How can I check if 'grep' doesn't have any output? - bash

Question

I need to check if the recipient username is in file /etc/passwd which contains all the users in my class, but I have tried a few different combinations of if statements and grep without success. The best I could come up with is below, but I don't think it's working properly.
My logic behind it is that if the grep is null, the user is invalid.
send_email()
{
message=
address=
attachment=
validuser=1
until [ "$validuser" = "0" ]
do
echo "Enter the email address: "
read address
if [ -z grep $address /etc/passwd ]
then
validuser=0
else
validuser=1
fi
echo -n "Enter the subject of the message: "
read message
echo ""
echo "Enter the file you want to attach: "
read attachment
mail -s "$message" "$address"<"$attachment"
done
press_enter
}

Answer 1

Just do a simple if like this:
if grep -q $address /etc/passwd
then
echo "OK";
else
echo "NOT OK";
fi
The -q option is used here just to make grep quiet (don't output...)

Answer 2

Use getent and check for grep's exit code. Avoid using /etc/passwd. Equivalent in the shell:
getent passwd | grep -q valid_user
echo $?
Output:
0
And:
getent passwd | grep -q invalid_user
echo $?
Output:
1

Answer 3

Your piece of code:
if [ -z grep $address /etc/passwd ]
You haven't saved the results of grep $address /etc/passwd in a variable. Before putting it in the if statement and then testing the variable to see if it is empty.
You can try it like this:
check_address=`grep $address /etc/passwd`
if [ -z "$check_address" ]
then
validuser=0
else
validuser=1
fi

Answer 4

The -z check is for variable strings, which your grep isn't giving. To give a value from your grep command, enclose it in $():
if [ -z $(grep $address /etc/passwd) ]

Answer 5

The easiest one will be this:
cat test123
# Output: 12345678
cat test123 | grep 123 >/dev/null && echo "grep result exist" || echo "grep result doesn't exist"
# Output: grep result exist
cat test123 | grep 999 >/dev/null && echo "grep result exist" || echo "grep result doesn't exist"
# Output: grep result doesn't exist

Answer 6

My problem was that the file I was trying to grep was a binary file. On windows, the first two characters in the file were little squares. On mac, the first two characters were question marks. When I used more or less on the file, I could see it was binary and when I used diff, it responded that the "Binary files foo.log and requirements.txt differ".
I used cat to display the contents of the file, highlighted and copied the text (minus the two question marks at the top, deleted the file, created a new file with touch and then used vi to paste the text back into the new file.
After that, grep worked fine.

Answer 7

Shorter:
until ! grep $address /etc/passwd ; do {
do_your_stuff
}