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I want to prefix all the lines of a file (except first line) using the first word in a file.


x.o: x.cpp /u/a.h 
/b.h \
/c.h \


x.o: x.cpp /u/a.h \
x.o: /b.h \
x.o: /c.h \

Can anyone please help me how to solve the above using sed?

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up vote 3 down vote accepted

You can do it like this with GNU sed:

sed -r '1 {h; s/ .*//; x}; 1!{G; s/([^\n]*)\n(.*)/\2 \1/}' infile

Or as a separate script:


1 {                         # run block for first line only
  h                         # save copy of line in hold space
  s/ .*//                   # remove redundant part
  x                         # swap prefix to hold space

1! {                        # when not first line
  G                         # append prefix to pattern space
  s/([^\n]*)\n(.*)/\2 \1/   # reorganize so prefix is the prefix

Run it like this:

 sed -rf parse.sed infile


x.o: x.cpp /u/a.h 
x.o: /b.h \
x.o: /c.h \
share|improve this answer
Thank you. This works great. – Naga May 24 '13 at 13:35
+1 I'm always impressed by non-trivial sed scripts. – glenn jackman May 24 '13 at 19:19

Perl solution:

perl -pe 'print $prefix; ($prefix) = /^(.+? )/ if 1..1' INPUT.TXT
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thank you. I'm not sure whether I can use above solution in a makefile. I want to use sed command to achieve above. – Naga May 24 '13 at 12:20
@Naga, there's nothing magic about sed in a makefile. perl (or awk or ...) is just an external tool like sed. – glenn jackman May 24 '13 at 19:17

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