> ## Documentation Index
> Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Configure user resource limits

> This guide explains how to configure user resource limits in Linux to prevent resource monopolization.

Managing resource usage per user prevents any single account from monopolizing CPU, memory, or processes. In this guide, you’ll learn how to configure limits via `/etc/security/limits.conf`, verify them, and understand each directive.

## Prerequisites

* A Linux distribution with PAM-enabled login (most modern distros).
* `sudo` privileges to edit `/etc/security/limits.conf`.

## 1. Back Up and Open limits.conf

<Callout icon="triangle-alert" color="#FF6B6B">
  Always back up system configuration files before editing.

  ```bash theme={null}
  sudo cp /etc/security/limits.conf{,.bak}
  ```
</Callout>

Open the file for editing:

```bash theme={null}
sudo vim /etc/security/limits.conf
```

You’ll see a template like:

```ini theme={null}
#<domain>    <type>  <item>      <value>
#*           soft    core       0
#*           hard    rss        10000
#@student    hard    nproc      20
#@faculty    soft    nproc      20
#@faculty    hard    nproc      50
#ftp         hard    nproc      0
#@student    -       maxlogins  4
```

***

## 2. Understanding limits.conf Fields

limits.conf uses four fields per line:

| Field  | Description                                                                  |
| ------ | ---------------------------------------------------------------------------- |
| domain | User (`trinity`), group (`@developers`), or `*` for all users.               |
| type   | `soft` (initial limit), `hard` (maximum ceiling), or `-` (both soft & hard). |
| item   | Resource type (e.g., `nproc`, `fsize`, `cpu`).                               |
| value  | Numeric limit (units vary per item).                                         |

### 2.1 Domain

* `username` (e.g., `trinity`)
* Group with `@` prefix (e.g., `@developers`)
* `*` for every user not otherwise matched

Example: limit user `trinity` to 10 processes:

```ini theme={null}
trinity    hard    nproc    10
```

### 2.2 Type

* `hard` — absolute maximum (cannot be exceeded).
* `soft` — initial/session limit (can increase up to hard).
* `-` — sets both `soft` and `hard`.

```ini theme={null}
# Hard only
trinity    hard    nproc    30

# Soft only
trinity    soft    nproc    10

# Both soft & hard
trinity    -       nproc    20
```

### 2.3 Item

Resource items you can limit:

| Item      | Description                                                                     | Example |
| --------- | ------------------------------------------------------------------------------- | ------- |
| nproc     | Max processes per user session                                                  | 10      |
| fsize     | Max file size (KB)                                                              | 1024    |
| cpu       | CPU time (minutes)                                                              | 1       |
| core      | Core dump file size (KB)                                                        | 0       |
| rss       | Resident set size (KB)                                                          | 10000   |
| maxlogins | Max concurrent logins per user                                                  | 4       |
| …         | See [man limits.conf](https://man7.org/linux/man-pages/man5/limits.conf.5.html) | –       |

Example: file-size and CPU restrictions for `trinity`:

```ini theme={null}
trinity    hard    fsize    1024   # 1 MB max file size
trinity    hard    cpu      1      # 1 minute CPU time
```

***

## 3. Exercise: Enforce a 3-Process Limit for “trinity”

1. Add the following (uncommented) line to `/etc/security/limits.conf`:

   ```ini theme={null}
   trinity    -    nproc    3
   ```

2. Save and exit.

### 3.1 Verify the Limit

```bash theme={null}
# Switch to trinity
sudo -iu trinity

# Attempt to list processes
ps | less
# Try spawning a fourth
ls -a | grep bash | less
logout
```

### 3.2 Check with ulimit

```bash theme={null}
ulimit -a
# ...
# max user processes        (-u)    3
```

You can adjust soft limits on the fly:

```bash theme={null}
# Raise soft limit up to hard (if below hard)
ulimit -u 5000
```

<Callout icon="lightbulb" color="#1CB2FE">
  Changes take effect on new sessions. Log out and back in to apply updates.\
  Use `ulimit -a` to inspect all current limits.
</Callout>

***

## 4. References and Further Reading

* [PAM Limits Module (limits.conf)](https://www.linux-pam.org/Linux-PAM-html/sag-pam_limits.html)
* [Linux Manual: limits.conf(5)](https://man7.org/linux/man-pages/man5/limits.conf.5.html)
* [ulimit Builtin](https://www.gnu.org/software/bash/manual/html_node/The-Set-Builtin.html)

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