help
850/644-4019
Unix Frequently Asked Questions
Version: $Id: part1,v 2.9 1996/06/11 13:07:56 tmatimar Exp $
These seven articles contain the answers to some Frequently Asked
Questions often seen in comp.unix.questions and comp.unix.shell.
Please don't ask these questions again, they've been answered plenty
of times already - and please don't flame someone just because they may
not have read this particular posting. Thank you.
This collection of documents is Copyright (c) 1994, Ted Timar, except
Part 6, which is Copyright (c) 1994, Pierre Lewis and Ted Timar.
All rights reserved. Permission to distribute the collection is
hereby granted providing that distribution is electronic, no money
is involved, reasonable attempts are made to use the latest version
and all credits and this copyright notice are maintained.
Other requests for distribution will be considered. All reasonable
requests will be granted.
All information here has been contributed with good intentions, but
none of it is guaranteed either by the contributors or myself to be
accurate. The users of this information take all responsibility for
any damage that may occur.
Many FAQs, including this one, are available on the archive site
rtfm.mit.edu in the directory pub/usenet/news.answers.
The name under which a FAQ is archived appears in the "Archive-Name:"
line at the top of the article. This FAQ is archived as
"unix-faq/faq/part[1-7]".
These articles are divided approximately as follows:
- General questions.
- Relatively basic questions, likely to be asked by beginners.
- Intermediate questions.
- Advanced questions, likely to be asked by people who thought they already knew all of the answers.
- Questions pertaining to the various shells, and the differences.
- An overview of Unix variants.
- An comparison of configuration management systems (RCS, SCCS).
Part 1
This article includes answers to: 1.1) Who helped you put this list together?
1.2) When someone refers to 'rn(1)' or 'ctime(3)', what does
the number in parentheses mean?
1.3) What does {some strange unix command name} stand for?
1.4) How does the gateway between "comp.unix.questions" and the
"info-unix" mailing list work?
1.5) What are some useful Unix or C books?
1.6) What happened to the pronunciation list that used to be
part of this document?
If you're looking for the answer to, say, question 1.5, and want to skip
everything else, you can search ahead for the regular expression "^1.5)".
While these are all legitimate questions, they seem to crop up in
comp.unix.questions or comp.unix.shell on an annual basis, usually
followed by plenty of replies (only some of which are correct) and then
a period of griping about how the same questions keep coming up. You
may also like to read the monthly article "Answers to Frequently Asked
Questions" in the newsgroup "news.announce.newusers", which will tell
you what "UNIX" stands for.
With the variety of Unix systems in the world, it's hard to guarantee
that these answers will work everywhere. Read your local manual pages
before trying anything suggested here. If you have suggestions or
corrections for any of these answers, please send them to to
tmatimar@isgtec.com.
----------------------------------------------------------------------
Subject: Who helped you put this list together?
Date: Thu Mar 18 17:16:55 EST 1993
1.1) Who helped you put this list together?
This document was one of the first collections of Frequently Asked
Questions. It was originally compiled in July 1989.
I took over the maintenance of this list. Almost all of the work
(and the credit) for generating this compilation was done by
Steve Hayman.
We also owe a great deal of thanks to dozens of Usenet readers who
submitted questions, answers, corrections and suggestions for this
list. Special thanks go to Maarten Litmaath, Guy Harris and
Jonathan Kamens, who have all made many especially valuable
contributions.
Part 5 of this document (shells) was written almost entirely by
Matthew Wicks <wicks@dcdmjw.fnal.gov>.
Part 6 of this document (Unix flavours) was written almost entirely by
Pierre (P.) Lewis <lew@bnr.ca>.
Where possible the author of each question and the date it was last
updated is given at the top. Unfortunately, I only started this
practice recently, and much of the information is lost. I was also
negligent in keeping track of who provided updates to questions.
Sorry to those who have made valuable contributions, but did not
receive the credit and recognition that they legitimately deserve.
I make this document available in *roff format (ms and mm macro
packages). Andrew Cromarty has also converted it into Texinfo format.
Marty Leisner <leisner@sdsp.mc.xerox.com> cleaned up the Texinfo
version.
Major contributors to this document who may or may not be
recognized elsewhere are:
Steve Hayman <shayman@Objectario.com>
Pierre Lewis <lew@bnr.ca>
Jonathan Kamens <jik@mit.edu>
Tom Christiansen <tchrist@mox.perl.com>
Maarten Litmaath <maart@nat.vu.nl>
Guy Harris <guy@auspex.com>
The formatted versions are available for anonymous ftp from
ftp.wg.omron.co.jp under pub/unix-faq/docs .
------------------------------
Subject: When someone refers to 'rn(1)' ... the number in parentheses mean?
Date: Tue, 13 Dec 1994 16:37:26 -0500
1.2) When someone refers to 'rn(1)' or 'ctime(3)', what does
the number in parentheses mean?
It looks like some sort of function call, but it isn't. These
numbers refer to the section of the "Unix manual" where the
appropriate documentation can be found. You could type
"man 3 ctime" to look up the manual page for "ctime" in section 3
of the manual.
The traditional manual sections are:
1 User-level commands
2 System calls
3 Library functions
4 Devices and device drivers
5 File formats
6 Games
7 Various miscellaneous stuff - macro packages etc.
8 System maintenance and operation commands
Some Unix versions use non-numeric section names. For instance,
Xenix uses "C" for commands and "S" for functions. Some newer
versions of Unix require "man -s# title" instead of "man # title".
Each section has an introduction, which you can read with "man #
intro" where # is the section number.
Sometimes the number is necessary to differentiate between a
command and a library routine or system call of the same name.
For instance, your system may have "time(1)", a manual page about
the 'time' command for timing programs, and also "time(3)", a
manual page about the 'time' subroutine for determining the
current time. You can use "man 1 time" or "man 3 time" to
specify which "time" man page you're interested in.
You'll often find other sections for local programs or even
subsections of the sections above - Ultrix has sections 3m, 3n,
3x and 3yp among others.
------------------------------
Subject: What does {some strange unix command name} stand for?
Date: Thu Mar 18 17:16:55 EST 1993
1.3) What does {some strange unix command name} stand for?
awk = "Aho Weinberger and Kernighan"
This language was named by its authors, Al Aho, Peter
Weinberger and Brian Kernighan.
grep = "Global Regular Expression Print"
grep comes from the ed command to print all lines matching a
certain pattern
g/re/p
where "re" is a "regular expression".
fgrep = "Fixed GREP".
fgrep searches for fixed strings only. The "f" does not stand
for "fast" - in fact, "fgrep foobar *.c" is usually slower than
"egrep foobar *.c" (Yes, this is kind of surprising. Try it.)
Fgrep still has its uses though, and may be useful when searching
a file for a larger number of strings than egrep can handle.
egrep = "Extended GREP"
egrep uses fancier regular expressions than grep. Many people
use egrep all the time, since it has some more sophisticated
internal algorithms than grep or fgrep, and is usually the
fastest of the three programs.
cat = "CATenate"
catenate is an obscure word meaning "to connect in a series",
which is what the "cat" command does to one or more files. Not
to be confused with C/A/T, the Computer Aided Typesetter.
gecos = "General Electric Comprehensive Operating Supervisor"
When GE's large systems division was sold to Honeywell,
Honeywell dropped the "E" from "GECOS".
Unix's password file has a "pw_gecos" field. The name is a
real holdover from the early days. Dennis Ritchie has reported:
"Sometimes we sent printer output or batch jobs
to the GCOS machine. The gcos field in the password file
was a place to stash the information for the $IDENT card.
Not elegant."
nroff = "New ROFF"
troff = "Typesetter new ROFF"
These are descendants of "roff", which was a re-implementation
of the Multics "runoff" program (a program that you'd use to
"run off" a good copy of a document).
tee = T
From plumbing terminology for a T-shaped pipe splitter.
bss = "Block Started by Symbol"
Dennis Ritchie says:
Actually the acronym (in the sense we took it up; it may
have other credible etymologies) is "Block Started by
Symbol." It was a pseudo-op in FAP (Fortran Assembly [-er?]
Program), an assembler for the IBM 704-709-7090-7094
machines. It defined its label and set aside space for a
given number of words. There was another pseudo-op, BES,
"Block Ended by Symbol" that did the same except that the
label was defined by the last assigned word + 1. (On these
machines Fortran arrays were stored backwards in storage
and were 1-origin.)
The usage is reasonably appropriate, because just as with
standard Unix loaders, the space assigned didn't have to be
punched literally into the object deck but was represented
by a count somewhere.
biff = "BIFF"
This command, which turns on asynchronous mail notification,
was actually named after a dog at Berkeley.
I can confirm the origin of biff, if you're interested.
Biff was Heidi Stettner's dog, back when Heidi (and I, and
Bill Joy) were all grad students at U.C. Berkeley and the
early versions of BSD were being developed. Biff was
popular among the residents of Evans Hall, and was known
for barking at the mailman, hence the name of the command.
Confirmation courtesy of Eric Cooper, Carnegie Mellon University
rc (as in ".cshrc" or "/etc/rc") = "RunCom"
"rc" derives from "runcom", from the MIT CTSS system, ca. 1965.
'There was a facility that would execute a bunch of
commands stored in a file; it was called "runcom" for "run
commands", and the file began to be called "a runcom."
"rc" in Unix is a fossil from that usage.'
Brian Kernighan & Dennis Ritchie, as told to Vicki Brown
"rc" is also the name of the shell from the new Plan 9
operating system.
Perl = "Practical Extraction and Report Language"
Perl = "Pathologically Eclectic Rubbish Lister"
The Perl language is Larry Wall's highly popular
freely-available completely portable text, process, and file
manipulation tool that bridges the gap between shell and C
programming (or between doing it on the command line and
pulling your hair out). For further information, see the
Usenet newsgroup comp.lang.perl.misc.
Don Libes' book "Life with Unix" contains lots more of these
tidbits.
------------------------------
Subject: How does the gateway between "comp.unix.questions" ... work ?
Date: Thu Mar 18 17:16:55 EST 1993
1.4) How does the gateway between "comp.unix.questions" and the
"info-unix" mailing list work?
"info-unix" and "unix-wizards" are mailing list versions of
comp.unix.questions and comp.unix.wizards respectively.
There should be no difference in content between the
mailing list and the newsgroup.
To get on or off either of these lists, send mail to
info-unix-request@brl.mil or unix-wizards-request@brl.mil.
Be sure to use the '-Request'. Don't expect an immediate response.
Here are the gory details, courtesy of the list's maintainer,
Bob Reschly.
==== postings to info-UNIX and UNIX-wizards lists ====
Anything submitted to the list is posted; I do not moderate
incoming traffic -- BRL functions as a reflector. Postings
submitted by Internet subscribers should be addressed to the list
address (info-UNIX or UNIX- wizards); the '-request' addresses
are for correspondence with the list maintainer [me]. Postings
submitted by USENET readers should be addressed to the
appropriate news group (comp.unix.questions or
comp.unix.wizards).
For Internet subscribers, received traffic will be of two types;
individual messages, and digests. Traffic which comes to BRL
from the Internet and BITNET (via the BITNET-Internet gateway) is
immediately resent to all addressees on the mailing list.
Traffic originating on USENET is gathered up into digests which
are sent to all list members daily.
BITNET traffic is much like Internet traffic. The main
difference is that I maintain only one address for traffic
destined to all BITNET subscribers. That address points to a list
exploder which then sends copies to individual BITNET
subscribers. This way only one copy of a given message has to
cross the BITNET-Internet gateway in either direction.
USENET subscribers see only individual messages. All messages
originating on the Internet side are forwarded to our USENET
machine. They are then posted to the appropriate newsgroup.
Unfortunately, for gatewayed messages, the sender becomes
"news@brl-adm". This is currently an unavoidable side-effect of
the software which performs the gateway function.
As for readership, USENET has an extremely large readership - I
would guess several thousand hosts and tens of thousands of
readers. The master list maintained here at BRL runs about two
hundred fifty entries with roughly ten percent of those being
local redistribution lists. I don't have a good feel for the
size of the BITNET redistribution, but I would guess it is
roughly the same size and composition as the master list.
Traffic runs 150K to 400K bytes per list per week on average.
------------------------------
Subject: What are some useful Unix or C books?
Date: Thu Mar 18 17:16:55 EST 1993
1.5) What are some useful Unix or C books?
Mitch Wright (mitch@cirrus.com) maintains a useful list of Unix
and C books, with descriptions and some mini-reviews. There are
currently 167 titles on his list.
You can obtain a copy of this list by anonymous ftp from
ftp.rahul.net (192.160.13.1), where it's "pub/mitch/YABL/yabl".
Send additions or suggestions to mitch@cirrus.com.
Samuel Ko (kko@sfu.ca) maintains another list of Unix books.
This list contains only recommended books, and is therefore
somewhat shorter. This list is also a classified list, with
books grouped into categories, which may be better if you are
looking for a specific type of book.
You can obtain a copy of this list by anonymous ftp from
rtfm.mit.edu, where it's "pub/usenet/news.answers/books/unix".
Send additions or suggestions to kko@sfu.ca.
If you can't use anonymous ftp, email the line "help" to
"ftpmail@decwrl.dec.com" for instructions on retrieving
things via email.
------------------------------
Subject: What happened to the pronunciation list ... ?
Date: Thu Mar 18 17:16:55 EST 1993
1.6) What happened to the pronunciation list that used to be part of this
document?
From its inception in 1989, this FAQ document included a
comprehensive pronunciation list maintained by Maarten Litmaath
(thanks, Maarten!). It was originally created by Carl Paukstis
<carlp@frigg.isc-br.com>.
It has been retired, since it is not really relevant to the topic
of "Unix questions". You can still find it as part of the
widely-distributed "Jargon" file (maintained by Eric S. Raymond,
eric@snark.thyrsus.com) which seems like a much more appropriate
forum for the topic of "How do you pronounce /* ?"
If you'd like a copy, you can ftp one from ftp.wg.omron.co.jp
(133.210.4.4), it's "pub/unix-faq/docs/Pronunciation-Guide".
Part 2
This article includes answers to: 2.1) How do I remove a file whose name begins with a "-" ?
2.2) How do I remove a file with funny characters in the filename ?
2.3) How do I get a recursive directory listing?
2.4) How do I get the current directory into my prompt?
2.5) How do I read characters from the terminal in a shell script?
2.6) How do I rename "*.foo" to "*.bar", or change file names
to lowercase?
2.7) Why do I get [some strange error message] when I
"rsh host command" ?
2.8) How do I {set an environment variable, change directory} inside a
program or shell script and have that change affect my
current shell?
2.9) How do I redirect stdout and stderr separately in csh?
2.10) How do I tell inside .cshrc if I'm a login shell?
2.11) How do I construct a shell glob-pattern that matches all files
except "." and ".." ?
2.12) How do I find the last argument in a Bourne shell script?
2.13) What's wrong with having '.' in your $PATH ?
2.14) How do I ring the terminal bell during a shell script?
2.15) Why can't I use "talk" to talk with my friend on machine X?
2.16) Why does calendar produce the wrong output?
If you're looking for the answer to, say, question 2.5, and want to skip
everything else, you can search ahead for the regular expression "^2.5)".
While these are all legitimate questions, they seem to crop up in
comp.unix.questions or comp.unix.shell on an annual basis, usually
followed by plenty of replies (only some of which are correct) and then
a period of griping about how the same questions keep coming up. You
may also like to read the monthly article "Answers to Frequently Asked
Questions" in the newsgroup "news.announce.newusers", which will tell
you what "UNIX" stands for.
With the variety of Unix systems in the world, it's hard to guarantee
that these answers will work everywhere. Read your local manual pages
before trying anything suggested here. If you have suggestions or
corrections for any of these answers, please send them to to
tmatimar@isgtec.com.
----------------------------------------------------------------------
Subject: How do I remove a file whose name begins with a "-" ?
Date: Thu Mar 18 17:16:55 EST 1993
2.1) How do I remove a file whose name begins with a "-" ?
Figure out some way to name the file so that it doesn't begin
with a dash. The simplest answer is to use
rm ./-filename
(assuming "-filename" is in the current directory, of course.)
This method of avoiding the interpretation of the "-" works with
other commands too.
Many commands, particularly those that have been written to use
the "getopt(3)" argument parsing routine, accept a "--" argument
which means "this is the last option, anything after this is not
an option", so your version of rm might handle "rm -- -filename".
Some versions of rm that don't use getopt() treat a single "-"
in the same way, so you can also try "rm - -filename".
------------------------------
Subject: How do I remove a file with funny characters in the filename ?
Date: Thu Mar 18 17:16:55 EST 1993
2.2) How do I remove a file with funny characters in the filename ?
If the 'funny character' is a '/', skip to the last part of this
answer. If the funny character is something else, such as a ' '
or control character or character with the 8th bit set, keep reading.
The classic answers are
rm -i some*pattern*that*matches*only*the*file*you*want
which asks you whether you want to remove each file matching
the indicated pattern; depending on your shell, this may not
work if the filename has a character with the 8th bit set (the
shell may strip that off);
and
rm -ri .
which asks you whether to remove each file in the directory.
Answer "y" to the problem file and "n" to everything else.
Unfortunately this doesn't work with many versions of rm. Also
unfortunately, this will walk through every subdirectory of ".",
so you might want to "chmod a-x" those directories temporarily
to make them unsearchable.
Always take a deep breath and think about what you're doing and
double check what you typed when you use rm's "-r" flag or a
wildcard on the command line;
and
find . -type f ... -ok rm '{}' \;
where "..." is a group of predicates that uniquely identify the
file. One possibility is to figure out the inode number of the
problem file (use "ls -i .") and then use
find . -inum 12345 -ok rm '{}' \;
or
find . -inum 12345 -ok mv '{}' new-file-name \;
"-ok" is a safety check - it will prompt you for confirmation of
the command it's about to execute. You can use "-exec" instead
to avoid the prompting, if you want to live dangerously, or if
you suspect that the filename may contain a funny character
sequence that will mess up your screen when printed.
What if the filename has a '/' in it?
These files really are special cases, and can only be created by
buggy kernel code (typically by implementations of NFS that don't
filter out illegal characters in file names from remote
machines.) The first thing to do is to try to understand exactly
why this problem is so strange.
Recall that Unix directories are simply pairs of filenames and
inode numbers. A directory essentially contains information
like this:
filename inode
file1 12345
file2.c 12349
file3 12347
Theoretically, '/' and '\0' are the only two characters that
cannot appear in a filename - '/' because it's used to separate
directories and files, and '\0' because it terminates a filename.
Unfortunately some implementations of NFS will blithely create
filenames with embedded slashes in response to requests from
remote machines. For instance, this could happen when someone on
a Mac or other non-Unix machine decides to create a remote NFS
file on your Unix machine with the date in the filename. Your
Unix directory then has this in it:
filename inode
91/02/07 12357
No amount of messing around with 'find' or 'rm' as described
above will delete this file, since those utilities and all other
Unix programs, are forced to interpret the '/' in the normal way.
Any ordinary program will eventually try to do
unlink("91/02/07"), which as far as the kernel is concerned means
"unlink the file 07 in the subdirectory 02 of directory 91", but
that's not what we have - we have a *FILE* named "91/02/07" in
the current directory. This is a subtle but crucial distinction.
What can you do in this case? The first thing to try is to
return to the Mac that created this crummy entry, and see if you
can convince it and your local NFS daemon to rename the file to
something without slashes.
If that doesn't work or isn't possible, you'll need help from
your system manager, who will have to try the one of the
following. Use "ls -i" to find the inode number of this bogus
file, then unmount the file system and use "clri" to clear the
inode, and "fsck" the file system with your fingers crossed.
This destroys the information in the file. If you want to keep
it, you can try:
create a new directory in the same parent directory as the one
containing the bad file name;
move everything you can (i.e. everything but the file with the
bad name) from the old directory to the new one;
do "ls -id" on the directory containing the file with the bad
name to get its inumber;
umount the file system;
"clri" the directory containing the file with the bad name;
"fsck" the file system.
Then, to find the file,
remount the file system;
rename the directory you created to have the name of the old
directory (since the old directory should have been blown away
by "fsck")
move the file out of "lost+found" into the directory with a
better name.
Alternatively, you can patch the directory the hard way by
crawling around in the raw file system. Use "fsdb", if you
have it.
------------------------------
Subject: How do I get a recursive directory listing?
Date: Thu Mar 18 17:16:55 EST 1993
2.3) How do I get a recursive directory listing?
One of the following may do what you want:
ls -R (not all versions of "ls" have -R)
find . -print (should work everywhere)
du -a . (shows you both the name and size)
If you're looking for a wildcard pattern that will match all ".c"
files in this directory and below, you won't find one, but you
can use
% some-command `find . -name '*.c' -print`
"find" is a powerful program. Learn about it.
------------------------------
Subject: How do I get the current directory into my prompt?
Date: Thu Mar 18 17:16:55 EST 1993
2.4) How do I get the current directory into my prompt?
It depends which shell you are using. It's easy with some
shells, hard or impossible with others.
C Shell (csh):
Put this in your .cshrc - customize the prompt variable the
way you want.
alias setprompt 'set prompt="${cwd}% "'
setprompt # to set the initial prompt
alias cd 'chdir \!* && setprompt'
If you use pushd and popd, you'll also need
alias pushd 'pushd \!* && setprompt'
alias popd 'popd \!* && setprompt'
Some C shells don't keep a $cwd variable - you can use
`pwd` instead.
If you just want the last component of the current directory
in your prompt ("mail% " instead of "/usr/spool/mail% ")
you can use
alias setprompt 'set prompt="$cwd:t% "'
Some older csh's get the meaning of && and || reversed.
Try doing:
false && echo bug
If it prints "bug", you need to switch && and || (and get
a better version of csh.)
Bourne Shell (sh):
If you have a newer version of the Bourne Shell (SVR2 or newer)
you can use a shell function to make your own command, "xcd" say:
xcd() { cd $* ; PS1="`pwd` $ "; }
If you have an older Bourne shell, it's complicated but not
impossible. Here's one way. Add this to your .profile file:
LOGIN_SHELL=$$ export LOGIN_SHELL
CMDFILE=/tmp/cd.$$ export CMDFILE
# 16 is SIGURG, pick a signal that's not likely to be used
PROMPTSIG=16 export PROMPTSIG
trap '. $CMDFILE' $PROMPTSIG
and then put this executable script (without the indentation!),
let's call it "xcd", somewhere in your PATH
: xcd directory - change directory and set prompt
: by signalling the login shell to read a command file
cat >${CMDFILE?"not set"} <<EOF
cd $1
PS1="\`pwd\`$ "
EOF
kill -${PROMPTSIG?"not set"} ${LOGIN_SHELL?"not set"}
Now change directories with "xcd /some/dir".
Korn Shell (ksh):
Put this in your .profile file:
PS1='$PWD $ '
If you just want the last component of the directory, use
PS1='${PWD##*/} $ '
T C shell (tcsh)
Tcsh is a popular enhanced version of csh with some extra
builtin variables (and many other features):
%~ the current directory, using ~ for $HOME
%/ the full pathname of the current directory
%c or %. the trailing component of the current directory
so you can do
set prompt='%~ '
BASH (FSF's "Bourne Again SHell")
\w in $PS1 gives the full pathname of the current directory,
with ~ expansion for $HOME; \W gives the basename of
the current directory. So, in addition to the above sh and
ksh solutions, you could use
PS1='\w $ '
or
PS1='\W $ '
------------------------------
Subject: How do I read characters from the terminal in a shell script?
Date: Thu Mar 18 17:16:55 EST 1993
2.5) How do I read characters from the terminal in a shell script?
In sh, use read. It is most common to use a loop like
while read line
do
...
done
In csh, use $< like this:
while ( 1 )
set line = "$<"
if ( "$line" == "" ) break
...
end
Unfortunately csh has no way of distinguishing between a blank
line and an end-of-file.
If you're using sh and want to read a *single* character from the
terminal, you can try something like
echo -n "Enter a character: "
stty cbreak # or stty raw
readchar=`dd if=/dev/tty bs=1 count=1 2>/dev/null`
stty -cbreak
echo "Thank you for typing a $readchar ."
------------------------------
Subject: How do I rename "*.foo" to "*.bar", or change file names to lowercase?
Date: Thu Mar 18 17:16:55 EST 1993
2.6) How do I rename "*.foo" to "*.bar", or change file names to lowercase?
Why doesn't "mv *.foo *.bar" work? Think about how the shell
expands wildcards. "*.foo" and "*.bar" are expanded before the
mv command ever sees the arguments. Depending on your shell,
this can fail in a couple of ways. CSH prints "No match."
because it can't match "*.bar". SH executes "mv a.foo b.foo
c.foo *.bar", which will only succeed if you happen to have a
single directory named "*.bar", which is very unlikely and almost
certainly not what you had in mind.
Depending on your shell, you can do it with a loop to "mv" each
file individually. If your system has "basename", you can use:
C Shell:
foreach f ( *.foo )
set base=`basename $f .foo`
mv $f $base.bar
end
Bourne Shell:
for f in *.foo; do
base=`basename $f .foo`
mv $f $base.bar
done
Some shells have their own variable substitution features, so
instead of using "basename", you can use simpler loops like:
C Shell:
foreach f ( *.foo )
mv $f $f:r.bar
end
Korn Shell:
for f in *.foo; do
mv $f ${f%foo}bar
done
If you don't have "basename" or want to do something like
renaming foo.* to bar.*, you can use something like "sed" to
strip apart the original file name in other ways, but the general
looping idea is the same. You can also convert file names into
"mv" commands with 'sed', and hand the commands off to "sh" for
execution. Try
ls -d *.foo | sed -e 's/.*/mv & &/' -e 's/foo$/bar/' | sh
A program by Vladimir Lanin called "mmv" that does this job
nicely was posted to comp.sources.unix (Volume 21, issues 87 and
88) in April 1990. It lets you use
mmv '*.foo' '=1.bar'
Shell loops like the above can also be used to translate file
names from upper to lower case or vice versa. You could use
something like this to rename uppercase files to lowercase:
C Shell:
foreach f ( * )
mv $f `echo $f | tr '[A-Z]' '[a-z]'`
end
Bourne Shell:
for f in *; do
mv $f `echo $f | tr '[A-Z]' '[a-z]'`
done
Korn Shell:
typeset -l l
for f in *; do
l="$f"
mv $f $l
done
If you wanted to be really thorough and handle files with `funny'
names (embedded blanks or whatever) you'd need to use
Bourne Shell:
for f in *; do
g=`expr "xxx$f" : 'xxx\(.*\)' | tr '[A-Z]' '[a-z]'`
mv "$f" "$g"
done
The `expr' command will always print the filename, even if it
equals `-n' or if it contains a System V escape sequence like `\c'.
Some versions of "tr" require the [ and ], some don't. It
happens to be harmless to include them in this particular
example; versions of tr that don't want the [] will conveniently
think they are supposed to translate '[' to '[' and ']' to ']'.
If you have the "perl" language installed, you may find this
rename script by Larry Wall very useful. It can be used to
accomplish a wide variety of filename changes.
#!/usr/bin/perl
#
# rename script examples from lwall:
# rename 's/\.orig$//' *.orig
# rename 'y/A-Z/a-z/ unless /^Make/' *
# rename '$_ .= ".bad"' *.f
# rename 'print "$_: "; s/foo/bar/ if <stdin> =~ /^y/i' *
$op = shift;
for (@ARGV) {
$was = $_;
eval $op;
die $@ if $@;
rename($was,$_) unless $was eq $_;
}
------------------------------
Subject: Why do I get [some strange error message] when I "rsh host command" ?
Date: Thu Mar 18 17:16:55 EST 1993
2.7) Why do I get [some strange error message] when I "rsh host command" ?
(We're talking about the remote shell program "rsh" or sometimes
"remsh" or "remote"; on some machines, there is a restricted shell
called "rsh", which is a different thing.)
If your remote account uses the C shell, the remote host will
fire up a C shell to execute 'command' for you, and that shell
will read your remote .cshrc file. Perhaps your .cshrc contains
a "stty", "biff" or some other command that isn't appropriate for
a non-interactive shell. The unexpected output or error message
from these commands can screw up your rsh in odd ways.
Here's an example. Suppose you have
stty erase ^H
biff y
in your .cshrc file. You'll get some odd messages like this.
% rsh some-machine date
stty: : Can't assign requested address
Where are you?
Tue Oct 1 09:24:45 EST 1991
You might also get similar errors when running certain "at" or
"cron" jobs that also read your .cshrc file.
Fortunately, the fix is simple. There are, quite possibly, a
whole *bunch* of operations in your ".cshrc" (e.g., "set
history=N") that are simply not worth doing except in interactive
shells. What you do is surround them in your ".cshrc" with:
if ( $?prompt ) then
operations....
endif
and, since in a non-interactive shell "prompt" won't be set, the
operations in question will only be done in interactive shells.
You may also wish to move some commands to your .login file; if
those commands only need to be done when a login session starts
up (checking for new mail, unread news and so on) it's better to
have them in the .login file.
------------------------------
Subject: How do I ... and have that change affect my current shell?
Date: Thu Mar 18 17:16:55 EST 1993
2.8) How do I {set an environment variable, change directory} inside
a program or shell script and have that change affect my
current shell?
In general, you can't, at least not without making special
arrangements. When a child process is created, it inherits a
copy of its parent's variables (and current directory). The
child can change these values all it wants but the changes won't
affect the parent shell, since the child is changing a copy of
the original data.
Some special arrangements are possible. Your child process could
write out the changed variables, if the parent was prepared to
read the output and interpret it as commands to set its own
variables.
Also, shells can arrange to run other shell scripts in the
context of the current shell, rather than in a child process, so
that changes will affect the original shell.
For instance, if you have a C shell script named "myscript":
cd /very/long/path
setenv PATH /something:/something-else
or the equivalent Bourne or Korn shell script
cd /very/long/path
PATH=/something:/something-else export PATH
and try to run "myscript" from your shell, your shell will fork
and run the shell script in a subprocess. The subprocess is also
running the shell; when it sees the "cd" command it changes *its*
current directory, and when it sees the "setenv" command it
changes *its* environment, but neither has any effect on the
current directory of the shell at which you're typing (your login
shell, let's say).
In order to get your login shell to execute the script (without
forking) you have to use the "." command (for the Bourne or Korn
shells) or the "source" command (for the C shell). I.e. you type
. myscript
to the Bourne or Korn shells, or
source myscript
to the C shell.
If all you are trying to do is change directory or set an
environment variable, it will probably be simpler to use a C
shell alias or Bourne/Korn shell function. See the "how do I get
the current directory into my prompt" section of this article for
some examples.
A much more detailed answer prepared by
xtm@telelogic.se (Thomas Michanek) can be found at
ftp.wg.omron.co.jp in /pub/unix-faq/docs/script-vs-env.
------------------------------
Subject: How do I redirect stdout and stderr separately in csh?
>From: msb@sq.com (Mark Brader)
Date: Mon, 26 Oct 1992 20:15:00 -0500
2.9) How do I redirect stdout and stderr separately in csh?
In csh, you can redirect stdout with ">", or stdout and stderr
together with ">&" but there is no direct way to redirect stderr
only. The best you can do is
( command >stdout_file ) >&stderr_file
which runs "command" in a subshell; stdout is redirected inside
the subshell to stdout_file, and both stdout and stderr from the
subshell are redirected to stderr_file, but by this point stdout
has already been redirected so only stderr actually winds up in
stderr_file.
If what you want is to avoid redirecting stdout at all, let sh
do it for you.
sh -c 'command 2>stderr_file'
------------------------------
Subject: How do I tell inside .cshrc if I'm a login shell?
Date: Thu Mar 18 17:16:55 EST 1993
2.10) How do I tell inside .cshrc if I'm a login shell?
When people ask this, they usually mean either
How can I tell if it's an interactive shell? or
How can I tell if it's a top-level shell?
You could perhaps determine if your shell truly is a login shell
(i.e. is going to source ".login" after it is done with ".cshrc")
by fooling around with "ps" and "$$". Login shells generally
have names that begin with a '-'. If you're really interested in
the other two questions, here's one way you can organize your
.cshrc to find out.
if (! $?CSHLEVEL) then
#
# This is a "top-level" shell,
# perhaps a login shell, perhaps a shell started up by
# 'rsh machine some-command'
# This is where we should set PATH and anything else we
# want to apply to every one of our shells.
#
setenv CSHLEVEL 0
set home = ~username # just to be sure
source ~/.env # environment stuff we always want
else
#
# This shell is a child of one of our other shells so
# we don't need to set all the environment variables again.
#
set tmp = $CSHLEVEL
@ tmp++
setenv CSHLEVEL $tmp
endif
# Exit from .cshrc if not interactive, e.g. under rsh
if (! $?prompt) exit
# Here we could set the prompt or aliases that would be useful
# for interactive shells only.
source ~/.aliases
------------------------------
Subject: How do I construct a ... matches all files except "." and ".." ?
Date: Thu Mar 18 17:16:55 EST 1993
2.11) How do I construct a shell glob-pattern that matches all files
except "." and ".." ?
You'd think this would be easy.
* Matches all files that don't begin with a ".";
.* Matches all files that do begin with a ".", but
this includes the special entries "." and "..",
which often you don't want;
.[!.]* (Newer shells only; some shells use a "^" instead of
the "!"; POSIX shells must accept the "!", but may
accept a "^" as well; all portable applications shall
not use an unquoted "^" immediately following the "[")
Matches all files that begin with a "." and are
followed by a non-"."; unfortunately this will miss
"..foo";
.??* Matches files that begin with a "." and which are
at least 3 characters long. This neatly avoids
"." and "..", but also misses ".a" .
So to match all files except "." and ".." safely you have to use
3 patterns (if you don't have filenames like ".a" you can leave
out the first):
.[!.]* .??* *
Alternatively you could employ an external program or two and use
backquote substitution. This is pretty good:
`ls -a | sed -e '/^\.$/d' -e '/^\.\.$/d'`
(or `ls -A` in some Unix versions)
but even it will mess up on files with newlines, IFS characters
or wildcards in their names.
In ksh, you can use: .!(.|) *
------------------------------
Subject: How do I find the last argument in a Bourne shell script?
Date: Thu Mar 18 17:16:55 EST 1993
2.12) How do I find the last argument in a Bourne shell script?
Answer by:
Martin Weitzel <@mikros.systemware.de:martin@mwtech.uucp>
Maarten Litmaath <maart@nat.vu.nl>
If you are sure the number of arguments is at most 9, you can use:
eval last=\${$#}
In POSIX-compatible shells it works for ANY number of arguments.
The following works always too:
for last
do
:
done
This can be generalized as follows:
for i
do
third_last=$second_last
second_last=$last
last=$i
done
Now suppose you want to REMOVE the last argument from the list,
or REVERSE the argument list, or ACCESS the N-th argument
directly, whatever N may be. Here is a basis of how to do it,
using only built-in shell constructs, without creating subprocesses:
t0= u0= rest='1 2 3 4 5 6 7 8 9' argv=
for h in '' $rest
do
for t in "$t0" $rest
do
for u in $u0 $rest
do
case $# in
0)
break 3
esac
eval argv$h$t$u=\$1
argv="$argv \"\$argv$h$t$u\"" # (1)
shift
done
u0=0
done
t0=0
done
# now restore the arguments
eval set x "$argv" # (2)
shift
This example works for the first 999 arguments. Enough?
Take a good look at the lines marked (1) and (2) and convince
yourself that the original arguments are restored indeed, no
matter what funny characters they contain!
To find the N-th argument now you can use this:
eval argN=\$argv$N
To reverse the arguments the line marked (1) must be changed to:
argv="\"\$argv$h$t$u\" $argv"
How to remove the last argument is left as an exercise.
If you allow subprocesses as well, possibly executing nonbuilt-in
commands, the `argvN' variables can be set up more easily:
N=1
for i
do
eval argv$N=\$i
N=`expr $N + 1`
done
To reverse the arguments there is still a simpler method, that
even does not create subprocesses. This approach can also be
taken if you want to delete e.g. the last argument, but in that
case you cannot refer directly to the N-th argument any more,
because the `argvN' variables are set up in reverse order:
argv=
for i
do
eval argv$#=\$i
argv="\"\$argv$#\" $argv"
shift
done
eval set x "$argv"
shift
------------------------------
Subject: What's wrong with having '.' in your $PATH ?
Date: Thu Mar 18 17:16:55 EST 1993
2.13) What's wrong with having '.' in your $PATH ?
A bit of background: the PATH environment variable is a list of
directories separated by colons. When you type a command name
without giving an explicit path (e.g. you type "ls", rather than
"/bin/ls") your shell searches each directory in the PATH list in
order, looking for an executable file by that name, and the shell
will run the first matching program it finds.
One of the directories in the PATH list can be the current
directory "." . It is also permissible to use an empty directory
name in the PATH list to indicate the current directory. Both of
these are equivalent
for csh users:
setenv PATH :/usr/ucb:/bin:/usr/bin
setenv PATH .:/usr/ucb:/bin:/usr/bin
for sh or ksh users
PATH=:/usr/ucb:/bin:/usr/bin export PATH
PATH=.:/usr/ucb:/bin:/usr/bin export PATH
Having "." somewhere in the PATH is convenient - you can type
"a.out" instead of "./a.out" to run programs in the current
directory. But there's a catch.
Consider what happens in the case where "." is the first entry
in the PATH. Suppose your current directory is a publically-
writable one, such as "/tmp". If there just happens to be a
program named "/tmp/ls" left there by some other user, and you
type "ls" (intending, of course, to run the normal "/bin/ls"
program), your shell will instead run "./ls", the other user's
program. Needless to say, the results of running an unknown
program like this might surprise you.
It's slightly better to have "." at the end of the PATH:
setenv PATH /usr/ucb:/bin:/usr/bin:.
Now if you're in /tmp and you type "ls", the shell will
search /usr/ucb, /bin and /usr/bin for a program named
"ls" before it gets around to looking in ".", and there
is less risk of inadvertently running some other user's
"ls" program. This isn't 100% secure though - if you're
a clumsy typist and some day type "sl -l" instead of "ls -l",
you run the risk of running "./sl", if there is one.
Some "clever" programmer could anticipate common typing
mistakes and leave programs by those names scattered
throughout public directories. Beware.
Many seasoned Unix users get by just fine without having
"." in the PATH at all:
setenv PATH /usr/ucb:/bin:/usr/bin
If you do this, you'll need to type "./program" instead
of "program" to run programs in the current directory, but
the increase in security is probably worth it.
------------------------------
Subject: How do I ring the terminal bell during a shell script?
>From: uwe@mpi-sb.mpg.de (Uwe Waldmann)
Date: Fri, 30 Apr 93 16:33:00 +0200
2.14) How do I ring the terminal bell during a shell script?
The answer depends on your Unix version (or rather on the kind of
"echo" program that is available on your machine).
A BSD-like "echo" uses the "-n" option for suppressing the final
newline and does not understand the octal \nnn notation. Thus
the command is
echo -n '^G'
where ^G means a _literal_ BEL-character (you can produce this in
emacs using "Ctrl-Q Ctrl-G" and in vi using "Ctrl-V Ctrl-G").
A SysV-like "echo" understands the \nnn notation and uses \c to
suppress the final newline, so the answer is:
echo '\007\c'
------------------------------
Subject: Why can't I use "talk" to talk with my friend on machine X?
>From: tmatimar@isgtec.com (Ted Timar)
Date: Thu Mar 18 17:16:55 EST 1993
2.15) Why can't I use "talk" to talk with my friend on machine X?
Unix has three common "talk" programs, none of which can talk with
any of the others. The "old" talk accounts for the first two types.
This version (often called otalk) did not take "endian" order into
account when talking to other machines. As a consequence, the Vax
version of otalk cannot talk with the Sun version of otalk.
These versions of talk use port 517.
Around 1987, most vendors (except Sun, who took 6 years longer than
any of their competitors) standardized on a new talk (often called
ntalk) which knows about network byte order. This talk works between
all machines that have it. This version of talk uses port 518.
There are now a few talk programs that speak both ntalk and one
version of otalk. The most common of these is called "ytalk".
------------------------------
Subject: Why does calendar produce the wrong output?
>From: tmatimar@isgtec.com (Ted Timar)
Date: Thu Sep 8 09:45:46 EDT 1994
2.16) Why does calendar produce the wrong output?
Frequently, people find that the output for the Unix calendar
program, 'cal' produces output that they do not expect.
The calendar for September 1752 is very odd:
September 1752
S M Tu W Th F S
1 2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
This is the month in which the US (the entire British Empire actually)
switched from the Julian to the Gregorian calendar.
The other common problem people have with the calendar program is
that they pass it arguments like 'cal 9 94'. This gives the calendar
for September of AD 94, NOT 1994.
Part 3
This article includes answers to: 3.1) How do I find the creation time of a file?
3.2) How do I use "rsh" without having the rsh hang around
until the remote command has completed?
3.3) How do I truncate a file?
3.4) Why doesn't find's "{}" symbol do what I want?
3.5) How do I set the permissions on a symbolic link?
3.6) How do I "undelete" a file?
3.7) How can a process detect if it's running in the background?
3.8) Why doesn't redirecting a loop work as intended? (Bourne shell)
3.9) How do I run 'passwd', 'ftp', 'telnet', 'tip' and other interactive
programs from a shell script or in the background?
3.10) How do I find the process ID of a program with a particular
name from inside a shell script or C program?
3.11) How do I check the exit status of a remote command
executed via "rsh" ?
3.12) Is it possible to pass shell variable settings into an awk program?
3.13) How do I get rid of zombie processes that persevere?
3.14) How do I get lines from a pipe as they are written instead of
only in larger blocks?
3.15) How do I get the date into a filename?
3.16) Why do some scripts start with #! ... ?
If you're looking for the answer to, say, question 3.5, and want to skip
everything else, you can search ahead for the regular expression "^3.5)".
While these are all legitimate questions, they seem to crop up in
comp.unix.questions or comp.unix.shell on an annual basis, usually
followed by plenty of replies (only some of which are correct) and then
a period of griping about how the same questions keep coming up. You
may also like to read the monthly article "Answers to Frequently Asked
Questions" in the newsgroup "news.announce.newusers", which will tell
you what "UNIX" stands for.
With the variety of Unix systems in the world, it's hard to guarantee
that these answers will work everywhere. Read your local manual pages
before trying anything suggested here. If you have suggestions or
corrections for any of these answers, please send them to to
tmatimar@isgtec.com.
----------------------------------------------------------------------
Subject: How do I find the creation time of a file?
Date: Thu Mar 18 17:16:55 EST 1993
3.1) How do I find the creation time of a file?
You can't - it isn't stored anywhere. Files have a last-modified
time (shown by "ls -l"), a last-accessed time (shown by "ls -lu")
and an inode change time (shown by "ls -lc"). The latter is often
referred to as the "creation time" - even in some man pages -
but that's wrong; it's also set by such operations as mv, ln,
chmod, chown and chgrp.
The man page for "stat(2)" discusses this.
------------------------------
Subject: How do I use "rsh" without having the rsh hang around ... ?
Date: Thu Mar 18 17:16:55 EST 1993
3.2) How do I use "rsh" without having the rsh hang around until the
remote command has completed?
(See note in question 2.7 about what "rsh" we're talking about.)
The obvious answers fail:
rsh machine command &
or rsh machine 'command &'
For instance, try doing rsh machine 'sleep 60 &' and you'll see
that the 'rsh' won't exit right away. It will wait 60 seconds
until the remote 'sleep' command finishes, even though that
command was started in the background on the remote machine. So
how do you get the 'rsh' to exit immediately after the 'sleep' is
started?
The solution - if you use csh on the remote machine:
rsh machine -n 'command >&/dev/null </dev/null &'
If you use sh on the remote machine:
rsh machine -n 'command >/dev/null 2>&1 </dev/null &'
Why? "-n" attaches rsh's stdin to /dev/null so you could run the
complete rsh command in the background on the LOCAL machine.
Thus "-n" is equivalent to another specific "< /dev/null".
Furthermore, the input/output redirections on the REMOTE machine
(inside the single quotes) ensure that rsh thinks the session can
be terminated (there's no data flow any more.)
Note: The file that you redirect to/from on the remote machine
doesn't have to be /dev/null; any ordinary file will do.
In many cases, various parts of these complicated commands
aren't necessary.
------------------------------
Subject: How do I truncate a file?
Date: Mon, 27 Mar 1995 18:09:10 -0500
3.3) How do I truncate a file?
The BSD function ftruncate() sets the length of a file.
(But not all versions behave identically.) Other Unix variants
all seem to support some version of truncation as well.
For systems which support the ftruncate function, there are
three known behaviours:
BSD 4.2 - Ultrix, SGI, LynxOS
- truncation doesn't grow file
- truncation doesn't move file pointer
BSD 4.3 - SunOS, Solaris, OSF/1, HP/UX, Amiga
- truncation can grow file
- truncation doesn't move file pointer
Cray - UniCOS 7, UniCOS 8
- truncation doesn't grow file
- truncation changes file pointer
Other systems come in four varieties:
F_CHSIZE - Only SCO
- some systems define F_CHSIZE but don't support it
- behaves like BSD 4.3
F_FREESP - Only Interative Unix
- some systems (eg. Interactive Unix) define F_FREESP but
don't support it
- behaves like BSD 4.3
chsize() - QNX and SCO
- some systems (eg. Interactive Unix) have chsize() but
don't support it
- behaves like BSD 4.3
nothing - no known systems
- there will be systems that don't support truncate at all
Moderator's Note: I grabbed the functions below a few years back.
I can no longer identify the original author.
S. Spencer Sun <spencer@ncd.com> has also
contributed a version for F_FREESP.
functions for each non-native ftruncate follow
/* ftruncate emulations that work on some System V's.
This file is in the public domain. */
#include
#include
#ifdef F_CHSIZE
int
ftruncate (fd, length)
int fd;
off_t length;
{
return fcntl (fd, F_CHSIZE, length);
}
#else
#ifdef F_FREESP
/* The following function was written by
kucharsk@Solbourne.com (William Kucharski) */
#include
#include
#include
int
ftruncate (fd, length)
int fd;
off_t length;
{
struct flock fl;
struct stat filebuf;
if (fstat (fd, &filebuf) < 0)
return -1;
if (filebuf.st_size < length)
{
/* Extend file length. */
if (lseek (fd, (length - 1), SEEK_SET) < 0)
return -1;
/* Write a "0" byte. */
if (write (fd, "", 1) != 1)
return -1;
}
else
{
/* Truncate length. */
fl.l_whence = 0;
fl.l_len = 0;
fl.l_start = length;
fl.l_type = F_WRLCK; /* Write lock on file space. */
/* This relies on the UNDOCUMENTED F_FREESP argument to
fcntl, which truncates the file so that it ends at the
position indicated by fl.l_start.
Will minor miracles never cease? */
if (fcntl (fd, F_FREESP, &fl) < 0)
return -1;
}
return 0;
}
#else
int
ftruncate (fd, length)
int fd;
off_t length;
{
return chsize (fd, length);
}
#endif
#endif
------------------------------
Subject: Why doesn't find's "{}" symbol do what I want?
Date: Thu Mar 18 17:16:55 EST 1993
3.4) Why doesn't find's "{}" symbol do what I want?
"find" has a -exec option that will execute a particular command
on all the selected files. Find will replace any "{}" it sees
with the name of the file currently under consideration.
So, some day you might try to use "find" to run a command on
every file, one directory at a time. You might try this:
find /path -type d -exec command {}/\* \;
hoping that find will execute, in turn
command directory1/*
command directory2/*
...
Unfortunately, find only expands the "{}" token when it appears
by itself. Find will leave anything else like "{}/*" alone, so
instead of doing what you want, it will do
command {}/*
command {}/*
...
once for each directory. This might be a bug, it might be a
feature, but we're stuck with the current behaviour.
So how do you get around this? One way would be to write a
trivial little shell script, let's say "./doit", that consists of
command "$1"/*
You could then use
find /path -type d -exec ./doit {} \;
Or if you want to avoid the "./doit" shell script, you can use
find /path -type d -exec sh -c 'command $0/*' {} \;
(This works because within the 'command' of "sh -c 'command' A B C ...",
$0 expands to A, $1 to B, and so on.)
or you can use the construct-a-command-with-sed trick
find /path -type d -print | sed 's:.*:command &/*:' | sh
If all you're trying to do is cut down on the number of times
that "command" is executed, you should see if your system has the
"xargs" command. Xargs reads arguments one line at a time from
the standard input and assembles as many of them as will fit into
one command line. You could use
find /path -print | xargs command
which would result in one or more executions of
command file1 file2 file3 file4 dir1/file1 dir1/file2
Unfortunately this is not a perfectly robust or secure solution.
Xargs expects its input lines to be terminated with newlines, so
it will be confused by files with odd characters such as newlines
in their names.
------------------------------
Subject: How do I set the permissions on a symbolic link?
Date: Thu Mar 18 17:16:55 EST 1993
3.5) How do I set the permissions on a symbolic link?
Permissions on a symbolic link don't really mean anything. The
only permissions that count are the permissions on the file that
the link points to.
------------------------------
Subject: How do I "undelete" a file?
Date: Thu Mar 18 17:16:55 EST 1993
3.6) How do I "undelete" a file?
Someday, you are going to accidentally type something like
"rm * .foo", and find you just deleted "*" instead of "*.foo".
Consider it a rite of passage.
Of course, any decent systems administrator should be doing
regular backups. Check with your sysadmin to see if a recent
backup copy of your file is available. But if it isn't, read
on.
For all intents and purposes, when you delete a file with "rm" it
is gone. Once you "rm" a file, the system totally forgets which
blocks scattered around the disk were part of your file. Even
worse, the blocks from the file you just deleted are going to be
the first ones taken and scribbled upon when the system needs
more disk space. However, never say never. It is theoretically
possible *if* you shut down the system immediately after the "rm"
to recover portions of the data. However, you had better have a
very wizardly type person at hand with hours or days to spare to
get it all back.
Your first reaction when you "rm" a file by mistake is why not
make a shell alias or procedure which changes "rm" to move files
into a trash bin rather than delete them? That way you can
recover them if you make a mistake, and periodically clean out
your trash bin. Two points: first, this is generally accepted
as a *bad* idea. You wil