Shell Programming and Scripting

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Regards,
Alister

Please enlighten me: why not sed?

If /etc/group occupies more than one disk block, it would be safer to use:

Code:

sed "/^$Grp:/s/$/,$Usr/" /etc/group > tmpfile
mv tmpfile /etc/group

to avoid the possibility of /etc/group being corrupt if someone tries to authenticate during the time it takes the "w" command to complete in the ed script I gave, but slufoot80 explicity said: not that he wanted to a add user to the end of a group in a copy of the /etc/group file.

Using the code above, you also have to be sure that tmpfile isn't an existing file and can't be created (hi-jacked) by a hacker between the sed and the mv. Note that you to run this in /etc; if tmpfile is on a different file system than /etc, the mv won't be an atomic operation any more than the w command in ed would have been.

@Don Cragun: Thanks for drawing the attention to these aspects. As I said, tinkering with system files is always a thing to consider twice.
Btw, you should add a s/:,/:/ to your sed cmd in case $Usr would be the first in $Grp.

Good catch. And doing that in the ed script would be something like:

Code:

ed -s /etc/group <<-EOF
	/^$Grp:/s/$/,$Usr/p
	.g/:,$Usr$/s//:$Usr/p
	w
	q
EOF

with the .g needed in front of the second s command to preserve the 0 exit status if $Grp was found and $Usr wasn't the first user in the group.

One should also make sure that the script isn't adding a user to the "nobody" group nor the "nogroup" group (or other groups depending on the OS on the machine), but I didn't bother because the idea of editing /etc/passwd, /etc/group, or any corresponding shadow files because usermod won't work (without explaining why usermod won't work) still just seems wrong.