how to append text at the end of a line in a file

mia_tech · 02-08-2014, 07:19 PM

guyes lets say that I want to append a username to an existing group in the /etc/group file.. how could I do it?

existing line: users:x:1010
would like: users:x:1010:nancy

apending user nancy to end of existing line

thanks

astrogeek · 02-08-2014, 07:24 PM

I am not certain I understand the question, but from what I think you mean...

For the general case, just open the file in an editor and add the text to the desired line, then save the file.

In the specific case of /etc/group, you should use usermod to set the user's group memberships which will make sure the file is updated accordingly.

Isaac Velando · 02-08-2014, 07:48 PM

A few notes. First, if you're definitely wanting to manually edit /etc/group, /etc/passwd, or /etc/sudoers you should use the secure edit commands for them, see:

Code:

man vipw
man vigr
man visudo

which adds failsafes to editing these critical files.

Second, as astrogeek suggested, again if editing these files is your actual goal then the administrative tools usermod, groupmod, and gpasswd may be of interest to you; again consult their man pages.

Finally, to answer the question posed in the title itself if the /etc/group was just a random example, you can use sed with a regular expression along the lines of:

Code:

sed -ie '3s/$/foo/' /path/to/file

which will append "foo" to the end of the third line of file. The form is '#s/pattern/replace/' and $ matches the end of a line which is why this works. For reference, ^ matches the beginning of a line.

chrism01 · 02-09-2014, 03:22 AM

Assuming a unique line pattern, instead of a known numbered line

Code:

sed -r -i -e 's/(users:x:1010)/\1:nancy/' t.t
 cat t.t
users:x:1010:nancy

With shell var

Code:

t='(users:x:1010)'
sed -r -i -e "s/$t/\1:nancy/" t.t
 cat t.t
users:x:1010:nancy

note double quotes

mia_tech · 02-09-2014, 06:55 PM

Quote:

Originally Posted by Isaac Velando

A few notes. First, if you're definitely wanting to manually edit /etc/group, /etc/passwd, or /etc/sudoers you should use the secure edit commands for them, see:

Code:

man vipw
man vigr
man visudo

which adds failsafes to editing these critical files.

Second, as astrogeek suggested, again if editing these files is your actual goal then the administrative tools usermod, groupmod, and gpasswd may be of interest to you; again consult their man pages.

Finally, to answer the question posed in the title itself if the /etc/group was just a random example, you can use sed with a regular expression along the lines of:

Code:

sed -ie '3s/$/foo/' /path/to/file

which will append "foo" to the end of the third line of file. The form is '#s/pattern/replace/' and $ matches the end of a line which is why this works. For reference, ^ matches the beginning of a line.

what if instead of hardcoding line 3 in your example I wanted to use a variable like $line? I tried

Code:

 sed -ie '$(echo $line) s/$/foo' /path

but no luck

Isaac Velando · 02-09-2014, 07:17 PM

Quote:

what if instead of hardcoding line 3 in your example I wanted to use a variable like $line? I tried

Code:

sed -ie '$(echo $line) s/$/foo' /path

but no luck

First, make sure to have a / at the end of your search/replace sed expression and no space between the line number and 's', so foo/ rather than foo and )s rather than ) s; it's a requirement of that format.

Second you'll need to use double quotes to have bash expand variables; single quotes will have bash interpret things as literals. So:

Code:

sed -ie "$(echo $line)s/$/foo/" /path

should do the trick.

dugan · 02-09-2014, 10:05 PM

Code:

gpasswd -a nancy users

Firerat · 02-09-2014, 10:56 PM

dugan is spot on

But for the sake of it

Code:

sed 's/users.*/&:nancy/' /path/to/file

& is the matched pattern
That _might_ be gnu sed specific, can't recall off the top of my head