[SOLVED] shellscript: compound tests (FYI)

zedmelon · 07-17-2006, 05:41 PM

I nearly put this in the programming forum. Sorry if it should be there, and mods feel free to move it.

I had a shell script question, and searching the forum didn't directly help, except I found someone reference the ABSG (Advanced Bash Scripting Guide), where I found my answer. I'm now posting for posterity.

I wanted to run a command block if either or each of the following was true:
(variable 1 = <STRING>)
(variable 2 = <var3> or variable 3 = <var6>)

I tried rearranging brackets and parenthesis to no avail. I knew there had to be a better way than two tests and a redundant 'else' clause...lame. The answer was in the use of a logical OR test joining two more traditional-looking tests.

So here it is; hope I'm not the only syntax-deprived bonehead out there. A short script and several runs with my varied input:

Code:

(16:32:25 bin) cat doubleif

#!/bin/sh
read one two three
if [ "$one" = "1" ] || [ "$two" = "2" -a "$three" = "3" ]
then    echo yep
else    echo nope
fi

(16:32:42 bin) doubleif
5 6 7
nope
(16:32:55 bin) doubleif
2 2 3
yep
(16:33:04 bin) doubleif
1 3 4
yep
(16:33:16 bin) doubleif
1
yep
(16:33:20 bin) doubleif
1 2 3
yep
(16:33:25 bin)

Curiosity now has me thinking on how to change the above "either or both" to "either, but not both." Back to ABSG...

bigearsbilly · 07-18-2006, 10:38 AM

Code:

[ "$one" = 1 -o "$two" = "2" -a "$three" = "3" ]

or (same thing but reassuring)

[ "$one" = 1 -o \( "$two" = "2" -a "$three" = "3" \) ]

unSpawn · 07-18-2006, 10:43 AM

change (..) to "either, but not both."
If you have no requirements, then how about:

Code:

res=0; read a b c
[ "$a" = "1" ] && res=$[$res+1]
[ "$b" = "2" ] && res=$[$res+2]
[ "$c" = "3" ] && res=$[$res+4]
case "$res" in 1|2|4) echo \
"yep";; *) echo "nope";; esac

zedmelon · 07-19-2006, 11:47 AM

I originally meant to post a quick tip, but then I yammered at the end--Dave Barry calls this "blitherer's disease."

Note to those who just found this thread: I was only curious about the superfluous problem. Feel free to solve it, but there are others who actually need help. My script is working and in place. That said, I love seeing my superiors' smarter code.

bigearsbilly, thanks for posting that. I'm evidently too impaired to realize escaping also works inside a test. I started by attempting to group the args, because I once read about problems controlling precedence with "-a" and "-o"--guess that guy was wrong.

unSpawn, thanks as well. Your "binary" code is a great idea, and tweaking it to do what I meant (not what I said ;-) was a snap.

I wasn't clear; by "both," I meant both tests from my first post. I just wondered if it's possible to do all this in a single test:

Code:

( ([a=1] -o [b=2 -a c=3]) but not ([a=1] -a [b=2 -a c=3]) )

This worked:

Code:

#!/bin/sh
res=0; read a b c
[ "$a" = "1" ] && res=$[$res+1]
[ "$b" = "2" -a "$c" = "3" ] && res=$[$res+2]
case "$res" in 1|2) echo \
"yep";; *) echo "nope";; esac

(10:37:07 bin) ./werks 
5 5 5                      <- neither
nope
(10:37:19 bin) ./werks 
5 2 3                      <- 2nd
yep
(10:37:23 bin) ./werks 
1 4 4                      <- 1st
yep
(10:37:30 bin) ./werks 
1 2 3                      <- both
nope