How to use the command ulimit (with examples)

The ulimit command is used to get and set user limits. It allows the user to manage various resource limits, such as the maximum number of open files or the maximum number of processes. By using the ulimit command, users can view the current limits for different resources and also modify them if needed.

Use case 1: Get the properties of all the user limits

Code:

ulimit -a

Motivation: This use case is helpful when a user wants to check the current limits set for all resources. It provides a comprehensive view of the limits imposed on the user.

Explanation: The -a option is used with ulimit to display all the current limits for the user. It retrieves and prints the soft and hard limits set for various resources.

Example output:

core file size          (blocks, -c) unlimited
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 7571
max locked memory       (kbytes, -l) 65536
...

Use case 2: Get hard limit for the number of simultaneously opened files

Code:

ulimit -H -n

Motivation: In certain situations, it is necessary to know the maximum number of files a user can open simultaneously. This use case provides the hard limit value for the number of simultaneously opened files.

Explanation: By using the -H option, ulimit retrieves the upper limit (hard limit) for a specified resource. The -n option specifically refers to the maximum number of simultaneously opened files.

Example output:

4096

Use case 3: Get soft limit for the number of simultaneously opened files

Code:

ulimit -S -n

Motivation: Sometimes it is beneficial to know the current soft limit for the number of files a user can open simultaneously. This allows users to determine if they are nearing the limit and need to adjust their workflow or request a limit increase.

Explanation: By using the -S option, ulimit retrieves the soft limit for a specified resource. The -n option specifically refers to the maximum number of simultaneously opened files.

Example output:

1024

Use case 4: Set max per-user process limit

Code:

ulimit -u 30

Motivation: Users might want to limit the maximum number of processes they can run simultaneously. This helps in resource management and preventing system overload. This use case allows setting the maximum process limit to a specific value.

Explanation: The -u option is used to set the maximum per-user process limit. In this example, we are setting the limit to 30 processes.

Example output: (No output is shown when setting the limit)

Conclusion:

The ulimit command is a powerful tool for managing resource limits for individual users. By knowing the current limits and being able to modify them, users can ensure efficient resource allocation and prevent system overload. Whether it is checking the limits for all resources, obtaining the hard or soft limit for a specific resource, or setting per-user process limits, the ulimit command provides the necessary functionality.