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

Just keep it simple :)

grep + echo should suffice:

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

-q be quiet

-x match the whole line

-F pattern is a plain string

https://linux.die.net/man/1/grep

Edit: incorporated @cerin and @thijs-wouters suggestions.

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Try this:

grep -q '^option' file && sed -i 's/^option.*/option=value/' file || echo 'option=value' >> file

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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.

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Using sed, the simplest syntax:

sed \
    -e '/^\(option=\).*/{s//\1value/;:a;n;ba;q}' \
    -e '$aoption=value' filename

This would replace the parameter if it exists, else would add it to the bottom of the file.

Use the -i option if you want to edit the file in-place.

If you want to accept and keep white spaces, and in addition to remove the comment, if the line already exists, but is commented out, write:

sed -i \
    -e '/^#\?\(\s*option\s*=\s*\).*/{s//\1value/;:a;n;ba;q}' \
    -e '$aoption=value' filename

Please note that neither option nor value must contain a slash /, or you will have to escape it to \/.

To use bash-variables $option and $value, you could write:

sed -i \
    -e '/^#\?\(\s*'${option//\//\\/}'\s*=\s*\).*/{s//\1'${value//\//\\/}'/;:a;n;ba;q}' \
    -e '$a'${option//\//\\/}'='${value//\//\\/} filename

The bash expression ${option//\//\\/} quotes slashes, it replaces all / with \/.

Note: Just trapped into a problem. In bash you may quote "${option//\//\\/}", but in the sh of busybox, this does not work, so you should avoid the quotes, at least in non-bourne-shells.

All combined in a bash function:

# call option with parameters: $1=name $2=value $3=file
function option() {
    name=${1//\//\\/}
    value=${2//\//\\/}
    sed -i \
        -e '/^#\?\(\s*'"${name}"'\s*=\s*\).*/{s//\1'"${value}"'/;:a;n;ba;q}' \
        -e '$a'"${name}"'='"${value}" $3
}

Explanation:

/^\(option=\).*/: Match lines that start with option= and (.*) ignore everything after the =. The \(…\) encloses the part we will reuse as \1later.

/^#?(\s*'"${option//////}"'\s*=\s*).*/: Ignore commented out code with # at the begin of line. \? means «optional». The comment will be removed, because it is outside of the copied part in \(…\). \s* means «any number of white spaces» (space, tabulator). White spaces are copied, since they are within \(…\), so you do not lose formatting.

/^\(option=\).*/{…}: If matches a line /…/, then execute the next command. Command to execute is not a single command, but a block {…}.

s//…/: Search and replace. Since the search term is empty //, it applies to the last match, which was /^\(option=\).*/.

s//\1value/: Replace the last match with everything in (…), referenced by \1and the textvalue`

:a;n;ba;q: Set label a, then read next line n, then branch b (or goto) back to label a, that means: read all lines up to the end of file, so after the first match, just fetch all following lines without further processing. Then q quit and therefore ignore everything else.

$aoption=value: At the end of file $, append a the text option=value

More information on sed and a command overview is on my blog:

https://marc.wäckerlin.ch/computer/stream-editor-sed-overview-and-reference

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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"

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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

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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

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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...

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Here is a one-liner sed which does the job inline. Note that it preserves the location of the variable and its indentation in the file when it exists. This is often important for the context, like when there are comments around or when the variable is in an indented block. Any solution based on "delete-then-append" paradigm fails badly at this.

   sed -i '/^[ \t]*option=/{h;s/=.*/=value/};${x;/^$/{s//option=value/;H};x}' test.conf

With a generic pair of variable/value you can write it this way:

   var=c
   val='12 34' # it handles spaces nicely btw
   sed -i '/^[ \t]*'"$var"'=/{h;s/=.*/='"$val"'/};${x;/^$/{s//c='"$val"'/;H};x}' test.conf

Finally, if you want also to keep inline comments, you can do it with a catch group. E.g. if test.conf contains the following:

a=123
# Here is "c":
  c=999 # with its own comment and indent
b=234
d=567

Then running this

var='c'
val='"yay"'
sed -i '/^[ \t]*'"$var"'=/{h;s/=[^#]*\(.*\)/='"$val"'\1/;s/'"$val"'#/'"$val"' #/};${x;/^$/{s//'"$var"'='"$val"'/;H};x}' test.conf

Produces that:

a=123
# Here is "c":
  c="yay" # with its own comment and indent
b=234
d=567

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As an awk-only one-liner:

awk -v s=option=value '/^option=/{$0=s;f=1} {a[++n]=$0} END{if(!f)a[++n]=s;for(i=1;i<=n;i++)print a[i]>ARGV[1]}' file

ARGV[1] is your input file. It is opened and written to in the for loop of theEND block. Opening file for output in the END block replaces the need for utilities like sponge or writing to a temporary file and then mving the temporary file to file.

The two assignments to array a[] accumulate all output lines into a. if(!f)a[++n]=s appends the new option=value if the main awk loop couldn't find option in file.

I have added some spaces (not many) for readability, but you really need just one space in the whole awk program, the space after print. If file includes # comments they will be preserved.

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use awk

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

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Here's an awk implementation

/^option *=/ { 
  print "option=value"; # print this instead of the original line
  done=1;               # set a flag, that the line was found
  next                  # all done for this line
}
{print}                 # all other lines -> print them
END {                   # end of file
  if(done != 1)         # haven't found /option=/ -> add it at the end of output
    print "option=value"
}

Run it using

awk -f update.awk < /etc/fdm_monitor.conf > /etc/fdm_monitor.conf.tmp && \
   mv /etc/fdm_monitor.conf.tmp /etc/fdm_monitor.conf

or

awk -f update.awk < /etc/fdm_monitor.conf | sponge /etc/fdm_monitor.conf

EDIT: As a one-liner:

awk '/^option *=/ {print "option=value";d=1;next}{print}END{if(d!=1)print "option=value"}' /etc/fdm_monitor.conf | sponge /etc/fdm_monitor.conf

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 sed -i 's/^option.*/option=value/g' /etc/fdm_monitor.conf
 grep -q "option=value" /etc/fdm_monitor.conf || echo "option=value" >> /etc/fdm_monitor.conf

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here is an awk one-liner:

 awk -v s="option=value" '/^option/{f=1;$0=s}7;END{if(!f)print s}' file

this doesn't do in-place change on the file, you can however :

awk '...' file > tmpfile && mv tmpfile file

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Using sed, you could say:

sed -e '/option=/{s/.*/option=value/;:a;n;:ba;q}' -e 'aoption=value' filename

This would replace the parameter if it exists, else would add it to the bottom of the file.

Use the -i option if you want to edit the file in-place:

sed -i -e '/option=/{s/.*/option=value/;:a;n;:ba;q}' -e 'aoption=value' filename

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sed -i '1 h
1 !H
$ {
   x
   s/^option.*/option=value/g
   t
   s/$/\
option=value/
   }' /etc/fdm_monitor.conf

Load all the file in buffer, at the end, change all occurence and if no change occur, add to the end

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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

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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

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I elaborated on kev's grep/sed solution by setting variables in order to reduce duplication.

Set the variables in the first line (hint: $_option shall match everything on the line up until the value [including any seperator like = or :]).

_file="/etc/ssmtp/ssmtp.conf" _option="mailhub=" _value="my.domain.tld" \
        sh -c '\
            grep -q "^$_option" "$_file" \
            && sed -i "s/^$_option.*/$_option$_value/" "$_file" \
            || echo "$_option$_value" >> "$_file"\
        '

Mind that the sh -c '...' just has the effect of widening the scope of the variables without the need for an export. (See Setting an environment variable before a command in bash not working for second command in a pipe)

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You can use this function to find and search config changes:

#!/bin/bash

#Find and Replace config values
find_and_replace_config () {
   file=$1
   var=$2
   new_value=$3
   awk -v var="$var" -v new_val="$new_value" 'BEGIN{FS=OFS="="}match($1, "^\\s*" var "\\s*") {$2=" " new_val}1' "$file" > output.tmp && sudo mv output.tmp $file
}

find_and_replace_config /etc/php5/apache2/php.ini max_execution_time 60

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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

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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

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