Bash Script: using if with -o and printing ONLY true evaluations

Question

I am using bash to script a basic check. It is in a java environment, and I'm pulling the xmx, xss and xms values from the environment, and then comparing them to a predefined standard. Here is a very simplified version:

xmxstd=1024
xmsstd=512
xssstd=256

xmxval=756
xmsval=512
xssval=128

if [ $xmsstd > $xms_val -o $xssstd > $xss_val -o $xssstd > $xss_val ]; then
    echo "<OFFENDING VALUE>"
fi

I don't know if this is possible in bash or not, but I just want to echo out whatever evaluates to be true in the above statement. Maybe I'm missing something stupid, but i've been all over the place and can't find anything on it and I don't want to end up writing this out in three different if's unless I have to.

Answer 1

The whole contents of the [ ] test is only going to be true or false, there's no way of probing the individual elements without splitting it up again.

You said you didn't want to use if, so how about this:

#!/bin/bash

xmxstd=1024
xmsstd=512
xssstd=256

xmxval=756
xmsval=512
xssval=128

(( xmxstd > xmxval )) && echo "xmx: $xmxval"
(( xmsstd > xmsval )) && echo "xms: $xmsval"
(( xssstd > xssval )) && echo "xss: $xssval"

---

edit: How about some associative arrays? This will only work on bash version >= 4.0 but I thought I'd give it a go:

#!/bin/bash

declare -A std
declare -A val
std=( [xmx]=1024 [xms]=512 [xss]=256 )
val=( [xmx]=756 [xms]=512 [xss]=128 )

for i in xmx xms xss
do 
    (( std[$i] > val[$i] )) && echo "$i: ${val[$i]}"
done

Answer 2

One method:

unset offenders
test $xmsstd -gt $xms_val && offenders+=' xms'
test $xmxstd -gt $xmx_val && offenders+=' xmx'
test $xssstd -gt $xss_val && offenders+=' xss'
echo offenders:$offenders

You could also "simplify" a little:

unset offenders
for v in xms xss xmx; do
        eval test \$${v}std -gt \$${v}_val && offenders+=" $v"
done
echo offenders:$offenders

Simplify is quoted there, because a very good argument can be made that this is complicating things rather than simplifying, and I definitely prefer the first, but if the number of variables grows it doesn't scale well. You can certainly use an array instead of a flat string to keep track of the variables that satisfy the constraint, but that is less portable.

Answer 3

Combining with -o (all but deprecated, by the way; even the POSIX spec recommends you use separate test commands joined by ||) is for when you don't care which one is true, as long as one is true. However, you do care which one is true, because you want to generate output tied to the true result. You'll have to have separate statements.

xmxstd=1024
xmsstd=512
xssstd=256

xmxval=756
xmsval=512
xssval=128

if [ $xmxstd > $xmxval ]; then
    echo "$xmxstd"
fi
if [ $xmsstd > $xmsval ]; then
    echo "$xmsstd"
fi
if [ $xssstd > $xssval ]; then
    echo "$xssstd"
fi

Answer 4

If using BASH you can make do:

for k in xms xss xmx; do
   kval="${k}val"
   kstd="${k}std"
   val="${!kval}"
   std="${!kstd}"
   (( std > val )) && echo "offending var: $kval with value: $val"
done