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.gzare two different files.
lsis a command,
LSis a mistake;
Further, some characters shouldn't be used: some of those are
$ # 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 -Fcommand. For example:
$ ls -F I_am_a_dir/ cindy.jpg cjpg* letter_to_Joe my_1st_script* old~
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.filewon't show up after the
/switch, Linux switches with
dir /s<tt> becomes
ls -R. Note that many DOS programs, like
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
/dev/modem, which points to either
To make a symbolic link:
$ ln -s <file_or_dir> <linkname>
$ 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).
/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),
o (other), X is either
-, perm is
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:
< > >> |
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
*is smarter under Linux:
*matches all files except the hidden ones;
.*matches all hidden files (but also the current directory `
.' and parent directory `
*.*matches only those that have a `
.' in the middle or that end with a dot;
p*rmatches both `peter' and `piper';
*c*matches both `picked' and `peck';
more, press <SPACE> to read through the file, `q' to exit.
lessis 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>&1redirects stderr to stdout, while
1>&2redirects stdout to stderr;
[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
RENAME; that is,
mv *.xxx *.yyywon't work. A REN-like command is available on ftp://metalab.unc.edu/pub/Linux/utils/file;
mv -ito 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
This is what the typical command line looks like:
$ command [-s1 [-s2] ... [-sn]] [par1 [par2] ... [parn]] [< input] [> output]
-sn are the program switches,
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:
the shell identifies the process with a job number (e.g.
$ progname [-switches] [parameters] [< input] [> output] &  123
; 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
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
$ 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:
xtermor 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.