Configure user resource limits

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

Always back up system configuration files before editing.

sudo cp /etc/security/limits.conf{,.bak}

Open the file for editing:

sudo vim /etc/security/limits.conf

You’ll see a template like:

#<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:

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.

# 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	–

Example: file-size and CPU restrictions for trinity:

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”

Add the following (uncommented) line to /etc/security/limits.conf:
trinity - nproc 3
Save and exit.

3.1 Verify the Limit

# 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

ulimit -a
# ...
# max user processes        (-u)    3

You can adjust soft limits on the fly:

# Raise soft limit up to hard (if below hard)
ulimit -u 5000

Changes take effect on new sessions. Log out and back in to apply updates.
Use ulimit -a to inspect all current limits.

Introduction

Operation of Running Systems

User and Group Management

Essential Commands

Configure user resource limits

Prerequisites

1. Back Up and Open limits.conf

2. Understanding limits.conf Fields

2.1 Domain

2.2 Type

2.3 Item

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

3.1 Verify the Limit

3.2 Check with ulimit

4. References and Further Reading

Watch Video

​Prerequisites

​1. Back Up and Open limits.conf

​2. Understanding limits.conf Fields

​2.1 Domain

​2.2 Type

​2.3 Item

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

​3.1 Verify the Limit

​3.2 Check with ulimit

​4. References and Further Reading

Watch Video

Prerequisites

1. Back Up and Open limits.conf

2. Understanding limits.conf Fields

2.1 Domain

2.2 Type

2.3 Item

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

3.1 Verify the Limit

3.2 Check with ulimit

4. References and Further Reading