What is ulimit
ulimit is a builtin shell command that allows us to restrict system resources, for example the number of processes, number of open files, maximum logins.
Display the limits
The see the current resource limits for the logged-in user, run:
ulimit -a
Example Output
-t: cpu time (seconds) unlimited
-f: file size (blocks) unlimited
-d: data seg size (kbytes) unlimited
-s: stack size (kbytes) 8192
-c: core file size (blocks) 0
-m: resident set size (kbytes) unlimited
-u: processes 22678
-n: file descriptors 2048
-l: locked-in-memory size (kbytes) 735404
-v: address space (kbytes) unlimited
-x: file locks unlimited
-i: pending signals 22678
-q: bytes in POSIX msg queues 819200
-e: max nice 0
-r: max rt priority 0
-N 15: unlimited
Setting resource limits
If you want to limit the number of processes for a user
ulimit -u 20000
# lets check the changes
ulimit -u
Output
20000
Flags
| Option | Resource Controlled |
|---|---|
-a |
Displays all current limits. |
-f |
Maximum size of files created by the shell (in blocks). |
-u |
Maximum number of processes a user can create. |
-n |
Maximum number of open file descriptors. |
-v |
Maximum virtual memory size (in kilobytes). |
-m |
Maximum resident memory size (in kilobytes). |
-c |
Maximum size of core files created (in blocks). |
-s |
Maximum size of the stack (in kilobytes). |
-l |
Maximum size of locked memory. |
Save changes to make limits persistent
If you want to permanently save the changes, edit /etc/security/limits.conf
You need to add the following:
<domain>: the target user or group(@group for groups)<type>:- soft - within range of hard limits
- hard - limits set by superuser
-for both hard and soft
<item>: - what do you want to limit<value>- the value you want to set
Example
To limit the user robert to a maximum of 4 simultaneous logins and restrict the student group to 20 processes:
sudo vim /etc/security/limits.conf
#<domain> <type> <item> <value>
robert - maxlogins 4
@student hard nproc 20
Conclusion
The ulimit is an essential tool to manage system resources.
Resources:
- https://ss64.com/bash/ulimit.html
man limits.confin Linux