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Cheat Sheet

This is a quick reference guide for useful comby options.

Useful command line options

-i	replace files in place
-d dir	only process files recursively and directories in dir
-zip zipfile	process files in a ZIP archive
-jobs n	run n processes in parallel
-matcher language	select the language to parse the file as (see more)
-exclude prefix	exclude files with names or paths that start with prefix
-exclude-dir prefix	same as -exclude but applies only to directories

– Exclude multiple prefixes with commas:

comby -exclude-dir vendor,node_modules,test ...

See comby -help for a complete list of flags.

How do I…

Run multiple search-and-replace templates?

Have a look at using configuration files. Alternatively, use a directory of rewrite templates with the -templates option and run it like so:

comby -f .go -templates /path/to/directory

It’s important to use the -f option for filtering files when using -templates, or comby will think it is an anonymous argument for a command line template.

The path/to/directory should contain folders, one for each rewrite pattern, that contain two files: one named match and the other named rewrite. An optional rule can be put in the same directory, in a file called rule. For example,

path/to/directory
├── some-rewrite
│   ├── match
│   └── rewrite
├── another-rewrite-with-rule
│   ├── match
│   ├── rule
│   └── rewrite
···

See the catalog directory layout for a sample catalog of templates.

Extract matches?

First way: Add -match-only -json-lines will print matched content, matched values for holes, and ranges in JSON format. One line is printed per match.

comby 'a :[1] c d' '' -stdin .txt -json-lines -match-only <<< "a b c d"

Use jq to extract values you’re interested in from the JSON. See the JSON schema for what the JSON output contains.

Second way: Add -newline-separated -stdout if you’re only interested in the matched fragments and don’t want to fiddle with JSON.

comby 'a :[1] c d' '' -stdin .txt -match-only -newline-separated -stdout <<< "a b c d"

This will print all matches terminal, one entry per line. If -match-only is removed, all rewritten-values are printed.

Review changes before accepting them?

Interactively review changes on the command line using -review, just like in codemod:

comby 'foo(:[1], :[2])' 'bar(:[2], :[1])' sub.js -review

You’ll see, for example:

------ sub.js
++++++ sub.js
@|-1,3 +1,3 ============================================================
-|function subtract(param1, param2) {
+|function subtract(param2, param1) {
 |  return param1 - param2;
 |}
Accept change (y = yes [default], n = no, e = edit original, E = apply+edit, q = quit)?

Get raw patches?

Add -diff to get patches as unified diffs.

comby 'foo(:[1], :[2])' 'bar(:[2], :[1])' -stdin -diff <<< 'foo(a, b)'

--- /dev/null
+++ /dev/null
@@ -1,1 +1,1 @@
-foo(a, b)
+bar(b, a)

Select the language to parse?

Comby will try and infer the language to parse based on file extensions, and will fall back to the generic parser if it does not recognize an extension. Set a language using the -matcher flag to force using a language parser, like -matcher .js or -matcher .c. See all languages by running comby -list. Note: you can use -matcher .js to parse files as a Javascript file even if they end in a different suffix, like my.custom.file.suffix

Pipe input to comby?

Add -stdin to have comby accept input from pipes, and -stdout to write to print to standard out.

echo 'foo(a, b)' | \
comby 'foo(:[1], :[2])' 'bar(:[2], :[1])' -stdin -stdout .js | \
comby 'bar(:[_])' 'qux(:[_])' -stdin -stdout .js

Specify rules on the command line?

Use the -rule option to specify rules:

comby 'foo(:[1])' 'bar(:[1])' .py -rule 'where :[1] == "self"'