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A packaged Mac backend that launches lsof by its bare name can log “lsof: command not found” even when lsof is installed. In the case documented by Mihai Perdum on DEV Community, the binary was at /usr/sbin/lsof, but the backend’s runtime PATH did not include /usr/sbin. The spawn failed, the code that identifies the process holding the port never ran, and the port stayed occupied. The fix has two parts: resolve lsof from known absolute locations before searching PATH, and signal only TCP listeners rather than every process touching the port.
Why the failure appears only in the packaged app
When you run a backend from Terminal, it inherits your login shell’s PATH, which usually includes the system administration directories. A packaged app does not inherit that environment. It receives whatever environment its launcher builds, and that can be much narrower. The lookup failure is easy to miss because nothing in a development run reproduces it.
What the author measured
- On the author’s host,
lsoflived at/usr/sbin/lsof. - The PATH the packaged backend actually received, as the author logged it, did not contain
/usr/sbin. - A bare-name spawn of
lsoftherefore failed, and the reclaim code never reached the step that inspects or signals the port holder.
These are the author’s machine measurements. Other Macs, macOS versions, and launchers may build a different PATH, so the only reliable check is the one below.
Check the PATH your shipped build uses
- At backend startup, log the PATH variable the process received, along with whether each known
lsoflocation exists. - Build the app and launch the packaged bundle the way users do, not through a development server.
- Read the log and compare it with the output of
echo $PATHin Terminal. The difference is the bug. - If
/usr/sbinis missing from the logged PATH, a barelsofcall will fail in that build.
Why the reclaim branch never ran
The reclaim logic depends on lsof output to find the process holding the port. When the spawn fails, there is no output to act on, so the branch is skipped or abandoned. The application may log a warning and carry on, which makes the failure look like a minor message rather than a broken feature.
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The author frames this as a design problem rather than a one-off bug:
“A branch that always fires is not an edge case, it is the design — and when the always-firing branch is the one that gives up, you have shipped the giving up.” — Mihai Perdum
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In practice, treat “the tool could not run” as a normal outcome with its own handling. It should stop the reclaim with an error that names the tool and the locations checked, not pass silently.
Resolve lsof without relying on PATH alone
The author’s preferred fix is to find the executable explicitly. The steps below follow his order.
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- Check known absolute locations first, in order:
/usr/sbin/lsof,/usr/bin/lsof, then/opt/homebrew/bin/lsof. These are candidates in the author’s code. They are not guaranteed to exist on every Mac, and a missing one is simply skipped. - If none of them exists, walk the PATH entries in order and check each directory for an executable named
lsof. Ignore empty PATH components. - If nothing is found, fail with an error that lists every candidate path checked, in order. A message such as “lsof not found” alone does not show where resolution went wrong; a list of checked paths does.
- Use the resolved absolute path for every later call to
lsof.
Widening PATH as supplemental hardening
The author also appended /usr/sbin:/sbin to the backend’s POSIX PATH. This is a reasonable extra safeguard because other system tools may be missing from the same narrow environment. It is not the primary fix. The resolver should still work if the launch environment changes again, which is why the absolute-path lookup comes first.
Signal only the listener
Finding lsof is only half the problem. The query that feeds the signal step decides which processes get killed. The author’s port probe on port 8874 compared two commands:
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| Command | Result in the author’s probe |
|---|---|
lsof -ti TCP:8874 |
Returned a listener PID and a connected client PID |
lsof -ti TCP:8874 -sTCP:LISTEN |
Returned only the listener PID |
The -t flag prints bare PIDs, and -i TCP:8874 selects TCP sockets on that port. Without a state filter, lsof reports any socket touching the port. A client with an established connection to the port is included, even though it is not the process holding the port open for listening. The author puts it this way:
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He also describes the state filter as the key safeguard:
“
-sTCP:LISTENis the whole defence:” — Mihai Perdum
A reclaim routine should therefore:
- Query with
-sTCP:LISTENbefore sending any signal. - Send signals only to the PIDs that query returns.
- Apply application-specific checks before killing anything, such as confirming that the PID belongs to a previous instance of your own backend. A successful
lsofrun does not by itself make a kill safe.
Verify the fix
- Confirm the startup log shows the absolute path that was resolved for
lsof. - Leave a client connected to a stale listener on the port, start the packaged app, and confirm that only the listener is terminated and the client connection survives.
- Confirm that a failed lookup produces an error listing every candidate path checked.
The author reports a test run of 43 passed, 0 failed, and 9 ignored for his project. Those results come from his codebase and are not independently reproduced here.
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The sources for this article are a single developer’s write-up and its published excerpts. The PATH contents, probe output, and test counts describe the author’s environment. The article does not establish how other macOS versions, launchers, or package formats set PATH, and it does not cite Apple documentation or lsof maintainer guidance. The quoted sentences are the author’s assertions. Check lsof behavior and the runtime PATH inside your own packaged build before acting on process IDs.
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