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

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
}

Marcellus · Accepted Answer · 2018-06-08T06:21:18.403

114

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...)

edited Jun 08 '18 at 06:21

answered Dec 10 '14 at 02:40

Marcellus

1,305
2
9
5

3

simplier solution if you don't want to keep result – alexandre-rousseau Sep 14 '17 at 06:39
1

How can this be combined with a test of variable value greater than 0? something like `if grep -q $address /etc/passwd && $val > 0` – user 923227 Sep 19 '17 at 20:51
4

@SumitKumarGhosh -- ```if grep -q "$address" /etc/passwd && [ "${val}" -gt 0 ]; then``` – storm_m2138 Apr 13 '18 at 22:48
grep -q not working in solaris.. any idea or alternative? – Stunner Sep 25 '18 at 08:17
I found my answer here - https://stackoverflow.com/questions/2480584/how-do-i-use-a-file-grep-comparison-inside-a-bash-if-else-statement – Smit Thakkar Jan 12 '23 at 16:22

score 23 · Answer 2 · edited Sep 16 '21 at 13:05

23

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:

And:

getent passwd | grep -q invalid_user
echo $?

Output:

edited Sep 16 '21 at 13:05

Peter Mortensen

30,738
21
105
131

answered Dec 10 '14 at 02:40

helloV

50,176
7
137
145

2

Why should `/etc/passwd` be avoided? – arcanemachine Mar 06 '22 at 19:56

score 19 · Answer 3 · edited Sep 16 '21 at 13:08

19

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

edited Sep 16 '21 at 13:08

Peter Mortensen

30,738
21
105
131

answered Dec 10 '14 at 04:03

repzero

8,254
2
18
40

score 12 · Answer 4 · answered Dec 10 '14 at 02:41

12

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) ]

answered Dec 10 '14 at 02:41

alienchow

383
2
7

2

Could you try adding double quotes? if [ -z "$(grep $address /etc/passwd)" ] Edit: Also, shift your entire chunk of echos and mailing commands into the else clause if you do not want them to be run for an invalid user. – alienchow Dec 10 '14 at 03:03

score 9 · Answer 5 · edited Sep 16 '21 at 13:09

9

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

edited Sep 16 '21 at 13:09

Peter Mortensen

30,738
21
105
131

answered Aug 31 '19 at 11:26

antigenius

121
1
3

score 1 · Answer 6 · answered Mar 23 '22 at 14:46

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.

score 0 · Answer 7 · answered Jan 20 '23 at 13:43

0

Shorter:

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

answered Jan 20 '23 at 13:43

jehon

1,404
15
21

score 0 · Answer 8 · answered Aug 22 '23 at 08:47

As alienchow said, You can use parantheses with $(). But in some cases, you need to use it in quotes. For example, in order to check if my kvm is intel or amd, I use this bash code:

if [ ! -z "$(lsmod | grep kvm_intel)" ]; then
    echo "kvm_intel found."
elif [ ! -z "$(lsmod | grep kvm_amd)" ]; then
    echo "kvm_amd found."
else
    echo "check your kvm with command 'lsmod | grep kvm' ."
fi

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

8 Answers8

Linked

Related