Need help understanding a command

henrtm05 · 09-26-2010, 12:30 AM

I have the following command to show all users belonging to a group.

grep :`grep ^mygroup /etc/group | cut -d: -f3`: /etc/passwd

I would like to understand what is happening here. The command starts with grep then space and a : and another : after -f3 what does the first grep do and what are the colons doing in this command. If someone could explain this it would be very helpful.

Thanks

malekmustaq · 09-26-2010, 01:07 AM

Hi hentrtm05,

You will get better perception if you first understand the elements of the script (series of commands). Please read the following manual pages in the terminal, type:

--$ man grep
--$ man cut
--$ man passwd
--$ man group

Then, when anyone try to explain to you about your script posted above you shall be ready enough to understand "WHY" and "WHAT'S" happening behind the screen.

Hope it helps.

sag47 · 09-26-2010, 01:35 AM

There's a couple of shell syntax to go over for you to fully understand.

The pipe | (located above the return key and is part of the backslash key) character is used to redirect the output of one command as input to another. Google shell pipe commands for solid examples.

The backtick ` (located above the tab key and is part of the tilde key ~) uses the output of a command as input in the middle of another command. The command within the backticks is evaluated first and then the overall command. Google shell backticks or see the following for an easy example.

Code:

echo 127.0.0.1
ping 127.0.0.1 -n 4
ping `echo 127.0.0.1` -n 4

Explanation of command
Original command

Code:

grep :`grep ^mygroup /etc/group | cut -d: -f3`: /etc/passwd

The first command executed is:

Code:

grep ^mygroup /etc/group

Grep outputs all lines which start with "mygroup" (^ means start with, read grep man page) from the file /etc/group. Lets say for example the output of that grep command is:

Code:

mygroup:S-1-5-32-545:545:

Now lets evaluate the code within the backticks as a whole.

Code:

grep ^mygroup /etc/group | cut -d: -f3

Cut will take the output of grep and chop it up using a colon : as a delimiter ( -d: ) and takes the 3rd field from the left ( -f3 ). So the output of the full command will be:

Code:

Now lets replace the backticks with the output of the commands within the backticks and the command may make a little more sense.

Code:

grep :545: /etc/passwd

Now I don't know if the final result of your command outputs anything but it doesn't in mine (I used a group found on my own system and still output nothing).

That's exactly how the shell code is acting. What're you using it for? Just curious.

Like malekmustaq stated, reading the man pages will clear all the command behavior up and tell you what the arguments are doing.

henrtm05 · 09-26-2010, 10:53 AM

I am using this command to take all users belonging to two different groups and creating one group containing all those users.

grep :`grep ^mygroup /etc/group | cut -d: -f3`: /etc/passwd

I am sorry, I should have been more specific on my question. I understood what all the commands did. I just didn't know what the colon after the first grep did and before the /etc/passwd did. I think I figured it out, the part in between the colons represent the group number pulled from /etc/group and the program will then use the colons as part of the search in /etc/passwd. An example would be :503: as my search criteria. I am using Linux Centos to perform this operation.

sag47 · 09-26-2010, 01:57 PM

Yes I beleive it is using the colons as search terms as well. I don't think they have any special meaning. Interesting command, I tried it again on my Linux netbook (was using cygwin when I was explaining it) and it works as expected.

Just to be sure it's a good practice to quote the expression and escape the characters. Though grep is smart enough to know not to use the : in its special commands because it doesn't match any cases (e.g. grep "[:alnum:]\+" same as grep "[0-9a-zA-Z]\+"); That may not always be the case when you use this command on other systems with older versions of grep.

Code:

grep "\:`grep ^mygroup /etc/group | cut -d: -f3`\:" /etc/passwd

Produces the same result.