How to set and check file descriptor limits

Topic: Servers linux

Check and raise the open file limit (nofile) for the shell with ulimit, and for a systemd service or system-wide with limits.conf or the unit file. Use this when you see 'too many open files' or when running high-connection services.

Intent: How-to

Quick answer

  • ulimit -n shows current process limit for open files; ulimit -Sn (soft) and ulimit -Hn (hard). In a shell: ulimit -n 65535 to raise for that session (up to hard limit).
  • Permanent for a user: add to /etc/security/limits.conf (e.g. deploy soft nofile 65535, deploy hard nofile 65535). For systemd service: add LimitNOFILE=65535 in the unit [Service] section.
  • System-wide default: /etc/security/limits.conf or a file in /etc/security/limits.d/. Verify with ulimit -n in the shell or cat /proc/PID/limits for a running process.

Prerequisites

Steps

  1. Check current limits

    ulimit -n (current process); ulimit -Sn and ulimit -Hn for soft and hard. For another process: cat /proc/PID/limits | grep open.

  2. Raise for shell session

    ulimit -n 65535 (cannot exceed hard limit). To raise hard limit, use ulimit -Hn 65535 first (if allowed by system limits), then ulimit -n 65535.

  3. Make persistent

    User: add deploy soft nofile 65535 and deploy hard nofile 65535 to /etc/security/limits.conf or a file in /etc/security/limits.d/. systemd unit: LimitNOFILE=65535 in [Service]; daemon-reload and restart.

Summary

Check limits with ulimit -n and /proc/PID/limits; raise for a session with ulimit; make persistent with limits.conf for users or LimitNOFILE for systemd units. Use this when you hit “too many open files” or when tuning high-connection services.

Prerequisites

Steps

Step 1: Check current limits

ulimit -n
ulimit -Sn
ulimit -Hn

For a process: cat /proc/PID/limits | grep open.

Step 2: Raise for shell session

ulimit -n 65535

Step 3: Make persistent

Add to /etc/security/limits.d/ for users, or LimitNOFILE= in the systemd unit; reload and restart.

Verification

  • ulimit -n (or /proc/PID/limits) shows the new limit; the app no longer reports “too many open files”.

Troubleshooting

Cannot raise above current hard — Edit limits.conf or systemd to set a higher hard limit; re-login or restart the service. systemd service ignores limits.conf — Set LimitNOFILE in the unit; limits.conf applies to PAM logins, not necessarily to services started by systemd.

Next steps

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