Using fs_usage to see what files a process is using#

Today I wanted to figure out where the vercel CLI tool on my Mac kept its authentication tokens.

I solved the problem using the macOS fs_usage command, which traces filesystem activity for everything or for a specific process.

I ran vercel login to start the tool running. Thankfully Vercel’s tool pauses at that point and asks you to select a login provider - which means you can find the PID of the new process in another window:

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$ vercel login
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Vercel CLI 28.16.11
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> Log in to Vercel (Use arrow keys)
4
❯ Continue with GitHub
5
  Continue with GitLab
6
  Continue with Bitbucket
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  Continue with Email
8
  Continue with SAML Single Sign-On
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  ─────────────────────────────────
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  Cancel

Then in another window:

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$ ps aux | grep vercel
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simon            87632   0.0  0.0 408644400   1552 s023  S+    7:58AM   0:00.00 grep vercel
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simon            87587   0.0  0.1 409432576 100576 s021  S+    7:58AM   0:00.41 node /opt/homebrew/bin/vercel login

Then I started fs_usage like this:

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sudo fs_usage -f pathname 87587

The -f pathname option filters to just show “Pathname-related events”. The PID comes at the end.

With that command running, I completed the login process in the other window. A whole bunch of output was dumped to the fs_usage window, including the following:

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08:02:07.011111  lstat64                   /Users/simon/Library/Application Support/com.vercel.cli                                0.000006   node.4017721
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08:02:07.011124  lstat64                   /Users/simon/Library/Application Support/com.vercel.cli/auth.json                      0.000011   node.4017721
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08:02:07.011587  stat64                    /Users/simon/Library/Application Support/com.vercel.cli/auth.json                      0.000009   node.4017721
4
08:02:07.011716  open     F=27   (_WC_T______X)  /Users/simon/Library/Application Support/com.vercel.cli/auth.json.1284651536     0.000074   node.4017721
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08:02:07.019564  close    F=27                                                                                                    0.000013   node.4017721

So the answer to my question is that Vercel store their authentication tokens in ~/Library/Application Support/com.vercel.cli/auth.json.

Sure enough:

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$ cat ~/Library/Application\ Support/com.vercel.cli/auth.json
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{
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  "// Note": "This is your Vercel credentials file. DO NOT SHARE!",
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  "// Docs": "https://vercel.com/docs/project-configuration#global-configuration/auth-json",
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  "token": "... redacted ..."
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}

More fs_usage#

Running sudo fs_usage without any other parameters displays a constant stream of everything happening on the machine - pretty overwhelming!

This includes network operations and disk I/O as well.

The -f filter option accepts the following values:

network - network-related events, such as accept, recfrom and sendto. These do not appear very readable, just presenting information like recvfrom F=13 B=0x2fc.
filesys filesystem events are also very low-level: fcntl, stat64, pread etc - but some of them do at least include the full path to the file being accessed.
pathname events are things like stat64, open, close, readlink - these all include the path and appear to be less verbose than filesys - I think pathname is the most useful filter.
exec events show when processes are started and stopped. I haven’t played with these enough to see them working yet.
diskio events show disk I/O operations and are even more low-level than filesys.
cachehit appears to show ALL of the above information, with the addition of CACHE_HIT rows.

So for most of my purposes it looks like sudo fs_usage -f pathname PID is the most useful command.

Newsletter

Using fs_usage to see what files a process is using#

More fs_usage#

See also these TILs#