tw@foobar.UUCP (Tom Walsh) (08/04/87)
hi, sortof like the scanf() contest...
i have a need to use popen(command, "r"). if command is not
found, i would like to dispose quietly of the shell's complaint:
sh: command: not found
the stderr of the command itself can be handled ala:
popen("/usr/bin/mynewcommand -flags 2> /dev/null", "r");
but if "mynewcommand" isn't around, the shell's stderr still
gets put on the *real* stderr.
question:
is there an easier way than:
.
.
.
realerr = dup(stderr);
freopen("/dev/null", "w", stderr);
popen();
do_stuff();
pclose();
fclose(stderr);
dup2(realerr, stderr);
.
.
.
to get rid of the shell's stderr????
-tw (Tom Walsh, (503)696-7105, Intel Corp; Hillsboro OR)
...tektronix!psu-cs!omosys! \
...seismo!vrdxhq!verdix!omepd!littlei! \
...intelisc! >foobar!tw
...inteloa! /
...omepd!littlei! /ugbinns@sunybcs.uucp (Leonard Binns) (08/07/87)
In article <124@foobar.UUCP> tw@foobar.UUCP (Tom Walsh) writes: > > hi, sortof like the scanf() contest... Oh no! :-) > i have a need to use popen(command, "r"). if command is not > found, i would like to dispose quietly of the shell's complaint: > sh: command: not found > the stderr of the command itself can be handled ala: > popen("/usr/bin/mynewcommand -flags 2> /dev/null", "r"); > > question: > is there an easier way than: Did you ever try: popen("/usr/bin/mynewcommand -flags 2>& /dev/null", "r"); ^^^ I think that should do the trick. Len Binns ugbinns@marvin.UUCP
dce@mips.UUCP (David Elliott) (08/07/87)
In article <4511@sunybcs.UUCP> ugbinns@marvin.UUCP (Leonard Binns) writes: >In article <124@foobar.UUCP> tw@foobar.UUCP (Tom Walsh) writes: > >> i have a need to use popen(command, "r"). if command is not >> found, i would like to dispose quietly of the shell's complaint: >> sh: command: not found >> the stderr of the command itself can be handled ala: >> popen("/usr/bin/mynewcommand -flags 2> /dev/null", "r"); >> >> question: >> is there an easier way than: > >Did you ever try: > popen("/usr/bin/mynewcommand -flags 2>& /dev/null", "r"); > ^^^ >I think that should do the trick. Why should it do the trick? It isn't even valid shell syntax! Remember that popen() uses /bin/sh, not your SHELL (and if AT&T changes this, I QUIT!). The >& is used in the sequence m>&n which says "make file descriptor m a duplicate of file descriptor n". The sequence 2>& /dev/null results in "/dev/null: bad file number". Anyway, the easiest way to do this is popen("( /usr/bin/bin/mynewcommand -flags ) 2>/dev/null", "r"); The sequence ( command ) runs the command in a subshell, so all redirection done outside of the parenthesis applies to the subshell. This is also useful in cases like: ( cat file1 ; echo separator ; cat file2 ) | filter -- David Elliott {decvax,ucbvax,ihnp4}!decwrl!mips!dce
chris@mimsy.UUCP (Chris Torek) (08/09/87)
>In article <124@foobar.UUCP> tw@foobar.UUCP (Tom Walsh) writes: >>[Using] popen(command, "r") ... i would like to dispose quietly of >>the shell's complaint: >> sh: command: not found In article <4511@sunybcs.UUCP> ugbinns@sunybcs.uucp (Leonard Binns) writes: >Did you ever try: > popen("/usr/bin/mynewcommand -flags 2>& /dev/null", "r"); This will not do it. (For one thing, it is syntactically invalid.) The shell prints `sh: <x>: not found' to the *shell's* standard error output, before any redirection. There are several possible solutions, but the easiest is to add one level of indirection: (void) sprintf(expandcmd, "sh -c '%s' 2>/dev/null", command); iop = popen(expandcmd, "r"); Note that this assumes `command' has no single quotes embedded in it. -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690) Domain: chris@mimsy.umd.edu Path: seismo!mimsy!chris
rayan@utegc.UUCP (08/10/87)
In article <7928@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) writes: # >In article <124@foobar.UUCP> tw@foobar.UUCP (Tom Walsh) writes: # >> ... i would like to dispose quietly of the shell's complaint: # >> sh: command: not found # # In article <4511@sunybcs.UUCP> ugbinns@sunybcs.uucp (Leonard Binns) writes: # > popen("/usr/bin/mynewcommand -flags 2>& /dev/null", "r"); # # This will not do it. ... [try:] # # (void) sprintf(expandcmd, "sh -c '%s' 2>/dev/null", command); # iop = popen(expandcmd, "r"); Seems too expensive. Try popen("exec 2> /dev/null ; /usr/bin/mynewcommand -flags", "r"); exec with no command does wonderful things to your shell. It is documented too, on BSD flavours at least. Also, it is built-in (obviously) so there is very little overhead involved. rayan -- Rayan Zachariassen AI group, University of Toronto
chris@mimsy.UUCP (Chris Torek) (08/10/87)
>In article <7928@mimsy.UUCP> I suggest: ># (void) sprintf(expandcmd, "sh -c '%s' 2>/dev/null", command); ># iop = popen(expandcmd, "r"); In article <8708100615.AA06203@ephemeral.ai.toronto.edu> rayan@utegc.UUCP writes: >Seems too expensive. Try > > popen("exec 2> /dev/null ; /usr/bin/mynewcommand -flags", "r"); This is a good idea. Unfortunately, it does not work; the shell has already set its own standard error to another descriptor, so that it can still print errors. -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690) Domain: chris@mimsy.umd.edu Path: seismo!mimsy!chris
jgy@hropus.UUCP (John Young) (08/11/87)
> >In article <7928@mimsy.UUCP> I suggest: > ># (void) sprintf(expandcmd, "sh -c '%s' 2>/dev/null", command); > ># iop = popen(expandcmd, "r"); > > In article <8708100615.AA06203@ephemeral.ai.toronto.edu> rayan@utegc.UUCP > writes: > >Seems too expensive. Try > > > > popen("exec 2> /dev/null ; /usr/bin/mynewcommand -flags", "r"); > > This is a good idea. Unfortunately, it does not work; the shell has > already set its own standard error to another descriptor, so that it > can still print errors. > -- > In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690) > Domain: chris@mimsy.umd.edu Path: seismo!mimsy!chris > It has always worked for me on System V systems.
rayan@utegc.UUCP (08/11/87)
>In article <7928@mimsy.UUCP> Chris suggests: ># (void) sprintf(expandcmd, "sh -c '%s' 2>/dev/null", command); ># iop = popen(expandcmd, "r"); In article <8708100615.AA06203@ephemeral.ai.toronto.edu> I write: >Seems too expensive. Try > > popen("exec 2> /dev/null ; /usr/bin/mynewcommand -flags", "r"); In article <7932@mimsy.UUCP> Chris replies: # This is a good idea. Unfortunately, it does not work; the shell has # already set its own standard error to another descriptor, so that it # can still print errors. Before posting my suggestion above, I tried it out. I'm on a SunOS 3.4 box, and using exec as shown did indeed get rid of the ": not found" message. But Chris usually knows what he's talking about (too...), so I just tried it on 4.3bsd. There his admonition holds true. Therefore I conclude that early v7-based sh's (as in 4.3bsd) require something like: 1. popen("( /usr/bin/mynewcommand -flags ) 2> /dev/null", "r"); while sysV-based sh's (as in SunOS) will work with: 2. popen("exec 2> /dev/null ; /usr/bin/mynewcommand -flags", "r"); Note that method 1. is less overhead (one less path search and exec), and less problematic vis-a-vis quoting, than the original suggestion at the top of this message. I thought of specifying the fd that the 4.3 sh dup's stderr to, instead of '2'. Unfortunately, that is fd 11, and only single digits may be used to specify the fd for a redirection. I'm glad this feature of juggling the fd's around was removed in sysV (presumably). Live and learn. rayan -- Rayan Zachariassen AI group, University of Toronto
allbery@ncoast.UUCP (Brandon Allbery) (08/13/87)
As quoted from <124@foobar.UUCP> by tw@foobar.UUCP (Tom Walsh): +--------------- | | hi, sortof like the scanf() contest... | i have a need to use popen(command, "r"). if command is not | found, i would like to dispose quietly of the shell's complaint: | sh: command: not found | the stderr of the command itself can be handled ala: | popen("/usr/bin/mynewcommand -flags 2> /dev/null", "r"); | but if "mynewcommand" isn't around, the shell's stderr still | gets put on the *real* stderr. +--------------- popen("exec 2> /dev/null; /usr/bin/mynewcommand -flags", "r"); -- Brandon S. Allbery, moderator of comp.sources.misc and comp.binaries.ibm.pc {{harvard,mit-eddie}!necntc,well!hoptoad,sun!mandrill!hal}!ncoast!allbery ARPA: necntc!ncoast!allbery@harvard.harvard.edu Fido: 157/502 MCI: BALLBERY <<ncoast Public Access UNIX: +1 216 781 6201 24hrs. 300/1200/2400 baud>> ** Site "cwruecmp" is changing its name to "mandrill". Please re-address ** *** all mail to ncoast to pass through "mandrill" instead of "cwruecmp". ***
ggs@ulysses.homer.nj.att.com (Griff Smith) (08/16/87)
chris@mimsy.UUCP: > (void) sprintf(expandcmd, "sh -c '%s' 2>/dev/null", command); > iop = popen(expandcmd, "r"); > rayan@utegc.UUCP: >Seems too expensive. Try > > popen("exec 2> /dev/null ; /usr/bin/mynewcommand -flags", "r"); > How about: popen("(/usr/bin/mynewcommand -flags) 2> /dev/null", "r"); Only costs an exra fork, no exec, not much extra typing. Did I miss something? -- Griff Smith AT&T (Bell Laboratories), Murray Hill Phone: 1-201-582-7736 UUCP: {allegra|ihnp4}!ulysses!ggs Internet: ggs@ulysses.uucp