How can I suppress output from grep, so that it only returns the exit status?

Just redirect output of grep to /dev/null:

grep sample test.txt > /dev/null

echo $?

You simply need to combine grep -q <pattern> with an immediate check of the exit code for last process to quit ($?).

You can use this to build up a command like this, for example:

uname -a | grep -qi 'linux' ; case "$?" in "0") echo "match" ;; "1") echo "no match" ;; *) echo "error" ;; esac

You can optionally suppress output from STDERR like so:

grep -qi 'root' /etc/shadow &> /dev/null ; case "$?" in "0") echo "match" ;; "1") echo "no match" ;; *) echo "error: $?" ;; esac

This will print error: 2 from the case statement (assuming we do not have privileges to read /etc/shadow or that the file does not exist) but the error message from grep will be redirected to /dev/null so we don't ever see it.

If you don't want to see any output, you need to redirect both stderr and stdout to /dev/null. The other answers are good but won't suppress errors, such as trying to read from a non-existent or unreadable (e.g. wrong permissions) file. Append &> /dev/null (works in bash and some other shells, but not all; >/dev/null 2>&1 may be more portable) so you won't see any output.

$ ls thisfiledoesnotexist
ls: cannot access thisfiledoesnotexist: No such file or directory
$ grep foo thisfiledoesnotexist
grep: thisfiledoesnotexist: No such file or directory
$ grep -q foo thisfiledoesnotexist
grep: thisfiledoesnotexist: No such file or directory
$ grep foo thisfiledoesnotexist >/dev/null
grep: thisfiledoesnotexist: No such file or directory
$ grep foo thisfiledoesnotexist &>/dev/null
$ echo $?
2
$ touch nopermissions
$ chmod 000 nopermissions
$ grep -q foo nopermissions
grep: nopermissions: Permission denied
$ grep foo nopermissions > /dev/null
grep: nopermissions: Permission denied
$ grep foo nopermissions &> /dev/null
$ echo $?
2
$ 

If you've already verified that the target file exists and is a file and you have permissions to read it, this isn't that important, but if you're being a little more careless and grepping recursively or using wildcards, you're more likely to see output on stderr.