To restart a Python process after it exits without paying for a supervisor, a small watchdog can check for it and launch it again. The key is identifying the right process: checking a known child PID is precise, while searching /proc or using pgrep can match the wrong process. For an unattended service, systemd is usually the better place to configure restarts.
Choose how to identify the process
There are two different problems: monitoring a child process that your Python program started, and finding a process that was started independently.
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| Method | What it tracks | Best suited to |
|---|---|---|
subprocess.Popen |
A child launched by the current Python process; its PID and exit state. | A Python watchdog that owns the process it launches. |
pgrep or /proc |
Processes selected by name or command line. | Checking for an independently launched process, provided the match is distinctive. |
systemd Restart= |
A process managed as a systemd service. | Unattended service lifecycle and restart handling. |
If your program launches the worker, keep the returned Popen object and use poll() to see whether it has exited. Python documents Popen.pid as the spawned child’s process ID; poll() reports whether it has completed, while wait() waits for completion. This avoids guessing identity from a process name. See the Python subprocess documentation.
A minimal watchdog pattern using pgrep
For an independently started process, a watchdog can look for a distinctive command-line string and start the program if no match is found. This illustrative shell loop checks every five seconds:
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#!/bin/sh
pattern='python3 /opt/myapp/worker.py'
while :; do
if ! pgrep -f -x "$pattern" >/dev/null; then
/usr/bin/python3 /opt/myapp/worker.py &
fi
sleep 5
doneReplace the executable and script path with the actual paths on your system. The quoted variable prevents shell word-splitting and glob expansion, and -x asks pgrep for an exact full-command-line match. Confirm that the command line used by the running process is exactly the pattern you specify; interpreter options, wrappers, or different paths can change it. This is a simple restart loop, not a guarantee that a worker is healthy or that the match is unique.
Why process matching can restart the wrong program
Default process-name matching is short
By default, pgrep matches a process name. Linux process names exposed through /proc/PID/stat are limited to 15 characters, so a longer name may be truncated and may not distinguish similar processes.
-f searches the full command line
With -f, pgrep searches the full command line exposed through /proc/PID/cmdline. That gives you more identifying detail, but a broad pattern can match unrelated processes. Use a distinctive exact command line when possible and check its behavior before relying on it. The pgrep manual documents the matching options and process-name limitation.
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Checking a PID is not the same as finding a process
Testing whether /proc/1234 exists answers whether that PID currently has a proc entry; it does not establish that the process is the intended worker. PIDs can be reused after a process exits. For a child you launched, use the Popen object instead. For an independently started program, match identifying details carefully rather than treating any live PID as the right one.
Use systemd for an unattended Linux service
If the goal is to keep a service running after an ordinary exit, configure it as a systemd service and choose a Restart= policy appropriate to the failure conditions you want systemd to handle. The policy determines which exits and failures trigger a restart; consult the systemd.service(5) manual for the available policies and unit behavior.
For example, a service unit can include a restart policy under its [Service] section:
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[Service]
ExecStart=/usr/bin/python3 /opt/myapp/worker.py
Restart=on-failureThis is a unit fragment, not a complete service file. It assumes the script and interpreter are at those paths; a complete unit also needs the appropriate service type and installation details for the host.
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Restart= and WatchdogSec= do different jobs
Restart= responds to process exit or failure conditions. By contrast, systemd’s WatchdogSec= expects the service to send regular liveness notifications using sd_notify(3). The systemd manual says, “The service must call sd_notify(3) regularly with "WATCHDOG=1" (i.e. the "keep-alive ping").” If those messages stop, systemd treats the service as failed. A process that is alive but stuck may need health reporting; a loop that only detects process absence will not detect that condition.
Handle shell commands safely
When Python invokes a shell explicitly, Python’s documentation places responsibility for quoting whitespace and shell metacharacters on the application. Prefer passing an argument sequence to subprocess rather than building a shell command string, and do not interpolate untrusted input into shell commands. This helps prevent malformed commands and shell injection; it does not solve the separate challenge of correctly identifying a process.
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