Regex sed issue - regex

Question

My sed expression looks as belos:
sed -i "s/-D CONSOLELOG /-D CONSOLELOG -fPIC /g" makefile.init
makefile.init
CFLAGS = -std=c99 -rdynamic -g -Wall -Wno-write-strings -D CONSOLELOG
Output after 1st Run( As expected)
CFLAGS = -std=c99 -rdynamic -g -Wall -Wno-write-strings -D CONSOLELOG -fPIC
2nd Run (Notice the extra fPIC at the end)
CFLAGS = -std=c99 -rdynamic -g -Wall -Wno-write-strings -D CONSOLELOG -fPIC -fPIC
I need to modify my sed expression to get output as in (1) irrespective of the number of times it is executed

Answer 1

This might work for you (GNU sed):
sed -ri 's/-D CONSOLELOG (-fPIC )?$/&-fPIC /' file
This would insert at most 2 -fPIC options following a -D CONSOLELOG option.

Answer 2

Sample changed for illustration purposes
$ cat ip.txt
42 foo baz
ijk baz xyz
$ sed -i 's/baz $/&123/' ip.txt
$ cat ip.txt
42 foo baz 123
ijk baz xyz
$ # further runs won't change input
$ sed -i 's/baz $/&123/' ip.txt
$ cat ip.txt
42 foo baz 123
ijk baz xyz
$ is a meta character to ensure matching only at end of line
so, matches elsewhere in the line won't be changed and hence applying the command again won't result in duplication
& in replacement section is backreference to entire matched string in search section
since there can only be one match at end of line, g modifier is not needed
To replace anywhere in the line(assuming only single match per line)
$ cat ip.txt
42 foo baz
ijk baz xyz
$ sed -i '/baz 123/! s/baz /&123/' ip.txt
$ cat ip.txt
42 foo baz 123
ijk baz 123xyz
$ # further runs won't change input
$ sed -i '/baz 123/! s/baz /&123/' ip.txt
$ cat ip.txt
42 foo baz 123
ijk baz 123xyz
sed commands can be qualified with addressing
here, /baz 123/! means lines not matching baz 123
Further reading: Difference between single and double quotes in Bash