ckchee@dgp.toronto.edu (Chuan Chee) (12/31/90)
I have SCO Xenix 2.2.3. What's the easiest way to "empty" a file while keeping its ownership (owner,group) and access permissions the same? Actually I only care about permissions (rw-rw-rw). I would like this done in Bourne shell (or possibly CSH). One other thing, this shell script is run under root. Here's the way I currently do it: rm -f $FILE touch $FILE chmod +w $FILE chown $OWNER $FILE chgrp $GRP $FILE Thanks in advance. ...Chuan Chee ckchee@dgp.utoronto.ca ckchee@dgp.toronto.edu (internet) ckchee@dgp.utoronto (bitnet)
karish@mindcraft.com (Chuck Karish) (12/31/90)
In article <1990Dec30.220722.29050@jarvis.csri.toronto.edu> ckchee@dgp.toronto.edu (Chuan Chee) writes: |I have SCO Xenix 2.2.3. What's the easiest way to "empty" a file |while keeping its ownership (owner,group) and access permissions the |same? Actually I only care about permissions (rw-rw-rw). > $FILE A redirect with no input truncates the file under Bourne shell. -- Chuck Karish karish@mindcraft.com Mindcraft, Inc. (415) 323-9000
donlash@uncle.uucp (Donald Lashomb) (12/31/90)
In article <1990Dec30.220722.29050@jarvis.csri.toronto.edu> ckchee@dgp.toronto.edu (Chuan Chee) writes: =I have SCO Xenix 2.2.3. What's the easiest way to "empty" a file =while keeping its ownership (owner,group) and access permissions the =same? Actually I only care about permissions (rw-rw-rw). =I would like this done in Bourne shell (or possibly CSH). =One other thing, this shell script is run under root. This works for me: > file # shell opens file for writing, and writes 0 bytes hope this is what you are looking for -Don
msb@hosmsb.ATT.COM (Mike Balenger) (01/01/91)
>>>>> On 31 Dec 90 03:07:23 GMT, ckchee@dgp.toronto.edu (Chuan Chee) said: ckchee> I have SCO Xenix 2.2.3. What's the easiest way to "empty" a ckchee> file while keeping its ownership (owner,group) and access ckchee> permissions the same? Actually I only care about permissions ckchee> (rw-rw-rw). I would like this done in Bourne shell (or ckchee> possibly CSH). One other thing, this shell script is run ckchee> under root. ckchee> Here's the way I currently do it: ckchee> rm -f $FILE ckchee> touch $FILE ckchee> chmod +w $FILE ckchee> chown $OWNER $FILE ckchee> chgrp $GRP $FILE This works in ksh and sh on SysV. I guess it should work for other UNIX look-alike's and shell look-alike's. > $FILE Since you don't remove the file, you don't need to recreate it. The file is truncated as a consequence of the defined behavior of output redirection, but since there is no command, no output is generated into the file. An added bonus, this method preserves links. They are lost with the "rm" in your original solution. You must, of course, have write permission to the file to try this. If that doesn't suite your fancy, try this > $FILE && { some machinations on ">", "mv", "cp" & "chmod" } The assorted machinations will only happen if the redirection fails -- most likely due to lack of write permission. This probably isn't a problem if you run a root, but may be useful for less-than-root uses. Note that the modification time is not changed if the file was already zero length. If you need that behavior, add touch $FILE -- ---------------------------------------------------------------------- <cute quote> Michael S. Balenger (908) 949-8789 <cute disclaimer> AT&T Bell Labs FAX: (908) 949-7512 M_Balenger@att.com Room 1L-405 msb@hos1cad.att.com Crawfords Corner Road att!hos1cad!msb Holmdel, NJ 07733-1988
segrove@PacBell.COM (S. E. Grove) (01/01/91)
Summary: > $FILE will do the job.
Stephen Grove Comm. Tech. ESS Pacific Bell
segrove@pbhya.PacBell.COM
PacBell.COM!{rtpkh0,rtpkh1}!segrove
--
Stephen Grove Comm. Tech. ESS Pacific Bell
seg@pbhya.PacBell.COM
segrove@pbhya.PacBell.COM (After 12-14-90)
PacBell.COM!{rtpkh0,rtpkh1}!segrovedberg@informix.com (David I. Berg) (01/01/91)
> I have SCO Xenix 2.2.3. What's the easiest way to "empty" a file > while keeping its ownership (owner,group) and access permissions the > same? How about: cat /dev/null >! $FILE ___ ___ dberg@cougar.informix.com / ) __ . __/ /_ ) _ __ Informix Software Inc. (303) 850-0210 _/__/ (_(_ (/ / (_(_ _/__> (-' -/~ (_- 5299 DTC Blvd #740; Englewood CO 80111 {uunet|pyramid}!infmx!dberg The opinions expressed herein do not necessarily reflect those of Informix Inc.
john@cstreet.com (John Poplett) (01/01/91)
In article <1990Dec30.220722.29050@jarvis.csri.toronto.edu> ckchee@dgp.toronto.edu (Chuan Chee) writes: >I have SCO Xenix 2.2.3. What's the easiest way to "empty" a file >while keeping its ownership (owner,group) and access permissions the >same? Actually I only care about permissions (rw-rw-rw). >I would like this done in Bourne shell (or possibly CSH). >One other thing, this shell script is run under root. > The simplest way to truncate a file using the Bourne shell is: > file Here's a short Bourne shell script that truncates files. Cut it out and save it to trunc.sh and run "make trunc" to get an executable script file. John ------ cut here -------- cut here -------- cut here -------- : # @(#)trunc.sh -- truncate or create empty files. Optionally, setting # owner or group ID. usage() { echo "usage: $0 [-o owner] [-g group] file1 [file2...]" 1>&2 exit 1 } if [ $# -lt 1 ] then usage fi set -- `getopt g:o: $*` if [ $? != 0 ] then usage fi for i in $* do case $i in -g) group=$2; shift; shift;; -o) owner=$2; shift; shift;; --) shift; break;; esac done for file in $* do > $file done if [ $group ] then chgrp $group $* fi if [ $owner ] then chown $owner $* fi exit 0 -- John Poplett @ C Street Software | Never make forecasts, especially 312 Wolff St. Oxnard, Ca. 93033 USA | about the future. (805) 486-7807 / john@cstreet.com | ~ Sam Goldwyn
jik@athena.mit.edu (Jonathan I. Kamens) (01/01/91)
Several people have suggested using
> $FILE
to truncate a file while retaining its old permissions. This will work under
sh or ksh or bash or other shells that allow null commands for redirection.
Unfortunately, csh and tcsh do not allow such null commands :-(. Therefore,
I'd like to point out another way to do this that works on most platforms I've
seen.
From the man page for cp(1):
DESCRIPTION
File1 is copied onto file2. By default, the mode and owner
of file2 are preserved if it already existed; otherwise the
mode of the source file modified by the current umask(2) is
used.
Therefore, you can use
cp /dev/null $FILE
to truncate a file and save its current permissions.
I hope this helps.
--
Jonathan Kamens USnail:
MIT Project Athena 11 Ashford Terrace
jik@Athena.MIT.EDU Allston, MA 02134
Office: 617-253-8085 Home: 617-782-0710allbery@NCoast.ORG (Brandon S. Allbery KB8JRR) (01/01/91)
As quoted from <1990Dec31.214030.7816@athena.mit.edu> by jik@athena.mit.edu (Jonathan I. Kamens): +--------------- | Several people have suggested using | > $FILE | to truncate a file while retaining its old permissions. This will work under | sh or ksh or bash or other shells that allow null commands for redirection. | Unfortunately, csh and tcsh do not allow such null commands :-(. Therefore, +--------------- % ls -s file 4 file % echo > file % ls -s file 0 file % _ Not that I like csh's echo behavior in this case, but it does get around the "null command" problem. ++Brandon -- Me: Brandon S. Allbery VHF/UHF: KB8JRR on 220, 2m, 440 Internet: allbery@NCoast.ORG Packet: KB8JRR @ WA8BXN America OnLine: KB8JRR AMPR: KB8JRR.AmPR.ORG [44.70.4.88] uunet!usenet.ins.cwru.edu!ncoast!allbery Delphi: ALLBERY
speelmo@bronze.ucs.indiana.edu (Lance Speelmon -- UCS) (01/11/91)
I think the easiest way to do this is: cp /dev/null <FILE> Lance In article <1990Dec30.220722.29050@jarvis.csri.toronto.edu> ckchee@dgp.toronto.edu (Chuan Chee) writes: >I have SCO Xenix 2.2.3. What's the easiest way to "empty" a file >while keeping its ownership (owner,group) and access permissions the >same? Actually I only care about permissions (rw-rw-rw). >I would like this done in Bourne shell (or possibly CSH). >One other thing, this shell script is run under root. > >Here's the way I currently do it: > rm -f $FILE > touch $FILE > chmod +w $FILE > chown $OWNER $FILE > chgrp $GRP $FILE > -- ============================================================================== | Lance Speelmon | University Computing Services | | speelmo@bronze.ucs.indiana.edu | Network Operations Center | ==============================================================================
]) (01/12/91)
>I think the easiest way to do this is: >cp /dev/null <FILE> That costs a fork/exec of cp... >>I have SCO Xenix 2.2.3. What's the easiest way to "empty" a file >>while keeping its ownership (owner,group) and access permissions the >>same? Actually I only care about permissions (rw-rw-rw). >>I would like this done in Bourne shell (or possibly CSH). >>One other thing, this shell script is run under root. >> >>Here's the way I currently do it: >> rm -f $FILE >> touch $FILE >> chmod +w $FILE >> chown $OWNER $FILE >> chgrp $GRP $FILE ...and that assumes that the FILE needs to be umask permissions plus write-permission for everyone, and costs five fork/execs. It also requires whatever processing preceeded the sample code to determin OWNER and GRP. Ideally, that processing would have included a setting of MODE so that the chmod +w $FILE would be replaced with a chmod $MODE $FILE to maintain the mode. What I usually do is : > $FILE at least in a SVR3 system, the permissions and ownerships are maintained. (I tested this by: echo a > junk; chmod 666 junk; chown someone_else junk; : > junk -- the ls -l junk results were unchanged across the : > junk command. My umask is 022.) Execution is controlled based on all applicable file permissions, and you don't go through a possibly long (from a computer's time sense) period when the file either doesn't exist at all or exists with root and root's current group and the wrong mode. Remember, too, that the datablocks for the file won't actually be freed until all processes using the file at the outset are finished. (Which is to say that if a process has it open on write, the old file will continue to grow until that other process is complete and the open-count on the file goes to 0; only *then* will you gain your disk-space back.) This is true regardless of the method applied for zeroing the contents out. ...Kris -- Kristopher Stephens, | (408-746-6047) | krs@uts.amdahl.com | KC6DFS Amdahl Corporation | | | [The opinions expressed above are mine, solely, and do not ] [necessarily reflect the opinions or policies of Amdahl Corp. ]
msb@hosmsb.ATT.COM (Mike Balenger) (01/16/91)
>>>>> On 12 Jan 91 01:20:03 GMT, krs@uts.amdahl.com (Kris Stephens [Hail Eris!]) said: krs> What I usually do is krs> : > $FILE The ":" doesn't seem to add anything here. The following is equivalent. (Or am I missing something?) > $FILE -- ---------------------------------------------------------------------- <cute quote> Michael S. Balenger (908) 949-8789 <cute disclaimer> AT&T Bell Labs FAX: (908) 949-7512 M_Balenger@att.com Room 1L-405 msb@hos1cad.att.com Crawfords Corner Road att!hos1cad!msb Holmdel, NJ 07733-1988