Shell Programming and Scripting

sed 's/abc.*/&.txt/g' <filename>

$
$ # The data file
$ cat -n f3
     1  abcdefghij
     2  the quick brown fox abc
     3  yet another line
     4  abcdef abcdef abcdef
     5  the abc quick brown abcd fox abcde
$
$ # conversion with sed
$ sed 's/abc.*/&.txt/g' f3
abcdefghij.txt
the quick brown fox abc.txt
yet another line
abcdef abcdef abcdef.txt
the abc quick brown abcd fox abcde.txt
$
$ # conversion with Perl
$ perl -pne 's/(abc.*)/$1.txt/g' f3
abcdefghij.txt
the quick brown fox abc.txt
yet another line
abcdef abcdef abcdef.txt
the abc quick brown abcd fox abcde.txt
$
$

perl -pe 's/(abc.*)/\1.txt/'

perl -nle 'if ($_ =~ m/abc /) { $_ =~ s/$/\.txt/; print $_}' file

$
$ cat f3
abc
abcd
$
$ perl -pne 's/(abc)$/$1.txt/' f3
abc.txt
abcd
$
$