How could I write a simple diff resp. patch script for applying additions and deletions to a list of lines in a file?
This could be a original file (it is sorted and each line is unique):
a
b
d
a simple patch file could look like this (or somehow as simple):
+ c
+ e
- b
The resulting file should look like (or in any other order, since sort could be applied anyways):
a
c
d
e
The normal patch formats can not be used since they include context, which might alter in this case.
Bash alternatives that read input files only once:
To generate patch you can:
comm -3 a.txt b.txt | sed 's/^\t/+ /;t;s/^/- /'
Because comm delimeters outputs from different files using tab, we can use that tab to detect if line should be added or removed.
To apply patch you can:
{ <patch.txt tee >(grep '^+ ' | cut -c3- >&5) |
grep '^- ' | cut -c3- | comm -13 - a.txt; } 5> >(cat)
The tee splits the input, that is the patch file, into two streams. The first part has + filtered and is outputted to file descriptor 5. The file descriptor 5 is opened to just >(cat) so it is just outputted on stdout. The second part has the minus - filtered and it is joined with a.txt and outputted. Because output should be line buffered, it should work.