Unix sed command to replace brackets in file

The issue with the command is the g at the end. It will cause all double quotes to be substituted with < on each line matching ^#include. Notice how the StackOverflow question that you link to is concerned with replacing the <> with "" (which could be done using y/<>/""/, or less efficiently using s/[<>]/"/g, on the relevant lines), not the other way around like you want.

If you had used first s/"/</ (no g) followed by s/"/> (no g here either), you would have been ok:

sed '/^#include/ { s/"/</; s/"/>/; }' file.c >file-new.c

The first substitution replaces the first double quote and the second substitution replaces the first one in the now modified string.

To correct the file (assuming you did an in-place edit), just replace the < at the end of the line with >:

sed '/^#include/ s/<$/>/' file.c >file-new.c

This matches <$ (an < at the end of the line), and replaces it with >.

Look at the resulting file-new.c and then replace file.c with it if it looks ok.

You need to replace two equal characters " for two distinct characters < and >. Your command doesn't work because it replaces all occurrences of one character " for one character only <. So try this:

# capture the final word between double quotes
# and replace it for itself enclosed in brackets
$ sed '/^#include/s/"\([^"]*\)"/<\1>/' file
#include <stdlib.h>
#include <graph.h>