What is the most portable syntax of these two choices?
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From: David Kirkby on 27 Jul 2010 17:42 I have this in a script, where $UNAME will be the output of 'uname', except on Cygwin, where it will be "CYGWIN". What of the following two lines is more portable? if [ "x$UNAME" != xSunOS ] && [ "x$UNAME" != xCYGWIN ] && [ "x$UNAME" ! = xHP-UX ]; then or if [ "$UNAME" = SunOS -a "$UNAME" = CYGWIN -a "$UNAME" = HP-UX ]; then I believe the former is more portable, but someone has suggested the use of the latter. Dave
From: David Kirkby on 27 Jul 2010 18:15 On Jul 27, 10:42 pm, David Kirkby <drkir...(a)gmail.com> wrote: > I have this in a script, where $UNAME will be the output of 'uname', > except on Cygwin, where it will be "CYGWIN". > > What of the following two lines is more portable? > > if [ "x$UNAME" != xSunOS ] && [ "x$UNAME" != xCYGWIN ] && [ "x$UNAME" ! > = xHP-UX ]; then > > or > > if [ "$UNAME" = SunOS -a "$UNAME" = CYGWIN -a "$UNAME" = HP-UX ]; then > > I believe the former is more portable, but someone has suggested the > use of the latter. > > Dave Sorry, that second line made no sense if [ "$UNAME" = SunOS -o "$UNAME" = CYGWIN -o "$UNAME" = HP-UX ]; then is what I was meaning.
From: Aragorn on 27 Jul 2010 20:09 On Wednesday 28 July 2010 00:15 in comp.unix.shell, somebody identifying as David Kirkby wrote... > On Jul 27, 10:42 pm, David Kirkby <drkir...(a)gmail.com> wrote: >> I have this in a script, where $UNAME will be the output of 'uname', >> except on Cygwin, where it will be "CYGWIN". >> >> What of the following two lines is more portable? >> >> if [ "x$UNAME" != xSunOS ] && [ "x$UNAME" != xCYGWIN ] && [ "x$UNAME" >> ! = xHP-UX ]; then >> >> or >> >> if [ "$UNAME" = SunOS -a "$UNAME" = CYGWIN -a "$UNAME" = HP-UX ]; >> then >> >> I believe the former is more portable, but someone has suggested the >> use of the latter. > > Sorry, that second line made no sense > > if [ "$UNAME" = SunOS -o "$UNAME" = CYGWIN -o "$UNAME" = HP-UX ]; then > > is what I was meaning. Well, both are Bourne-compatible expressions, but they are not interchangeable. In your first expression, whatever follows after "then" will only get executed if the value of $UNAME matches none of the listed values, while in the second command, what follows after "then" will get executed if one of the tested conditions is true. So whatever follows after "then" would have to be defined differently, depending on which of the two commands you use. But I'm assuming that you know this and that the portability or perhaps rather even the "recommendability" of the method is your sole concern. Technically, using... if [ condition1 -o condition2 -o condition3 ]; then .... evaluates to true if either single condition is true, but also when any combination of conditions evaluates to true. In the example given, where it concerns the uname of any given environment, there is of course little to no chance that you would get a combination of different unames - no system will be *and* SunOS *and* HP-UX at the same time - but the construct of the evaluation itself does allow for that. Your first line however... if [ not_condition1 ] && [ not_condition2] && [ not_condition3 ]; then .... while being more elaborate in its evaluation, is more exclusive, because all three of the conditions have to be met individually as being "not true" before whatever follows after "then" is executed. Therefore, personally, I would also opt for that first line if a strict and exclusive evaluation is required for the success of the script. It is, in my humble opinion, the more portable of the two, in that such a construct can be more easily applied to multiple kinds of scripts, while the second line (with the "-o") leaves more room for unforeseen circumstances. -- *Aragorn* (registered GNU/Linux user #223157)
From: Thomas 'PointedEars' Lahn on 28 Jul 2010 05:35 David Kirkby wrote: > David Kirkby wrote: >> I have this in a script, where $UNAME will be the output of 'uname', >> except on Cygwin, where it will be "CYGWIN". >> >> What of the following two lines is more portable? >> >> if [ "x$UNAME" != xSunOS ] && [ "x$UNAME" != xCYGWIN ] && [ "x$UNAME" ! >> = xHP-UX ]; then >> >> or >> >> if [ "$UNAME" = SunOS -a "$UNAME" = CYGWIN -a "$UNAME" = HP-UX ]; then >> >> I believe the former is more portable, but someone has suggested the >> use of the latter. > > Sorry, that second line made no sense > > if [ "$UNAME" = SunOS -o "$UNAME" = CYGWIN -o "$UNAME" = HP-UX ]; then > > is what I was meaning. "$UNAME" might start with a `-', although I have found that to be explicitly recommended against in BSD (passwd(5)); so the first variant is safer, both with regard to prepending letter and using `&&' instead of `-a'/`-o'. I do not think there is much of a difference in portability between the two approaches. However, I think that `test' might be more portable than `[', and that you should choose the boolean operator depending on what you think is the more likely case; shortcut evaluation can increase overall efficiency. HTH -- PointedEars
From: Sven Mascheck on 28 Jul 2010 14:24
David Kirkby wrote: > David Kirkby wrote: >> What of the following two lines is more portable? >> >> if [ "x$UNAME" != xSunOS ] && [ "x$UNAME" != xCYGWIN ] && [ "x$UNAME" ! >> = xHP-UX ]; then >> >> or >> >> if [ "$UNAME" = SunOS -a "$UNAME" = CYGWIN -a "$UNAME" = HP-UX ]; then >> > > Sorry, that second line made no sense > > if [ "$UNAME" = SunOS -o "$UNAME" = CYGWIN -o "$UNAME" = HP-UX ]; then That's weird. Did you mean to ask if [ "x$UNAME" != xSunOS ] && .... or if [ "x$UNAME" != xSunOS -a ... but missed both the ! for negation and the extra-x in the second line (let alone the update)? :-) |