once you have your .tar file, you would probably want to compress it with gzip. This will create a file <directory>.tar.gz, which is sometimes called <directory>.tgz for brevity.

A second kind of archiving utility is cpio. cpio is actually more powerful than tar, but is considered to be more cryptic to use. The principles of cpio are quite similar and its use is left as an exercise.

4.18 The PATH Where Commands Are Searched For

When you type a command at the shell prompt, it has to be read off disk out of one or other directory. On UNIX, all such executable commands are located in one of about four directories. A file is located in the directory tree according to its type, rather than according to what software package it belongs to. For example, a word processor may have its actual executable stored in a directory with all other executables, while its font files are stored in a directory with other fonts from all other packages.

The shell has a procedure for searching for executables when you type them in. If you type in a command with slashes, like /bin/cp, then the shell tries to run the named program, cp, out of the /bin directory. If you just type cp on its own, then it tries to find the cp command in each of the subdirectories of your PATH. To see what your PATH is, just type

 
echo $PATH

You will see a colon separated list of four or more directories. Note that the current directory  .  is not listed. It is important that the current directory not be listed for reasons of security. Hence, to execute a command in the current directory, we hence always ./<command>.

To append, for example, a new directory /opt/gnome/bin to your PATH, do

 
 
PATH="$PATH:/opt/gnome/bin"
export PATH

LINUX supports the convenience of doing this in one line:

 
export PATH="$PATH:/opt/gnome/bin"

There is a further command, which, to check whether a command is locatable from the PATH. Sometimes there are two commands of the same name in different directories of the PATH. [This is more often true of Solaris systems than LINUX.] Typing which <command> locates the one that your shell would execute. Try:

 
 
 
 
which ls
which cp mv rm
which which
which cranzgots

which is also useful in shell scripts to tell if there is a command at all, and hence check whether a particular package is installed, for example, which netscape.

4.19 The -- Option

If a file name happens to begin with a - then it would be impossible to use that file name as an argument to a command. To overcome this circumstance, most commands take an option --. This option specifies that no more options follow on the command-line--everything else must be treated as a literal file name. For instance

 
 
touch -- -stupid_file_name
rm -- -stupid_file_name


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