Dodo the stupid editor
This post continues on from my previous post  which ended with the following
Not being able to use either `ed` or `sed` left me a little dissapointed, The c program worked but in it's current form it is very brittle and isn't helped by all the noise of manual error checking, this all requires far too much effort to debug and maintain. I want a more general solution. Ideally I want a dumbed down `ed`-like that allows editing of the file in place, without having to read or write any of the lines I don't care about, taking care of all of the noise for me, a domain specific language for editing a file in place. Hopefully details and some code will come in a later post.
This scriptable editor (
language) was named
Dodo  is a simple scriptable inplace file editor
That is to say that dodo is an interpreter for a small domain specific language for making small byte-by-byte changes in place to large text files.
Dodo is a wrapper around
Dodo performs all edits in place on the file, keeping no temporary copies, and not loading the contents of the file into memory.
This allows for dodo to efficiently edit files far too large to fit into memory.
This also means that there is no undo, and thus I only recommend using dodo on files that you can 'reproduce',
my use case being making small changes to SQL dumps (which I can reproduce again with
A quick example
if we have a file called example.txt with the following contents
hello world how are you
we then run the following program:
./dodo example.txt <<EOF p # print (upto) 100 bytes p5 # print 5 bytes ('hello') e/hello/ # expect string 'hello' b6 # goto byte 6 in file e/world/ # expect string 'world' w/marge/ # write string 'marge' (writes over 'world') q # quit EOF
which is also equivalent to:
echo 'pp5e/hello/b6e/world/w/marge/' | ./dodo example.txt
this program will output:
'hello world how are you ' 'hello'
and leave example.txt with the following contents:
hello marge how are you
The real test
The root issue dodo was designed to deal with was the one pointed out in my previous post 
Writing a dodo script equivalent to the one from that post  is the real test
and here it is:
b62 e/SET lock_timeout = 0;/ w/--/ b406 e/CREATE EXTENSION IF NOT EXISTS plpgsql WITH SCHEMA pg_catalog;/ w/CREATE PROCEDURAL LANGUAGE plpgsql;---------------------------/ q
which can be read as:
- seek to byte 62
- expect that the string under the cursor equals 'SET lock_timeout = 0;'
- write over the first 2 bytes with '--' (thus commenting it out as this is SQL)
- seek to byte 402 within file (absolute from beginning)
- check that the string under the cusor equsl 'CREATE EXTENSION IF NOT EXISTS plpgsql WITH SCHEMA pg_catalog;'
- overwrite that string with 'CREATE PROCEDURAL LANGUAGE plpgsql;---------------------------/
- exit dodo
Dodo is yet another niche tool with a sharp edge,
with no undo history it makes it easy to shoot yourself in the foot,
but I do find it indispensable in editing large files when the usual toolset fails (
Dodo was also quite fun to write as it is a very simple interpreter built for a single purpose.
You can find more about dodo on it's github page .