Debug Linux Process

Debug Linux Process.md

Show a list of all running nginx processes, along with their PIDs and full command lines

ps -C nginx -o pid,cmd

• ps -C nginx: select processes with the command name nginx.
• -o pid,cmd: show only the PID and COMMAND columns.

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List the PIDs of all processes with the command name nginx, sorted by start time (oldest first).

ps -C nginx -o pid= --sort=+start_time

• -C nginx: selects all processes named exactly nginx.

• -o pid=: outputs only the PID (no headers).

• --sort=+start_time: sorts processes from oldest to newest.

Useful for: grabbing the master Nginx PID (it's the oldest one).

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Show all nginx processes that are running under the www-data user.

ps -u www-data -C nginx

• -u www-data: restrict to processes owned by the www-data user.
• -C nginx: match the command name nginx.

Useful for: confirming that worker processes (which run as www-data) are alive.

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Retrieve the oldest (first started) process ID of any process named nginx

pgrep -xo nginx

• pgrep: searches for processes by name.
• -x: matches the exact name (nginx, not nginx-something).
• -o: returns the oldest PID (usually the master process in Nginx).

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Search for and return the PID of the nginx master process

pgrep -f 'nginx: master process'

• pgrep -f: matches the full command line of each process.
• 'nginx: master process': string found in the command name of the master process. Useful for: getting the correct master PID to inspect limits or send signals (e.g., reload).

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Print the resource limits (like max open files) for the nginx master process

cat /proc/$(pgrep -xo nginx)/limits

• /proc/<PID>/limits: is a virtual file showing what the kernel limits are for that specific process.
• $(pgrep -xo nginx): injects the master PID dynamically.