HERBER@FNALA.FNAL.GOV ("Randolph J. Herber, CD/DCD/SP, x2966") (03/06/91)
Frequently Asked Questions: UNIX OS w/answers
This article was obtained from a USENET news group (conference).
[Last changed: $Date: 90/08/01 23:44:07 $ by $Author: sahayman $]
This article contains the answers to some Frequently Asked Questions
often seen in comp.unix.questions and comp.unix.wizards. 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 article includes answers to:
1) How do I remove a file whose name begins with a "-" ?
2) How do I remove a file with funny characters in the filename ?
3) How do I get a recursive directory listing?
4) How do I get the current directory into my prompt?
5) How do I read characters from a terminal without requiring the user
to hit RETURN?
6) How do I read characters from the terminal in a shell script?
7) How do I check to see if there are characters to be read without
actually reading?
8) How do I find the name of an open file?
9) How do I rename "*.foo" to "*.bar", or change file names
to lowercase?
10) Why do I get [some strange error message] when I
"rsh host command" ?
11) How do I find out the creation time of a file?
12) How do I use "rsh" without having the rsh hang around
until the remote command has completed?
13) How do I truncate a file?
14) How do I {set an environment variable, change directory} inside a
shell script and have that change affect my current shell?
15) Why doesn't find's "{}" symbol do what I want?
16) How do I redirect stdout and stderr separately in csh?
17) How do I set the permissions on a symbolic link?
18) When someone refers to 'rn(1)' or 'ctime(3)', what does
the number in parentheses mean?
19) What does {awk,grep,fgrep,egrep,biff,cat,gecos,nroff,troff,tee,bss}
stand for?
20) How does the gateway between "comp.unix.questions" and the
"info-unix" mailing list work?
21) How do I "undelete" a file?
22) How can a process detect if it's running in the background?
23) How can an executing program determine its own pathname?
24) How do I pronounce "vi" , or "!", or "/*", or ...?
If you're looking for the answer to, say, question 14, and want to skip
everything else, you can search ahead for the regular expression "^14)".
While these are all legitimate questions, they seem to crop up in
comp.unix.questions 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
sahayman@iuvax.cs.indiana.edu or iuvax!sahayman.
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".
2) How do I remove a file with funny characters in the filename ?
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.
If none of these work, find your system manager.
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.
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 some signal that isn't 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
%d or %/ the full pathname of the current directory
%c or %. the trailing component of the current directory
so you can do
set prompt='%~ '
5) How do I read characters from a terminal without requiring the user
to hit RETURN?
Check out cbreak mode in BSD, ~ICANON mode in SysV.
If you don't want to tackle setting the terminal parameters
yourself (using the "ioctl(2)" system call) you can let the stty
program do the work - but this is slow and inefficient, and you
should change the code to do it right some time:
#include <stdio.h>
main()
{
int c;
printf("Hit any character to continue\n");
/*
* ioctl() would be better here; only lazy
* programmers do it this way:
*/
system("/bin/stty cbreak"); /* or "stty raw" */
c = getchar();
system("/bin/stty -cbreak");
printf("Thank you for typing %c.\n", c);
exit(0);
}
You might like to check out the documentation for the "curses"
library of portable screen functions. Often if you're interested
in single-character I/O like this, you're also interested in doing
some sort of screen display control, and the curses library
provides various portable routines for both functions.
6) 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 ."
7) How do I check to see if there are characters to be read without
actually reading?
Certain versions of UNIX provide ways to check whether
characters are currently available to be read from a file
descriptor. In BSD, you can use select(2). You can also use
the FIONREAD ioctl (see tty(4)), which returns the number of
characters waiting to be read, but only works on terminals,
pipes and sockets. In System V Release 3, you can use poll(2),
but that only works on streams. In Xenix - and therefore
Unix SysV r3.2 and later - the rdchk() system call reports
whether a read() call on a given file descriptor will block.
There is no way to check whether characters are available to be
read from a FILE pointer. (You could poke around inside stdio data
structures to see if the input buffer is nonempty, but that wouldn't
work since you'd have no way of knowing what will happen the next
time you try to fill the buffer.)
Sometimes people ask this question with the intention of writing
if (characters available from fd)
read(fd, buf, sizeof buf);
in order to get the effect of a nonblocking read. This is not the
best way to do this, because it is possible that characters will
be available when you test for availability, but will no longer
be available when you call read. Instead, set the O_NDELAY flag
(which is also called FNDELAY under BSD) using the F_SETFL option
of fcntl(2). Older systems (Version 7, 4.1 BSD) don't have O_NDELAY;
on these systems the closest you can get to a nonblocking read is
to use alarm(2) to time out the read.
8) How do I find the name of an open file?
In general, this is too difficult. The file descriptor may
be attached to a pipe or pty, in which case it has no name.
It may be attached to a file that has been removed. It may
have multiple names, due to either hard or symbolic links.
If you really need to do this, and be sure you think long
and hard about it and have decided that you have no choice,
you can use find with the -inum and possibly -xdev option,
or you can use ncheck, or you can recreate the functionality
of one of these within your program. Just realize that
searching a 600 megabyte filesystem for a file that may not
even exist is going to take some time.
9) 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 called "ren" that does this job nicely was posted
to comp.sources.unix some time ago. It lets you use
ren '*.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 $f | tr '[A-Z]' '[a-z]'`
done
If you wanted to be really thorough and handle files with
`funny' names (embedded blanks or whatever) you'd need to use
Bourne Shell:
for f in *; do
eval mv '"$f"' \"`echo "$f" | tr '[A-Z]' '[a-z]'`\"
done
(Some versions of "tr" require the [ and ], some don't. It happens
to be harmless to include them in this particular example; versions of
tr that don't want the [] will conveniently think they are supposed
to translate '[' to '[' and ']' to ']').
If you have the "perl" language installed, you may find this rename
script by Larry Wall very useful. It can be used to accomplish a
wide variety of filename changes.
#!/usr/bin/perl
#
# rename script examples from lwall:
# rename 's/\.orig$//' *.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 $_;
}
10) 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";
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.
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.
11) How do I find out 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.
12) How do I use "rsh" without having the rsh hang around until the
remote command has completed?
(See note in question 10 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.
13) How do I truncate a file?
The BSD function ftruncate() sets the length of a file. Xenix -
and therefore SysV r3.2 and later - has the chsize() system call.
For other systems, the only kind of truncation you can do is
truncation to length zero with creat() or open(..., O_TRUNC).
14) How do I {set an environment variable, change directory} inside a
shell script and have that change affect my current shell?
You can't, unless you use a special command to run the script in
the context of the current shell rather than in a child program.
The process environment (including environment variables and
current directory) is inherited by child programs but cannot be
passed back to parent programs.
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.
15) 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.
16) 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.
Sometimes it's easier to let sh do the work for you.
sh -c 'command >stdout_file 2>stderr_file'
17) 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.
18) 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.
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.
19) What does {awk,grep,fgrep,egrep,biff,cat,gecos,nroff,troff,tee,bss}
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 System"
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
Don Libes' book "Life with Unix" contains lots more of these
tidbits.
20) 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.
21) 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 comprised 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 will
become dependent upon this behaviour of "rm", and you will find
yourself someday on a normal system where "rm" is really "rm", and
you will get yourself in trouble. Second, you will eventually find
that the hassle of dealing with the disk space and time involved in
maintaining the trash bin, it might be easier just to be a bit more
careful with "rm". For starters, you should look up the "-i" option
to "rm" in your manual.
If you are still undaunted, then here is a possible simple answer. You
can create yourself a "can" command which moves files into a
trashcan directory. In csh(1) you can place the following commands
in the ".login" file in your home directory:
alias can 'mv \!* ~/.trashcan' # junk file(s) to trashcan
alias mtcan 'rm -f ~/.trashcan/*' # irretrievably empty trash
if ( ! -d ~/.trashcan ) mkdir ~/.trashcan # ensure trashcan exists
You might also want to put a:
rm -f ~/.trashcan/*
in the ".logout" file in your home directory to automatically empty
the trash when you log out. (sh and ksh versions are left as an
exercise for the reader.)
MIT's Project Athena has produced a comprehensive
delete/undelete/expunge/purge package, which can serve as a
complete replacement for rm which allows file recovery. This
package was posted to comp.sources.unix (volume 18, issue 73).
22) How can a process detect if it's running in the background?
First of all: do you want to know if you're running in the background,
or if you're running interactively? If you're deciding whether or
not you should print prompts and the like, that's probably a better
criterion. Check if standard input is a terminal:
sh: if [ -t 0 ]; then ... fi
C: if(isatty(0)) { ... }
In general, you can't tell if you're running in the background.
The fundamental problem is that different shells and different
versions of UNIX have different notions of what "foreground" and
"background" mean - and on the most common type of system with a
better-defined notion of what they mean, programs can be moved
arbitrarily between foreground and background!
UNIX systems without job control typically put a process into the
background by ignoring SIGINT and SIGQUIT and redirecting the standard
input to "/dev/null"; this is done by the shell.
Shells that support job control, on UNIX systems that support job
control, put a process into the background by giving it a process group
ID different from the process group to which the terminal belongs. They
move it back into the foreground by setting the terminal's process group
ID to that of the process. Shells that do *not* support job control, on
UNIX systems that support job control, typically do what shells do on
systems that don't support job control.
23) How can an executing program determine its own pathname?
Your program can look at argv[0]; if it begins with a "/",
it is probably the absolute pathname to your program, otherwise
your program can look at every directory named in the environment
variable PATH and try to find the first one that contains an
executable file whose name matches your program's argv[0]
(which by convention is the name of the file being executed).
By concatenating that directory and the value of argv[0] you'd
probably have the right name.
You can't really be sure though, since it is quite legal for one
program to exec() another with any value of argv[0] it desires.
It is merely a convention that new programs are exec'd with the
executable file name in argv[0].
For instance, purely a hypothetical example:
#include <stdio.h>
main()
{
execl("/usr/games/rogue", "vi Thesis", (char *)NULL);
}
The executed program thinks its name (its argv[0] value) is
"vi Thesis". (Certain other programs might also think that
the name of the program you're currently running is "vi Thesis",
but of course this is just a hypothetical example, don't
try it yourself :-)
24) How do I pronounce "vi" , or "!", or "/*", or ...?
You can start a very long and pointless discussion by wondering
about this topic on the net. Some people say "vye", some say
"vee-eye" (the vi manual suggests this) and some Roman numerologists
say "six". How you pronounce "vi" has nothing to do with whether
or not you are a true Unix wizard.
Similarly, you'll find that some people pronounce "char" as "care",
and that there are lots of ways to say "#" or "/*" or "!" or
"tty" or "/etc". No one pronunciation is correct - enjoy the regional
dialects and accents.
Since this topic keeps coming up on the net, here is a comprehensive
pronunciation list that has made the rounds in the past. This list
is maintained by Maarten Litmaath, maart@cs.vu.nl .
The Pronunciation Guide
-----------------------
version 2.2
Names derived from UNIX are marked with *, names derived from C are marked
with +, names derived from (Net)Hack are marked with & and names deserving
futher explanation are marked with a #. The explanations will be given at
the very end.
------------------------------------------------------------------------------
-- SINGLE CHARACTERS --
SPACE, blank, ghost&
! EXCLAMATION POINT, exclamation (mark), (ex)clam, excl, wow, hey, boing,
bang#, shout, yell, shriek, pling, factorial, ball-bat, smash, cuss,
store#, potion&, not*+, dammit*#
" QUOTATION MARK, (double) quote, dirk, literal mark, rabbit ears,
double ping, double glitch, amulet&, web&, inverted commas
# CROSSHATCH, pound, pound sign, number, number sign, sharp, octothorpe#,
hash, (garden) fence, crunch, mesh, hex, flash, grid, pig-pen,
tictactoe, scratch (mark), (garden) gate, hak, oof, rake, sink&,
corridor&, unequal#, punch mark
$ DOLLAR SIGN, dollar, cash, currency symbol, buck, string#, escape#,
ding, big-money, gold&, Sonne#
% PERCENT SIGN, percent, mod+, shift-5, double-oh-seven, grapes, food&
& AMPERSAND, and, amper, address+, shift-7, andpersand, snowman,
bitand+, donald duck#, daemon&, background*, pretzel
' APOSTROPHE, (single) quote, tick, prime, irk, pop, spark, glitch,
lurker above&
* ASTERISK, star, splat, spider, aster, times, wildcard*, gear, dingle,
(Nathan) Hale#, bug, gem&, twinkle, funny button#, pine cone, glob*
() PARENTHESES, parens, round brackets, bananas, ears, bowlegs
( LEFT PARENTHESIS, (open) paren, so, wane, parenthesee, open, sad,
tool&
) RIGHT PARENTHESIS, already, wax, unparenthesee, close (paren), happy,
thesis, weapon&
+ PLUS SIGN, plus, add, cross, and, intersection, door&, spellbook&
, COMMA, tail, trapper&
- HYPHEN, minus (sign), dash, dak, option, flag, negative (sign), worm,
bithorpe#
. PERIOD, dot, decimal (point), (radix) point, spot, full stop,
put#, floor&
/ SLASH, stroke, virgule, solidus, slant, diagonal, over, slat, slak,
across#, compress#, reduce#, replicate#, spare, divided-by, wand&,
forward slash, shilling#
: COLON, two-spot, double dot, dots, chameleon&
; SEMICOLON, semi, hybrid, giant eel&, go-on#
<> ANGLE BRACKETS, angles, funnels, brokets, pointy brackets
< LESS THAN, less, read from*, from*, in*, comesfrom*, crunch,
sucks, left chevron#, open pointy (brack[et]), bra#, upstairs&, west
> GREATER THAN, more, write to*, into/toward*, out*, gazinta*, zap,
blows, right chevron#, closing pointy (brack[et]), ket#, downstairs&,
east
= EQUAL SIGN, equal(s), gets, becomes, quadrathorpe#, half-mesh, ring&
? QUESTION MARK, question, query, whatmark, what, wildchar*, huh, ques,
kwes, quiz, quark, hook, scroll&
@ AT SIGN, at, each, vortex, whirl, whirlpool, cyclone, snail, ape, cat,
snable-a#, trunk-a#, rose, cabbage, Mercantile symbol, strudel#,
fetch#, shopkeeper&, human&, commercial-at
[] BRACKETS, square brackets, U-turns, edged parentheses
[ LEFT BRACKET, bracket, bra, (left) square (brack[et]), opensquare,
armor&
] RIGHT BRACKET, unbracket, ket, right square (brack[et]), unsquare, close,
mimic&
\ BACKSLASH, reversed virgule, bash, (back)slant, backwhack, backslat,
escape*, backslak, bak, scan#, expand#, opulent throne&, slosh, slope,
blash
^ CIRCUMFLEX, caret, carrot, (top)hat, cap, uphat, party hat, housetop,
up arrow, control, boink, chevron, hiccup, power, to-the(-power), fang,
sharkfin, and#, xor+, wok, trap&, pointer#, pipe*, upper-than#
_ UNDERSCORE, underline, underbar, under, score, backarrow, flatworm, blank,
chain&, gets#, dash#
` GRAVE, (grave/acute) accent, backquote, left/open quote, backprime,
unapostrophe, backspark, birk, blugle, backtick, push, backglitch,
backping, execute#, boulder&, rock&
{} BRACES, curly braces, squiggly braces, curly brackets, squiggle brackets,
Tuborgs#, ponds, curly chevrons#, squirrly braces, hitchcocks#,
chippendale brackets#
{ LEFT BRACE, brace, curly, leftit, embrace, openbrace, begin+,
fountain&
} RIGHT BRACE, unbrace, uncurly, rytit, bracelet, close, end+, a pool&
| VERTICAL BAR, pipe*, pipe to*, vertical line, broken line#, bar, or+,
bitor+, vert, v-bar, spike, to*, gazinta*, thru*, pipesinta*, tube,
mark, whack, gutter, wall&
~ TILDE, twiddle, tilda, tildee, wave, squiggle, swung dash, approx,
wiggle, enyay#, home*, worm, not+
-- MULTIPLE CHARACTER STRINGS --
!? interrobang (one overlapped character)
*/ asterslash+, times-div#
/* slashterix+, slashaster
:= becomes#
<- gets
<< left-shift+, double smaller
<> unequal#
>> appends*, cat-astrophe, right-shift+, double greater
-> arrow+, pointer to+, hiccup+
#! sh'bang, wallop
\!* bash-bang-splat
() nil#
&& and+, and-and+, amper-amper, succeeds-then*
|| or+, or-or+, fails-then*
-- NOTES --
! bang comes from old card punch phenom where punching ! code made a
loud noise; however, this pronunciation is used in the (non-
computerized) publishing and typesetting industry in the U.S.
too, so ...
Alternatively it could have come from comic books, where the
words each character utters are shown in a "balloon" near that
character's head. When one character shoots another, it is
common to see a balloon pointing at the barrel of the gun to
denote that the gun had been fired, not merely aimed.
That balloon contained the word "!" -- hence, "!" == "Bang!"
! store from FORTH
! dammit as in "quit, dammit!" while exiting vi and hoping one hasn't
clobbered a file too badly
# octothorpe from Bell System (orig. octalthorpe)
# unequal e.g. Modula-2
$ string from BASIC
$ escape from TOPS-10
$ Sonne In the "socialist" countries they used and are using all kinds
of IBM clones (hardware + sw). It was a common practice just
to rename everything (IBM 360 --> ESER 1040 etc.).
Of course the "dollar" sign had to be renamed - it became the
"international currency symbol" which looks like a circle with
4 rays spreading from it:
____
\/ \/
/ \
\ /
/\____/\
Because it looks like a (small) shining sun, in the German
Democratic Republic it was usually called "Sonne" (sun).
& donald duck from the Danish "Anders And", which means "Donald Duck"
* splat from DEC "spider" glyph
* Nathan Hale "I have but one asterisk for my country."
* funny button at Pacific Bell, * was referred to by employees as the "funny
button", which did not please management at all when it became
part of the corporate logo of Pacific Telesis, the holding
company ...
*/ times-div from FORTH
= quadrathorpe half an octothorpe
- bithorpe half a quadrathorpe (So what's a monothorpe?)
. put Victor Borge's Phonetic Punctuation which dates back to the
middle 1950's
/ across APL
/ compress APL
/ reduce APL
/ replicate APL
/ shilling from the British currency symbol
:= becomes e.g. Pascal
; go-on Algol68
< left chevron from the military: worn vertically on the sleeve to signify
rating
< bra from quantum mechanics
<> unequal e.g. Pascal
> right chevron see "< left chevron"
> ket from quantum mechanics
@ snable-a from Danish; may translate as "trunk-a"
@ trunk-a "trunk" = "elephant nose"
@ strudel as in Austrian apple cake
@ fetch from FORTH
\ scan APL
\ expand APL
^ and from formal logic
^ pointer from PASCAL
^ upper-than cf. > and <
_ gets some alternative representation of underscore resembles a
backarrow
_ dash as distinct from '-' == minus
` execute from shell command substitution
{} Tuborgs from advertizing for well-known Danish beverage
{} curly chevr. see "< left chevron"
{} hitchcocks from the old Alfred Hitchcock show, with the stylized profile
of the man
{} chipp. br. after Chippendale chairs
| broken line EBCDIC has two vertical bars, one solid and one broken.
~ enyay from the Spanish n-tilde
() nil LISP
--
Steve Hayman Workstation Manager Computer Science Department Indiana U.
sahayman@iuvax.cs.indiana.edu (812) 855-6984
NeXT Mail: sahayman@spurge.bloomington.in.us