test - linux command

Question

I'm just wondering about test Linux command.

Is there an output from test -f FILE that I could grab using a variable?

How do I know whether this test perform successfully or not?

Answer 1

The test command sets its exit code. You can examine the value in $? but usually you don't have to. Instead, you use a control flow construct such as while or if which does this behind the scenes for you.

while test -e "$myfile"; do
   if grep -q 'fnord' "$myfile"; then
       echo "Success!  Play again."
       dd if=/dev/urandom count=1 | base64 >"$myfile"
   else
       break
   fi
done

Observe how while examines the exit code from test and if examines the exit code from grep. Each of these programs -- and generally any useful Unix utility -- is written so that it returns a zero exit code on success, otherwise a non-zero value.

test is perhaps somewhat odd in that that's all it does. Historically, it was a bit of a kitchen sink to avoid having to burden the shell's syntax with a lot of string and file property tests (which may in retrospect have been a slightly unhappy design decision -- this trips up beginners still today).

You also see a lot of beginners who have not picked up this idiom, and write pretzel code like

test -e "$myfile"
while [ $? -eq 0 ]; do
    ....

Yo dawg, I hear you like running test so I run test to test the output from the first test.