Is is OK to unset a variable that is not set ?
in [Shell]
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From: Dave on 14 Dec 2009 12:05 I have a shell script written by someone else, which starts #!/usr/bin/env bash then has the line: unset DISTUTILS_DEBUG It's a bit pointless, as DISTUTILS_DEBUG is not set in the script, but there is some code commented out, where is was obviously set at one time. I've added set -e in this script, and find the script is exiting. Putting a few echo statements in, I see: echo "about to unset DISTUTILS_DEBUG" unset DISTUTILS_DEBUG echo "unset DISTUTILS_DEBUG" The "about to unset DISTUTILS_DEBUG" is printed, but the unset DISTUTILS_DEBUG is not, so I assume it's the fact one tries to unset something that is not set is what's causing the problem. Is this behavior to be expected? It does not happen under OS X, Linux or Solaris, but is causing a problem on HP-UX 11.11. If it's not ok to do this, how can I test if the variable is set, before trying to unset it? I know how to test if a variable is of zero length (use -z in test), but that is not quite the same as a variable being set or not. Dave -- I respectfully request that this message is not archived by companies as unscrupulous as 'Experts Exchange' . In case you are unaware, 'Experts Exchange' take questions posted on the web and try to find idiots stupid enough to pay for the answers, which were posted freely by others. They are leeches.
From: Icarus Sparry on 14 Dec 2009 12:20 On Mon, 14 Dec 2009 17:05:52 +0000, Dave wrote: > I have a shell script written by someone else, which starts > > > #!/usr/bin/env bash > > > then has the line: > > unset DISTUTILS_DEBUG > > It's a bit pointless, as DISTUTILS_DEBUG is not set in the script, but > there is some code commented out, where is was obviously set at one > time. > > I've added > > set -e set -x would probably be more useful. > in this script, and find the script is exiting. Putting a few echo > statements in, I see: > > echo "about to unset DISTUTILS_DEBUG" > unset DISTUTILS_DEBUG > echo "unset DISTUTILS_DEBUG" > > The "about to unset DISTUTILS_DEBUG" > > is printed, but the > > unset DISTUTILS_DEBUG > > is not, so I assume it's the fact one tries to unset something that is > not set is what's causing the problem. > > Is this behavior to be expected? It does not happen under OS X, Linux or > Solaris, but is causing a problem on HP-UX 11.11. > > If it's not ok to do this, how can I test if the variable is set, before > trying to unset it? I know how to test if a variable is of zero length > (use -z in test), but that is not quite the same as a variable being set > or not. The normal way is to use the ${DISTUTILS_DEBUG+yes} variable expansion. This will evaluate to "yes" if the variable is set, even if it is the zero length string, and nothing otherwise. So hopefully test -z "${DISUTILE_DEBUG+yes} && unset DISTUTILS_DEBUG will fix your problem. What version of bash are you running on your HP-UX machine?
From: Dave on 14 Dec 2009 12:22 Dave wrote: > I have a shell script written by someone else, which starts > > > #!/usr/bin/env bash > > > then has the line: > > unset DISTUTILS_DEBUG > > It's a bit pointless, as DISTUTILS_DEBUG is not set in the script, but > there is some code commented out, where is was obviously set at one time. > > I've added > > set -e > > in this script, and find the script is exiting. Putting a few echo > statements in, I see: > > echo "about to unset DISTUTILS_DEBUG" > unset DISTUTILS_DEBUG > echo "unset DISTUTILS_DEBUG" > > The "about to unset DISTUTILS_DEBUG" > > is printed, but the > > unset DISTUTILS_DEBUG > > is not, so I assume it's the fact one tries to unset something that is > not set is what's causing the problem. > > Is this behavior to be expected? It does not happen under OS X, Linux or > Solaris, but is causing a problem on HP-UX 11.11. > > If it's not ok to do this, how can I test if the variable is set, before > trying to unset it? I know how to test if a variable is of zero length > (use -z in test), but that is not quite the same as a variable being set > or not. > > > > Dave > > Looking at POSIX 2004, http://www.opengroup.org/onlinepubs/009695399/utilities/unset.html it says: "Unsetting a variable or function that was not previously set shall not be considered an error and does not cause the shell to abort." so it looks like this is a place where HP-UX is not POSIX 2004 compatible. Dave -- I respectfully request that this message is not archived by companies as unscrupulous as 'Experts Exchange' . In case you are unaware, 'Experts Exchange' take questions posted on the web and try to find idiots stupid enough to pay for the answers, which were posted freely by others. They are leeches.
From: Stachu 'Dozzie' K. on 14 Dec 2009 12:24 On 14.12.2009, Dave wrote: > I have a shell script written by someone else, which starts > > > #!/usr/bin/env bash > > > then has the line: > > unset DISTUTILS_DEBUG > > It's a bit pointless, as DISTUTILS_DEBUG is not set in the script, Not really pointless. Author doesn't know in what environment his script will be run. This unset ensures the script will have DISTUTILS_DEBUG unset. Personally I prefer to set variables in such cases to empty string. > but there is > some code commented out, where is was obviously set at one time. > > I've added > > set -e > > in this script, and find the script is exiting. Putting a few echo statements > in, I see: > > echo "about to unset DISTUTILS_DEBUG" > unset DISTUTILS_DEBUG > echo "unset DISTUTILS_DEBUG" > > The "about to unset DISTUTILS_DEBUG" > > is printed, but the > > unset DISTUTILS_DEBUG > > is not, so I assume it's the fact one tries to unset something that is not set > is what's causing the problem. > > Is this behavior to be expected? It is not expected. http://www.opengroup.org/onlinepubs/009695399/utilities/unset.html #v+ Unsetting a variable or function that was not previously set shall not be considered an error and does not cause the shell to abort. #v- Maybe DISTUTILS_DEBUG is set read-only somewhere? > It does not happen under OS X, Linux or > Solaris, but is causing a problem on HP-UX 11.11. > > If it's not ok to do this, how can I test if the variable is set, before trying > to unset it? Just set it to anything (empty string will be OK, I think) and then unset. And leave the comment what the hell is such a strange code piece for. > I know how to test if a variable is of zero length (use -z in > test), but that is not quite the same as a variable being set or not. -- Stanislaw Klekot
From: Dave on 14 Dec 2009 12:28 Icarus Sparry wrote: > On Mon, 14 Dec 2009 17:05:52 +0000, Dave wrote: > >> I have a shell script written by someone else, which starts >> >> >> #!/usr/bin/env bash >> >> >> then has the line: >> >> unset DISTUTILS_DEBUG >> >> It's a bit pointless, as DISTUTILS_DEBUG is not set in the script, but >> there is some code commented out, where is was obviously set at one >> time. >> >> I've added >> >> set -e > > set -x > > would probably be more useful. > >> in this script, and find the script is exiting. Putting a few echo >> statements in, I see: >> >> echo "about to unset DISTUTILS_DEBUG" >> unset DISTUTILS_DEBUG >> echo "unset DISTUTILS_DEBUG" >> >> The "about to unset DISTUTILS_DEBUG" >> >> is printed, but the >> >> unset DISTUTILS_DEBUG >> >> is not, so I assume it's the fact one tries to unset something that is >> not set is what's causing the problem. >> >> Is this behavior to be expected? It does not happen under OS X, Linux or >> Solaris, but is causing a problem on HP-UX 11.11. >> >> If it's not ok to do this, how can I test if the variable is set, before >> trying to unset it? I know how to test if a variable is of zero length >> (use -z in test), but that is not quite the same as a variable being set >> or not. > > The normal way is to use the > ${DISTUTILS_DEBUG+yes} > variable expansion. This will evaluate to "yes" if the variable is set, > even if it is the zero length string, and nothing otherwise. > > So hopefully > test -z "${DISUTILE_DEBUG+yes} && unset DISTUTILS_DEBUG > will fix your problem. What version of bash are you running on your HP-UX > machine? > bash-2.04$ bash --version GNU bash, version 2.04.0(1)-release (hppa2.0w-hp-hpux11.00) Copyright 1999 Free Software Foundation, Inc. bash-2.04$ Is this likely to be a problem with HP-UX, or bash? The bash command is /opt/OpenSource/bin/bash so from the location, one might guess HP do not support 'bash' I appreciate a newer version of bash might not suffer this problem, but I'd rather sort out issues which occur with old versions of commands too - not everyone runs the latest. The code I am using is checked by quite a few people on the more common linux and OS X distributions, so most people will not find issues like this. -- I respectfully request that this message is not archived by companies as unscrupulous as 'Experts Exchange' . In case you are unaware, 'Experts Exchange' take questions posted on the web and try to find idiots stupid enough to pay for the answers, which were posted freely by others. They are leeches.
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