Linux has a structure of directories and files very similar to that of DOS/Win. Files have filenames that obey special rules, are stored in directories, some are executable, and among these most have command switches. Moreover, you can use wildcard characters, redirection, and piping. There are only a few minor differences:
NOTENOUG.TXT. Under Linux we can do better. If you installed Linux
using a file system like ext2 or umsdos, you can use longer filenames (up to
255 characters), and with more than one dot: for example,
This_is.a.VERY_long.filename. Please note that I used both upper
and lower case characters: in fact...
FILENAME.tar.gz and filename.tar.gz
are two different files. ls is a command, LS is a mistake;
$ # the following command makes a directory called "My old files"
$ mkdir "My old files"
$ ls
My old files bin tmp
!*$&#.
*' at the end of their name when you issue the ls -F
command. For example:
$ ls -F
I_am_a_dir/ cindy.jpg cjpg* letter_to_Joe my_1st_script* old~
cjpg* and my_1st_script* are executables, that
is ``programs''. Under DOS, backup files end in .BAK, while under Linux they
end with a tilde `~'. Further, a file whose name starts with
a dot is considered as hidden. Example: the file
.I.am.a.hidden.file won't show up after the ls command;
/switch, Linux
switches with -switch or --switch. Example: dir
/s<tt> becomes ls -R. Note that many DOS programs, like
PKZIP or ARJ, use UNIX-style switches.
You can now jump to Section Translating Commands from DOS to Linux, but if I were you I'd read on.
UNIX has a type of file that doesn't exist under DOS: the symbolic link.
This can be thought of as a pointer to a file or to a directory, and can be
used instead of the file or directory it points to; it's similar to Windows
shortcuts. Examples of symbolic links are /usr/X11, which points to
/usr/X11R6; /dev/modem, which points to either
/dev/ttyS0 or /dev/ttyS1.
To make a symbolic link:
$ ln -s <file_or_dir> <linkname>
Example:
$ ln -s /usr/doc/g77/DOC g77manual.txt
Now you can refer to g77manual.txt instead of
/usr/doc/g77/DOC. Links appear like this in directory listings:
$ ls -F
g77manual.txt@
$ ls -l
(several things...) g77manual.txt -> /usr/doc/g77/DOC
DOS files and directories have the following attributes: A (archive), H (hidden), R (read-only), and S (system). Only H and R make sense under Linux: hidden files start with a dot, and for the R attribute, read on.
Under UNIX a file has ``permissions'' and an owner, who in turn belongs to a ``group''. Look at this example:
$ ls -l /bin/ls
-rwxr-xr-x 1 root bin 27281 Aug 15 1995 /bin/ls*
The first field contains the permissions of the file /bin/ls, which
belongs to root, group bin. Leaving the remaining information aside,
remember that -rwxr-xr-x means, from left to right:
- is the file type (- = ordinary file, d =
directory, l = link, etc); rwx are the permissions for the
file owner (read, write, execute); r-x are the permissions for the
group of the file owner (read, execute); (I won't cover the concept of
group, you can survive without it as long as you're a beginner ;-)
r-x are the permissions for all other users (read, execute).
The directory /bin has permissions, too: see Section
Directories Permissions for further
details. This is why you can't delete the file /bin/ls unless you
are root: you don't have the permission to do so. To change a file's
permissions, the command is:
$ chmod <whoXperm> <file>
where who is u (user, that is owner), g (group),
o (other), X is either + or -, perm is r
(read), w (write), or x (execute). Common examples of
chmod use are the following:
$ chmod +x file
this sets the execute permission for the file.
$ chmod go-rw file
this removes read and write permission for everyone but the owner.
$ chmod ugo+rwx file
this gives everyone read, write, and execute permission.
# chmod +s file
this makes a so-called ``setuid'' or ``suid'' file---a file that everyone can execute with its owner's privileges. Typically, you'll come across root suid files; these are often important system files, like the X server.
A shorter way to refer to permissions is with digits: rwxr-xr-x can
be expressed as 755 (every letter corresponds to a bit: --- is 0,
--x is 1, -w- is 2, -wx is 3...). It looks
difficult, but with a bit of practice you'll understand the concept. root,
being the superuser, can change everyone's file permissions. RMP.
On the left, the DOS commands; on the right, their Linux counterpart.
ATTRIB: chmod
COPY: cp
DEL: rm
MOVE: mv
REN: mv
TYPE: more, less, cat
Redirection and plumbing operators: < > >> |
Wildcards: * ?
nul: /dev/null
prn, lpt1: /dev/lp0 or /dev/lp1; lpr
DOS Linux
---------------------------------------------------------------------
C:\GUIDO>ATTRIB +R FILE.TXT $ chmod 400 file.txt
C:\GUIDO>COPY JOE.TXT JOE.DOC $ cp joe.txt joe.doc
C:\GUIDO>COPY *.* TOTAL $ cat * > total
C:\GUIDO>COPY FRACTALS.DOC PRN $ lpr fractals.doc
C:\GUIDO>DEL TEMP $ rm temp
C:\GUIDO>DEL *.BAK $ rm *~
C:\GUIDO>MOVE PAPER.TXT TMP\ $ mv paper.txt tmp/
C:\GUIDO>REN PAPER.TXT PAPER.ASC $ mv paper.txt paper.asc
C:\GUIDO>PRINT LETTER.TXT $ lpr letter.txt
C:\GUIDO>TYPE LETTER.TXT $ more letter.txt
C:\GUIDO>TYPE LETTER.TXT $ less letter.txt
C:\GUIDO>TYPE LETTER.TXT > NUL $ cat letter.txt > /dev/null
n/a $ more *.txt *.asc
n/a $ cat section*.txt | less
Notes:
* is smarter under Linux: * matches all files
except the hidden ones; .* matches all hidden files (but also the
current directory `.' and parent directory `..': beware!);
*.* matches only those that have a `.' in the middle or
that end with a dot; p*r matches both `peter' and `piper';
*c* matches both `picked' and `peck';
more, press <SPACE> to read through the
file, `q' to exit. less is more intuitive and lets you use the
arrow keys;
UNDELETE, so think twice before
deleting anything;
< > >>, Linux has
2> to redirect error messages (stderr); moreover,
2>&1 redirects stderr to stdout, while 1>&2
redirects stdout to stderr;
[]. Usage: [abc]*
matches files starting with a, b, c; *[I-N1-3] matches files ending
with I, J, K, L, M, N, 1, 2, 3;
lpr <file> prints a file in background. To check the
status of the print queue, use lpq; to remove a file from the print
queue, use lprm;
RENAME; that is, mv *.xxx
*.yyy won't work. A REN-like command is available on
ftp://metalab.unc.edu/pub/Linux/utils/file;
cp -i and mv -i to be warned when a file is
going to be overwritten.
To run a program, type its name as you would do under DOS. If the directory
(Section
Using Directories) where the program
is stored is included in the PATH (Section
System Initialisation Files), the program will start. Exception:
unlike DOS, under Linux a program located in the current directory won't run
unless the directory is included in the PATH. Escamotage: being
prog your program, type ./prog.
This is what the typical command line looks like:
$ command [-s1 [-s2] ... [-sn]] [par1 [par2] ... [parn]] [< input] [> output]
where -s1, ..., -sn are the program switches,
par1, ..., parn are the program parameters. You can issue
several commands on the command line:
$ command1 ; command2 ; ... ; commandn
That's all about running programs, but it's easy to go a step beyond. One of the main reasons for using Linux is that it is a multitasking os---it can run several programs (from now on, processes) at the same time. You can launch processes in background and continue working straight away. Moreover, Linux lets you have several sessions: it's like having many computers to work on at once!
su - <loginname>. Example: su - root. This
is useful, for instance, when you need to perform a task that only root can
do.
exit. If there are stopped jobs (see
later), you'll be warned.
&'
at the end of the command line:
$ progname [-switches] [parameters] [< input] [> output] &
[1] 123
[1]; see
below), and with a PID (Process Identification Number; 123 in our example).
ps ax. This will
output a list of currently running processes.
kill <PID>. You
may need to kill a process when you don't know how to quit it the right
way.... Unless you're root, you can't kill other people's processes.
Sometimes, a process will only be killed by kill -SIGKILL
<PID>.
In addition, the shell allows you to stop or temporarily suspend a process,
send a process to background, and bring a process from background to
foreground. In this context, processes are called ``jobs''.
jobs. Here the jobs are
identified by their job number, not by their PID.
bg
<%job> (it becomes a job).
fg <%job>. To bring
to foreground the last job sent to background, simply type fg.
kill <%job> where <job> may
be 1, 2, 3,...
Using these commands you can format a disk, zip a bunch of files, compile a program, and unzip an archive all at the same time, and still have the prompt at your disposal. Try this with Windows, just to see the difference in performance (if it doesn't crash, of course).
To run a program on a remote machine whose name is
remote.machine.edu:
$ telnet remote.machine.edu
After logging in, start your favourite program. Needless to say, you must have a shell account on the remote machine.
If you have X11, you can even run an X application on a remote computer,
displaying it on your X screen. Let remote.machine.edu be the
remote X computer and let local.linux.box be your Linux machine. To
run from local.linux.box an X program that resides on
remote.machine.edu, do the following:
xterm or equivalent terminal emulator,
then type:
$ xhost +remote.machine.edu
$ telnet remote.machine.edu
remote:$ DISPLAY=local.linux.box:0.0
remote:$ progname &
DISPLAY..., you may have to write: setenv DISPLAY
local.linux.box:0.0. It depends on the remote shell.)
Et voila! Now progname will start on remote.machine.edu
and will be displayed on your machine. Don't try this over the modem though,
for it's too slow to be usable. Moreover, this is a crude and insecure
method: please read the ``Remote X Apps mini-HOWTO'' at
http://www.linuxdoc.org/HOWTO/mini/Remote-X-Apps.html.