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

Just keep it simple :)

grep + echo should suffice:

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

Edit: incorporated @Cerin suggestion.

NOTE: for "editors" changing my answer: if you have an alternative solution give your own answer! Editing is intended for clarifying, formatting, better English and so on! Don't change the answer given by others! (thank you)

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  I would add the -q switch to grep to suppress output: grep -vq ... – Dave Kirby Aug 24 '10 at 16:33
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  FYI, using -v and && doesn't look to do the trick, whereas || without -v works. – bPizzi Aug 25 '10 at 8:21
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  This only works for very simple lines. Otherwise, grep interprets most non-alpha characters in your line as patterns, causing it to not detect the line in the file. – Cerin Jul 10 '14 at 20:34
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  Note, using -F seems to fix my issue. – Cerin Jul 10 '14 at 20:46
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  Beautiful solution. This also works for triggering more complicated expressions, of course. Mine uses the echo to trigger a cat of a multiline heredoc into a config file. – Eric Lawler Jun 15 '15 at 16:10

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"

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  If it's to a file where you need root permissions: sudo grep -qF "$LINE" "$FILE" || echo "$LINE" | sudo tee -a "$FILE" – nebffa Dec 8 '17 at 0:50
• This actually doesn't work when the line starts with a -. – hyperknot Feb 6 at 22:03
• @zsero: good point! I added -- in the grep command, so it will not interpret a $LINE starting with a dash as options any more. – rubo77 Feb 7 at 1:05

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

• For a command that replaces a partial string with the complete/updated config file entry, or appends the line as needed: sed -i -e '\|session.*pam_mkhomedir.so|h; ${x;s/mkhomedir//;{g;tF};a\' -e 'session\trequired\tpam_mkhomedir.so umask=0022' -e '};:F;s/.*mkhomedir.*/session\trequired\tpam_mkhomedir.so umask=0022/g;' /etc/pam.d/common-session – bgStack15 May 31 '16 at 16:01
• Nice, it helped me a lot +1. could you please, explain your sed expression ? – Rahul Jun 10 '16 at 10:39
• I'd love to see an annotated explanation of this solution. I've used sed for years but mostly just the s command. The hold and pattern space swaps are beyond me. – nortally Jun 24 '16 at 15:50

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:

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

use awk

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

• Is it 'file file' or just 'file' ? – webdevguy Apr 29 '14 at 13:48
• It's file file because it makes two passes over the same file. NR==FNR is true on the first pass, but not the second. This is a common Awk idiom. – tripleee Mar 15 '17 at 14:12

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 controlls from your actuall entry, i.e. to put a backslash infront of ^$/*?+().

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

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  Nice one-liner, short and easy to read. Works great for updating etc/hosts: sed -i.bak -e '$a\' -e "$NEW_IP\t\t$HOST.domain\t$HOST" -e "/.*$HOST/d" /etc/hosts – Noam Manos Apr 26 at 16:38

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

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

• This doesn't look correct, it will lose the other lines from the file when the config line is missing. – tripleee Mar 15 '17 at 14:13