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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.

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Prerequisites

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

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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

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2. Understanding limits.conf Fields

limits.conf uses four fields per line:
FieldDescription
domainUser (trinity), group (@developers), or * for all users.
typesoft (initial limit), hard (maximum ceiling), or - (both soft & hard).
itemResource type (e.g., nproc, fsize, cpu).
valueNumeric limit (units vary per item).

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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

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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

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2.3 Item

Resource items you can limit:
ItemDescriptionExample
nprocMax processes per user session10
fsizeMax file size (KB)1024
cpuCPU time (minutes)1
coreCore dump file size (KB)0
rssResident set size (KB)10000
maxloginsMax concurrent logins per user4
…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

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

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

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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

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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.

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4. References and Further Reading

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