Editing 200 Files with Bash and Perl

I recently had to change 189 files in our code base, all in almost the same way. Rather than doing it manually, I decided to brush up on my command-line text manipulation ... and ended up taking it further than I expected.

The Mission

The changes were pretty simple. In our API code, we have TypeScript definitions for every endpoint. They look something like this:

1interface API {
2 "/api/widget/:widgetId": {
3 GET: {
4 params: {
5 widgetId: MongoId;
6 };
7 response: WidgetResponse;
8 }
9 }

You'll notice the params are defined twice: once in the URL key string (as :widgetId) and again in the GET attribute (under params); we are moving to a TypeScript template literal string parser to get the type information out of the URL key string itself, and so I wanted to remove the params key from these definitions. But with 189 files to change, the usual manual approach wasn't so inviting.

So, I set myself the challenge of doing it via the command line.

Step 1: Remove the lines

I'll be honest, when I started, this was the only step I had in mind. I needed to do a multi-line find-and-replace, to remove params: { ... }; a quick grep showed me that this pattern was unique to the places I wanted to change; however, I could have narrowed the set of files I was searching to just our endpoints in src/resources if necessary. For doing the replacement, I thought sed might be the right tool, but new lines can be challenging to work with ... so I ended up learning my first bit of perl to make this work.

Here's what I ended up doing (I've added line breaks for readability):

1grep -r --files-with-matches "params: {" ./src | while read file;
2 do
3 perl -0777 -pi -e 's/ *params: {[^}]*};\n//igs' "$file";
4 done

This one-liner uses grep to recursively search my src directory to find all the files that have the pattern I want to remove. Actually, I usually reach for ag (the silver searcher) or ripgrep, but grep is already available pretty much everywhere. Then, we'll loop over the files and use perl to replace that content.

Like I said, this was my first line of perl, but I'm fairly sure it won't be my last. This technique of using perl for find-and-replace logic is called a perl pie. Here's what it does:

  • 0777 means perl will read in the entire file
  • p wraps that one-liner in the conventional perl script wrapper.
  • i means that perl will change the file in place; if you aren't making this change in a git repo like I am, you can do something like i.backup and perl will create a copy of the original file, so you aren't making an irreversible change.
  • e expects an argument that is your one-line program

Oh, and the program itself:

1s/ *params: {[^}]*};\n//igs

This is typical 's/find/replace/flags' syntax, and you know how regexes work. The flags are global, case-insensitive, and single-line (where . will also match newlines).

So, this changed the 189 files, in exactly the way I wanted. At this point, I was feeling great about my change. Reviewed the changes, committed it and started the git push.

Step 2: Remove unused imports

Not so fast. Our pre-push hooks caught a TypeScript linting issue:

1error TS6133: 'MongoId' is declared but its value is never read.
35 import { MongoId } from "our-types";
4 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Ah, yeah, that makes sense. URL parameters are strings, but we have a MongoId type that's a branded string. I forgot about this step, but that's why we have pre-push checks! We'll need to remove those imports.

How can we do this? Well, let's get a list of the files we changed in our most recent commit:

1git show --name-only | grep ^src

We add the grep to only find the files within our top-level src directory (and to remove the commit information).

Then, we need to find all the files that include MongoId only once. If a file references MongoId multiple times, then we don't want to remove the import, because clearly we're still using it. If the file only references MongoId once, we can remove the import ... but we have to consider that it might not be the only thing we're importing on that line. For starters, grep's -c flag to count the number of occurrences per file.

1for file in $(git show --name-only | grep ^src)
2 do
3 grep -c MongoId "$file"
4 done

A simple for loop works here, because I know the only whitespace is the linebreaks between the file names. Once we have the count, we can check to see that there's only 1 match:

1for file in $(git show --name-only | grep ^src)
2 do
3 if [ $(grep -c MongoId "$file") = 1 ]; then; echo "..."; fi
4 done

We're using an if statement here, to check that the occurrence count is 1. If it is, we want to do something. But what? Remember, we might be importing multiple things on that line, so that leaves us with three possible actions:

  1. Remove the whole line when MongoId is the only item imported.
  2. Remove MongoId, when it's the first item imported on that line. Don't miss that following comma!
  3. Remove , MongoId when it's not the first item on the that line. Don't miss the preceding comma!

There are many ways we could do this, so let's have some fun with reading input from the command line! To be clear, this isn't the best way to do it. We could easily match our three cases above with perl or sed. But we've already used that pattern in this project, and reading input in a shell script is an incredibly useful tool to have in your toolbox.

At this point, we probably want to move this into an actual shell script, instead of running it like a one-off on the command line:

3for file in $(git show --name-only | grep ^src)
4 do
5 if [ $(grep -c MongoId "$file") = 1 ]
6 then
7 echo ""
8 echo "====================="
9 echo "1 - remove whole line"
10 echo "2 - remove first import"
11 echo "3 - remove other import"
12 echo ""
13 echo "file: $file"
14 echo "line: $(grep MongoId "$file" | grep -v "^//")"
15 echo -n "> "
17 read choice
19 echo "your choice: $choice"
21 case "$choice" in
22 1)
23 sed -i '' "/MongoId/d" "$file";
24 ;;
25 2)
26 perl -i -pe "s/MongoId, ?//" "$file";
27 ;;
28 3)
29 perl -i -pe "s/, ?MongoId//" "$file";
30 ;;
31 *)
32 echo "nothing, skipping line"
33 ;;
34 esac
35 fi

Don't be intimidated by this, it's mostly echo statements. But we're doing some pretty cool stuff here.

Inside our if statement, we start by echoing some instructions, as well as the file name and the line that we're about to operate on. Then, we read an input from the command line. At this point, the script will pause and wait for us to type some input. Once we hit <enter> the script will resume and assign the value we entered to our choice variable.

Once we have determined our choice, we can do the correct replacement using the bash equivalent of a switch/case statement. For case 1, we're using sed's delete line command d. For cases 2 and 3, we'll use perl instead of sed, because it will operate only on the matched text, and not on the whole line. Finally, the default case will do nothing.

Running this script, we can now walk through the files, one by one, and review each change. It reduces our work to one keystroke per file, which is way less than opening each file, finding the line, removing the right stuff.

And that's it! While we don't use command-line editing commands every day, keeping these skills sharp will speed up your workflow when the right task comes along.

Andrew Burgess

Andrew loves full stack engineering, improving complex code bases, and building great teams.

    Next Post
    Previous Post