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So I am trying to tell if the machine has a battery, then run different commands based on whether or not it does. Here's what I have so far:

upower -d >> /tmp/batstat
if [ grep -F BAT0 /tmp/batstat ]; then
    echo "BAT0 PRESENT" >> /tmp/batstat
else
    echo "NO BAT0" >> /tmp/batstat
fi

However, this returns the error "bash: [: too many arguments" while writing NO BAT0 to /tmp/batstat as expected. Running only the grep works properly, so I don't understand why it doesn't work as an if condition.

If there is a better way to check for a battery, I am 100% open to it.

  • [ is interpreting grep -F BAT0 /tmp/batstat as four separate arguments, not the one or two it's expecting. I'm sure there are more idiomatic ways to do it, but I'd do it like so: [ "$(grep -F 'text' test.txt)" != "" ]. Not an answer, because unsure about battery: what OS are you running? – John N Dec 15 '16 at 19:01
  • I'm using Debian Jessie. – user5104897 Dec 15 '16 at 19:31
  • Also, when I try using your recommendation with BAT0 in place of text (with or without single quotes), I get the following error: bash: $'NO BAT0\nNO BAT0\nNO BAT0\nNO BAT0\nNO BAT0\nNO BAT0': command not found – user5104897 Dec 15 '16 at 19:36
3
if upower -d | grep -qF BAT0 ; then
   echo "BAT0 PRESENT"
else
   echo "NO BAT0"
fi

if will take a series of commands and works on the status of the final one, in this case the grep. grep returns true if it matches, and false otherwise.

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grep -F BAT0 /tmp/batstat is in no way a CONDITIONAL EXPRESSION (as detailed in the fine manual) for the test builtin of bash. Since the output of grep is unused, the -q option and relying on the exit code likely suffices.

grep -F -q BAT0 batstat
if [[ $? -eq 0 ]]; then
    echo yea
else
    echo nay
fi

Also, /tmp/batstat is likely a very poor choice, especially where a nefarious local user has access. Consider instead using mktemp(1), or not locating the file under /tmp.

  • I didn't include this in the post but the script immediately does rm /tmp/batstat as soon as it is finished with the commands in the if statement. I only need to check if upower -d mentions BAT0 for this one if statement. This is for a Debian preseed installer so I'm not sure where else to write to. – user5104897 Dec 15 '16 at 19:43
  • I am thinking of using something like this: d-i upower -d >> /tmp/batstat d-i grep -F -q BAT0 /tmp/batstat d-i if [[ $? -eq 0 ]]; then echo yea else echo nay fi d-i rm /tmp/batstat – user5104897 Dec 15 '16 at 19:44

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