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Archive-name: unix-faq/faq/part2
Version: $Id: part2,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:
1.*) General questions.
2.*) Relatively basic questions, likely to be asked by beginners.
3.*) Intermediate questions.
4.*) Advanced questions, likely to be asked by people who thought
they already knew all of the answers.
5.*) Questions pertaining to the various shells, and the differences.
6.*) An overview of Unix variants.
7.*) An comparison of configuration management systems (RCS, SCCS).
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.
------------------------------
End of unix/faq Digest part 2 of 7
**********************************
--
Ted Timar - tmatimar@isgtec.com
ISG Technologies Inc., 6509 Airport Road, Mississauga, Ontario, Canada L4V 1S7
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