I would like to update a large number of C++ source files with an extra include directive before any existing #includes. For this sort of task, I normally use a small bash script with sed to re-write the file.
How do I get sed
to replace just the first occurrence of a string in a file rather than replacing every occurrence?
If I use
sed s/#include/#include "newfile.h"\n#include/
it replaces all #includes.
Alternative suggestions to achieve the same thing are also welcome.
Best Answer
A
sed
script that will only replace the first occurrence of "Apple" by "Banana"Example
This is the simple script: Editor's note: works with GNU
sed
only.The first two parameters
0
and/Apple/
are the range specifier. Thes/Apple/Banana/
is what is executed within that range. So in this case "within the range of the beginning (0
) up to the first instance ofApple
, replaceApple
withBanana
. Only the firstApple
will be replaced.Background: In traditional
sed
the range specifier is also "begin here" and "end here" (inclusive). However the lowest "begin" is the first line (line 1), and if the "end here" is a regex, then it is only attempted to match against on the next line after "begin", so the earliest possible end is line 2. So since range is inclusive, smallest possible range is "2 lines" and smallest starting range is both lines 1 and 2 (i.e. if there's an occurrence on line 1, occurrences on line 2 will also be changed, not desired in this case).GNU
sed adds its own extension of allowing specifying start as the "pseudo"line 0
so that the end of the range can beline 1
, allowing it a range of "only the first line" if the regex matches the first line.Or a simplified version (an empty RE like
//
means to re-use the one specified before it, so this is equivalent):And the curly braces are optional for the
s
command, so this is also equivalent:All of these work on GNU
sed
only.You can also install GNU sed on OS X using homebrew
brew install gnu-sed
.