Subject: Frequently Asked Questions about Unix - with Answers [Monthly posting] [Last changed: $Date: 91/01/03 14:27:19 $ by $Author: sahayman $] This article contains the answers to some Frequently Asked Questions often seen in comp.unix.questions. 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 program or 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 tell inside .cshrc if I'm a login shell? 25) Why doesn't redirecting a loop work as intended? (Bourne shell) 26) How do I use popen() to open a process for reading AND writing? 27) How do I run 'passwd', 'ftp', 'telnet', 'tip' and other interactive programs from a shell script or in the background? 28) How do I sleep() in a C program for less than one second? 29) How can I get setuid shell scripts to work? 30) What are some useful Unix or C books? 31) How do I construct a shell glob-pattern that matches all files except "." and ".." ? 32) 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"} < 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 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 $l done If you wanted to be really thorough and handle files with `funny' names (embedded blanks or whatever) you'd need to use Bourne Shell: for f in *; do eval mv '"$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 =~ /^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 2>&1 ", 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. [Note: The newsgroup "comp.unix.wizards" was recently deleted, but the "Unix-Wizards" mailing list still exists. I'm not really sure how this is all going to sort itself out.] 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 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 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 "$$"; 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 25) Why doesn't redirecting a loop work as intended? (Bourne shell) Take the following example: foo=bar while read line do # do something with $line foo=bletch done < /etc/passwd echo "foo is now: $foo" Despite the assignment ``foo=bletch'' this will print ``foo is now: bar'' in many implementations of the Bourne shell. Why? Because of the following, often undocumented, feature of historic Bourne shells: redirecting a control structure (such as a loop, or an ``if'' statement) causes a subshell to be created, in which the structure is executed; variables set in that subshell (like the ``foo=bletch'' assignment) don't affect the current shell, of course. The POSIX 1003.2 Shell and Tools Interface standardization committee forbids the behaviour described above, i.e. in P1003.2 conformant Bourne shells the example will print ``foo is now: bletch''. In historic (and P1003.2 conformant) implementations you can use the following `trick' to get around the redirection problem: foo=bar # make file descriptor 9 a duplicate of file descriptor 0 (stdin); # then connect stdin to /etc/passwd; the original stdin is now # `remembered' in file descriptor 9; see dup(2) and sh(1) exec 9<&0 < /etc/passwd while read line do # do something with $line foo=bletch done # make stdin a duplicate of file descriptor 9, i.e. reconnect it to # the original stdin; then close file descriptor 9 exec 0<&9 9<&- echo "foo is now: $foo" This should always print ``foo is now: bletch''. Right, take the next example: foo=bar echo bletch | read foo echo "foo is now: $foo" This will print ``foo is now: bar'' in many implementations, ``foo is now: bletch'' in some others. Why? Generally each part of a pipeline is run in a different subshell; in some implementations though, the last command in the pipeline is made an exception: if it is a builtin command like ``read'', the current shell will execute it, else another subshell is created. Draft 10 of POSIX 1003.2 allows both behaviours; future drafts may explicitly specify only one of them though. 26) How do I use popen() to open a process for reading AND writing? The problem with trying to pipe both input and output to an arbitrary slave process is that deadlock can occur, if both processes are waiting for not-yet-generated input at the same time. Deadlock can be avoided only by having BOTH sides follow a strict deadlock-free protocol, but since that requires cooperation from the processes it is inappropriate for a popen()-like library function. The 'expect' distribution includes a library of functions that a C programmer can call directly. One of the functions does the equivalent of a popen for both reading and writing. It uses ptys rather than pipes, and has no deadlock problem. It's portable to both BSD and SV. See the next answer for more about 'expect'. 27) How do I run 'passwd', 'ftp', 'telnet', 'tip' and other interactive programs from a shell script or in the background? The shell itself cannot interact with interactive tty-based programs like these. Fortunately some programs have been written to manage the connection to a pseudo-tty so that you can run these sorts of programs in a script. 'expect' is a one such program, which you can ftp pub/expect.shar.Z from durer.cme.nist.gov. The following expect script is an example of a non-interactive version of passwd(1). # username is passed as 1st arg, password as 2nd set password [index $argv 2] spawn passwd [index $argv 1] expect "*password:" send "$password\r" expect "*password:" send "$password\r" expect eof Another solution is provided by the 'pty' program, which runs a program under a pty session and was posted to comp.sources.unix, volume 23, issue 31. You can also ftp pub/flat/pty-* from stealth.acf.nyu.edu . A pty-based solution using named pipes to do the same as the above might look like this: #!/bin/sh /etc/mknod out.$$ p; exec 2>&1 ( exec 4/dev/null ) | ( pty passwd "$1" >out.$$ ) Here, 'waitfor' is a simple C program that searches for its argument in the input, character by character. You can ftp pub/flat/misc-waitfor.c from stealth.acf.nyu.edu . A simpler pty solution (which has the drawback of not synchronizing properly with the passwd program) is #!/bin/sh ( sleep 5; echo "$2"; sleep 5; echo "$2") | pty passwd "$1" 28) How do I sleep() in a C program for less than one second? The first thing you need to be aware of is that all you can specify is a MINIMUM amount of delay; the actual delay will depend on scheduling issues such as system load, and could be arbitrarily large if you're unlucky. There is no standard library function that you can count on in all environments for "napping" (the usual name for short sleeps). The following code is adapted from Doug Gwyn's System V emulation support for 4BSD and exploits the 4BSD select() system call. On System V you might be able to use poll() in a similar way. /* nap -- support routine for 4.2BSD system call emulations last edit: 29-Oct-1984 D A Gwyn */ extern int select(); int nap( usec ) /* returns 0 if ok, else -1 */ long usec; /* delay in microseconds */ { static struct /* `timeval' */ { long tv_sec; /* seconds */ long tv_usec; /* microsecs */ } delay; /* _select() timeout */ delay.tv_sec = usec / 1000000L; delay.tv_usec = usec % 1000000L; return select( 0, (long *)0, (long *)0, (long *)0, &delay ); } 29) How can I get setuid shell scripts to work? [ This is a long answer, but it's a complicated and frequently-asked question. Thanks to Maarten Litmaath for this answer, and for the "indir" program mentioned below. ] Let us first assume you are on a UNIX variant (e.g. 4.3BSD or SunOS) that knows about so-called `executable shell scripts'. Such a script must start with a line like: #!/bin/sh The script is called `executable' because just like a real (binary) executable it starts with a so-called `magic number' indicating the type of the executable. In our case this number is `#!' and the OS takes the rest of the first line as the interpreter for the script, possibly followed by 1 initial option like: #!/bin/sed -f Suppose this script is called `foo' and is found in /bin, then if you type: foo arg1 arg2 arg3 the OS will rearrange things as though you had typed: /bin/sed -f /bin/foo arg1 arg2 arg3 There is one difference though: if the setuid permission bit for `foo' is set, it will be honored in the first form of the command; if you really type the second form, the OS will honor the permission bits of /bin/sed, which is not setuid, of course. ---------- OK, but what if my shell script does NOT start with such a `#!' line or my OS does not know about it? Well, if the shell (or anybody else) tries to execute it, the OS will return an error indication, as the file does not start with a valid magic number. Upon receiving this indication the shell ASSUMES the file to be a shell script and gives it another try: /bin/sh shell_script arguments But we have already seen that a setuid bit on `shell_script' will NOT be honored in this case! ---------- Right, but what about the security risks of setuid shell scripts? Well, suppose the script is called `/etc/setuid_script', starting with: #!/bin/sh Now let us see what happens if we issue the following commands: $ cd /tmp $ ln /etc/setuid_script -i $ PATH=. $ -i We know the last command will be rearranged to: /bin/sh -i But this command will give us an interactive shell, setuid to the owner of the script! Fortunately this security hole can easily be closed by making the first line: #!/bin/sh - The `-' signals the end of the option list: the next argument `-i' will be taken as the name of the file to read commands from, just like it should! --------- There are more serious problems though: $ cd /tmp $ ln /etc/setuid_script temp $ nice -20 temp & $ mv my_script temp The third command will be rearranged to: nice -20 /bin/sh - temp As this command runs so slowly, the fourth command might be able to replace the original `temp' with `my_script' BEFORE `temp' is opened by the shell! There are 4 ways to fix this security hole: 1) let the OS start setuid scripts in a different, secure way - System V R4 and 4.4BSD use the /dev/fd driver to pass the interpreter a file descriptor for the script 2) let the script be interpreted indirectly, through a frontend that makes sure everything is all right before starting the real interpreter - if you use the `indir' program from comp.sources.unix the setuid script will look like this: #!/bin/indir -u #?/bin/sh /etc/setuid_script 3) make a `binary wrapper': a real executable that is setuid and whose only task is to execute the interpreter with the name of the script as an argument 4) make a general `setuid script server' that tries to locate the requested `service' in a database of valid scripts and upon success will start the right interpreter with the right arguments. --------- Now that we have made sure the right file gets interpreted, are there any risks left? Certainly! For shell scripts you must not forget to set the PATH variable to a safe path explicitly. Can you figure out why? Also there is the IFS variable that might cause trouble if not set properly. Other environment variables might turn out to compromise security as well, e.g. SHELL... Furthermore you must make sure the commands in the script do not allow interactive shell escapes! Then there is the umask which may have been set to something strange... Etcetera. You should realise that a setuid script `inherits' all the bugs and security risks of the commands that it calls! All in all we get the impression setuid shell scripts are quite a risky business! You may be better off writing a C program instead! 30) What are some useful Unix or C books? Mitch Wright (mitch@hq.af.mil) maintains a useful list of Unix and C books, with descriptions and some mini-reviews. There are currently 77 titles on his list. You can obtain a copy of this list by anonymous ftp from iuvax.cs.indiana.edu (129.79.254.192), where it's "pub/Unix-C-Booklist". If you can't use anonymous ftp, email the line "help" to "mailserv@iuvax.cs.indiana.edu" for instructions on retrieving things via email. Send additions or suggestions to mitch@hq.af.mil . 31) 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) 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" . Many people are willing to use .??* to match all dotfiles (or * .??* to match all files) even though that pattern doesn't get everything - it has the advantage of being easy to type. If you really do want to be sure, you'll need to employ an external program or two and use backquote substitution. This is pretty good: `ls -a | sed -e '/^\.$/d' -e '/^\.\.$/d'` but even it will mess up on files with newlines in their names. 32) 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. Send updates to Steve Hayman, sahayman@cs.indiana.edu. Special thanks to Maarten Litmaath for his work in maintaining this list in the past. The Pronunciation Guide ----------------------- version 2.4 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, widgets < LESS THAN, less, read from*, from*, in*, comesfrom*, crunch, sucks, left chevron#, open pointy (brack[et]), bra#, upstairs&, west, (left|open) widget > GREATER THAN, more, write to*, into/toward*, out*, gazinta*, zap, blows, right chevron#, closing pointy (brack[et]), ket#, downstairs&, east, (right|close) widget = EQUAL SIGN, equal(s), gets, becomes, quadrathorpe#, half-mesh, ring& ? QUESTION MARK, question, query, whatmark, what, wildchar*, huh, ques, kwes, quiz, quark, hook, scroll&, interrogation point @ AT SIGN, at, each, vortex, whirl, whirlpool, cyclone, snail, ape (tail), cat, snable-a#, trunk-a#, rose, cabbage, Mercantile symbol, strudel#, fetch#, shopkeeper&, human&, commercial-at, monkey (tail) [] 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#, sneak ` 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