Okay, so I'm learning Bash, and there's that exercise;
"Write a script that checks every ten seconds if the user 'user000' is logged in."
My idea is to grep a who, but I don't know how to incorporate this into a script. I tried things like
if [ `who | grep "user000"` ] then things
but it returns the matched lines with grep, not true/false.
You want grep -q. That's "quiet mode"; just sets status based on whether there were any matches, doesn't output anything. So:
if who | grep -q "user000"; then things; fi
grep does not support -q
who | grep -q "user000" && things. (Sorry. I would have just edited your answer if I had the ability)You can do
who | grep "user000" > /dev/null 2>&1
# You can use "-q" option of grep instead of redirecting to /dev/null
# if your grep support it. Mine does not.
if [ "$?" -eq "0" ]
then ...
This uses $? - a Shell variable which stores the return/exit code of the last command that was exected. grep exits with return code "0" on success and non-zero on failure (e.g. no lines found returns "1" ) - a typical arrangement for a Unix command, by the way.
>&! creates a file called "!". Perhaps you mean ... 2>&1 > /dev/nullIf you're testing the exit code of a pipe or command in a if or while, you can leave off the square brackets and backticks (you should use $() instead of backticks anyway):
if who | grep "user000" > /dev/null 2>&1
then
things-to-do
fi
Most answers have the right idea, but really you want to drop all output from grep, including errors. Also, a semicolon is required after the ] for an if:
if who | grep 'user000' >/dev/null 2>&1; then
do things
fi
If you are using GNU grep, you can use the -s and -q options instead:
if who | grep -sq 'user000'; then
do things
fi
EDIT: dropped brackets; if only needs brackets for comparison ops
It's probably not the most elegant incantation, but I tend to use:
if [ `who | grep "user000" | wc -l` = "1" ]; then ....
who list one mention per login shell?To write a Bash script that checks if the user 'user000' is logged in every ten seconds, you can use a loop and the who command combined with grep. The grep command returns a non-zero exit status if the pattern is not found, which you can use to determine if the user is logged in.
# Infinite loop to check every 10 seconds
while true; do
# Check if user000 is logged in
if who | grep -q "user000"; then
echo "user000 is logged in."
else
echo "user000 is not logged in."
fi
# Wait for 10 seconds before checking again
sleep 10
done
You can save this script to a file, for example, check.sh, and run it by making it executable and executing it:
chmod +x user.sh
./user.sh
