How can I check if 'grep' doesn't have any output?
Solution 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...)
Solution 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
Solution 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
Solution 4
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
Solution 5
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 $address /etc/passwd) ]
Duncan
Updated on September 17, 2021Comments
-
Duncan over 2 years
I need to check if the recipient username is in file /etc/passwd which contains all the users in my class, but I have tried a few different combinations of if statements and grep without success. The best I could come up with is below, but I don't think it's working properly.
My logic behind it is that if the grep is null, the user is invalid.
send_email() { message= address= attachment= validuser=1 until [ "$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 }