Test if command output contains string with "set -e"

Question

I need to check utilized RAM percentage from shell script. But I want to use "set -e" for it and this makes me a problem.

There are two version of free utility that has slightly different output. In order to use awk on output, I check which one of free command is, by grepping output for string "buffers" which appears in only one of them. If there is no such string, command has exit status = 1 which kills my script from executing due to set -e parameter.

I tried also adding "|| true" after grep command but this always sets exit code = 0, making my grep test useless.

set -e
free -m | grep 'buffers' &> /dev/null || true
if [ $? == 0 ]; then
    MEMORY=$(free -m | awk 'NR==3{printf "%.0f%%", $3*100/($3+$4)}')
else
    MEMORY=$(free -m | awk 'NR==2{printf "%.0f%%", ($2-$7)*100/$2}')
fi
echo $MEMORY

So how I can do this properly without turning off set -e parameter? Also any better idea then presented to get percentage of utilized RAM into variable is welcome.

Using set -e is not universally considered a good idea -- ignore the parable and skip to the exercises below if in a hurry. Reviewing the incompatibilities between set -e implementations across various shells may further make the point of how set -e causes bugs, as opposed to only solving them. — Charles Duffy
– Charles Duffy, Commented Nov 21, 2017 at 22:25
Also, [ $? == 0 ] is gratuitously incompatible, as == is not a valid comparison operator in POSIX test; use = instead, if you must compare $? at all (as opposed to just putting the content to be tested into the body of the if). — Charles Duffy
– Charles Duffy, Commented Nov 21, 2017 at 22:27

janos · Accepted Answer · 2017-11-21 22:29:49Z

3

You can avoid the failed command causing an exit due to set -e if you move it into the if:

if free -m | grep 'buffers' &> /dev/null; then
    MEMORY=$(free -m | awk 'NR==3{printf "%.0f%%", $3*100/($3+$4)}')
else
    MEMORY=$(free -m | awk 'NR==2{printf "%.0f%%", ($2-$7)*100/$2}')
fi
echo $MEMORY

Also, it's not so great that free -m is executed twice. You could store it in a variable for repeated processing:

free=$(free -m)
if [[ $free =~ buffers ]]; then
    MEMORY=$(awk 'NR==3{printf "%.0f%%", $3*100/($3+$4)}' <<< "$free")
else
    MEMORY=$(awk 'NR==2{printf "%.0f%%", ($2-$7)*100/$2}' <<< "$free")
fi
echo $MEMORY

edited Nov 21, 2017 at 22:29

answered Nov 21, 2017 at 22:22

janos

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1 Comment

glenn jackman Over a year ago

+1. There are certain places where set -e cannot reach, and the command list of if is one of them (gnu.org/software/bash/manual/bashref.html#The-Set-Builtin)

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