[linux] Appending a line to a file only if it does not already exist

I need to add the following line to the end of a config file:

include "/configs/projectname.conf"

to a file called lighttpd.conf

I am looking into using sed to do this, but I can't work out how.

How would I only insert it if the line doesn't already exist?

If, one day, someone else have to deal with this code as "legacy code", then that person will be grateful if you write a less exoteric code, such as

grep -q -F 'include "/configs/projectname.conf"' lighttpd.conf
if [ $? -ne 0 ]; then
  echo 'include "/configs/projectname.conf"' >> lighttpd.conf
fi

Here's a sed version:

sed -e '\|include "/configs/projectname.conf"|h; ${x;s/incl//;{g;t};a\' -e 'include "/configs/projectname.conf"' -e '}' file

If your string is in a variable:

string='include "/configs/projectname.conf"'
sed -e "\|$string|h; \${x;s|$string||;{g;t};a\\" -e "$string" -e "}" file

use awk

awk 'FNR==NR && /configs.*projectname\.conf/{f=1;next}f==0;END{ if(!f) { print "your line"}} ' file file

If you want to run this command using a python script within a Linux terminal...

import os,sys
LINE = 'include '+ <insert_line_STRING>
FILE = <insert_file_path_STRING>                                
os.system('grep -qxF $"'+LINE+'" '+FILE+' || echo $"'+LINE+'" >> '+FILE)

The $ and double quotations had me in a jungle, but this worked. Thanks everyone

The answers using grep are wrong. You need to add an -x option to match the entire line otherwise lines like #text to add will still match when looking to add exactly text to add.

So the correct solution is something like:

grep -qxF 'include "/configs/projectname.conf"' foo.bar || echo 'include "/configs/projectname.conf"' >> foo.bar

This would be a clean, readable and reusable solution using grep and echo to add a line to a file only if it doesn't already exist:

LINE='include "/configs/projectname.conf"'
FILE='lighttpd.conf'
grep -qF -- "$LINE" "$FILE" || echo "$LINE" >> "$FILE"

If you need to match the whole line use grep -xqF

Add -s to ignore errors when the file does not exist, creating a new file with just that line.

I needed to edit a file with restricted write permissions so needed sudo. working from ghostdog74's answer and using a temp file:

awk 'FNR==NR && /configs.*projectname\.conf/{f=1;next}f==0;END{ if(!f) { print "your line"}} ' file > /tmp/file
sudo mv /tmp/file file

another sed solution is to always append it on the last line and delete a pre existing one.

sed -e '$a\' -e '<your-entry>' -e "/<your-entry-properly-escaped>/d"

"properly-escaped" means to put a regex that matches your entry, i.e. to escape all regex controls from your actual entry, i.e. to put a backslash in front of ^$/*?+().

this might fail on the last line of your file or if there's no dangling newline, I'm not sure, but that could be dealt with by some nifty branching...

If writing to a protected file, @drAlberT and @rubo77 's answers might not work for you since one can't sudo >>. A similarly simple solution, then, would be to use tee --append (or, on MacOS, tee -a):

LINE='include "/configs/projectname.conf"'
FILE=lighttpd.conf
grep -qF "$LINE" "$FILE"  || echo "$LINE" | sudo tee --append "$FILE"

Using sed: It will insert at the end of line. You can also pass in variables as usual of course.

grep -qxF "port=9033" $light.conf
if [ $? -ne 0 ]; then
  sed -i "$ a port=9033" $light.conf
else
    echo "port=9033 already added"
fi

Using oneliner sed

grep -qxF "port=9033" $lightconf || sed -i "$ a port=9033" $lightconf

Using echo may not work under root, but will work like this. But it will not let you automate things if you are looking to do it since it might ask for password.

I had a problem when I was trying to edit from the root for a particular user. Just adding the $username before was a fix for me.

grep -qxF "port=9033" light.conf
if [ $? -ne 0 ]; then
  sudo -u $user_name echo "port=9033" >> light.conf
else
    echo "already there"    
fi