Ubuntu – How to use time command in bash, to sum sys + user time, and output it as milliseconds

The problem was that grive command was outputting to the error stream, STDERR (File descriptor 2), which is normally used for passing error messages while the STDOUT (File descriptor 1) is used for showing actual (upon success) output of any program (strictly speaking).

I have not used grive, but according to your words the developer might have used STDERR only to show any messages from the command.

Now the command substitution you have used :

VARABLENAME=$(command)

will save the STDOUT of command to VARABLENAME, not STDERR. As a result the variable was empty and you were seeing messages on terminal.

I have suggested :

VARABLENAME="$(command 2>&1)"

this will save both the STDOUT and STDERR to the variable (in your case just STDERR as there is no STDOUT), So you would get desired result using echo "${VARIABLENAME}".

Also as there is no STDOUT you can save just the STDERR to the variable (generalization, true for any such program) :

VARABLENAME="$(command 2>&1 >/dev/null)"

Basically, you want this structure

if grep -q "un-clean"  /path/to/log_file.log ;
then
    # put some command in case we find result is unclean
else 
    # if the output is ok, do something else
fi

All it does is to silently (without printing to screen) check if there is a match of string "unclean" in the file . If there is we do the if part, otherwise - we do else part.

Here's an example:

$> if grep -q 'root' /etc/passwd ; then  echo "This user exists" ; else echo "This user doesn't exist"; fi    
This user exists
$> if grep -q 'NOEXIST' /etc/passwd ; then  echo "This user exists" ; else echo "This user doesn't exist"; fi 
This user doesn't exist

What also can be done is to start command you want from script, but in background. That way we can have it's PID. Here's what I mean

$> echo "HelloWorld"   &                                                                                      
[1] 6876

Adding & causes echo "HelloWorld" run in background, and we have it's PID stored in $! variable. Thus, we can do something like,

some-command  &
CMD_PID=$!
if grep -q "un-clean"  /path/to/log_file.log ;
then
         kill -TERM $CMD_PID
else 
        # if the output is ok, do something else
fi