How to set ulimit in linux

Linux ulimit Command

Of course, because of its nature, working with “ulimit” requires admin access (when changing value). Moreover, it’ll only work on systems that allow control through the shell. Without further ado, let’s get started with “ulimit”!

Location

Fire up the terminal and run the following command.

This shows up an error, right? Why? It’s because “ulimit” isn’t a separate binary. It’s embedded into the shell itself.

Basic usage

Just run the command by itself.

As the output suggests, the current user has “unlimited” amount of resources to be accessed. Essentially, this means the current user can consume all the resources the current system supports.

To get the report in details, add the “-a” flag at the end. This will print all the resource limits for the current user.

Note: There are 2 types of resource limitation: “hard” and “soft”. Hard resource limit defines the physical limit that the user can reach. The “soft” resource limit is manageable by the user. Its value can go up to the “hard” limit.

The system resources are defined in a configuration file located at “/etc/security/limits.conf”. “ulimit”, when called, will report these values.

Checking different limits of a user

How about displaying the individual parameters? Then call “ulimit” with the respective parameters.

Note: Not all of these commands will work as not every system incorporates the features.

To check the value of max “core file size”, use the flag “-c”. The result will be shown in blocks.

For max “data seg size” (in kilobytes), use the flag “-d”.

In scheduling, priority matters. Check out the max scheduling priority of the current user with the help of the “-e” flag.

To check the maximum stack size of the current user, use the flag “-s”.

For the maximum number of user processes, use the flag “-u”.

For the maximum number of threads, use the flag “-T”.

For getting the size of virtual memory, use the following one.

Let’s check out the socket buffer size!

This one is an interesting one. The following command will report the time each process is allowed to run for.

File descriptors are another interesting part of the Linux ecosystem. To check out how many file descriptors a process can have, run the following one.

For the complete list of “ulimit” flags, check out the help page of “ulimit”.

Setting values

Up until now, we’ve seen how to check the values of the resource limits of the current system. Now, it’s time to see how to manually modify these values.

Note: For changing the “hard” limit, it’ll require admin access, i.e. “root” permission. Make sure not to mess things up!

We mentioned the “limits.conf” file, right? It’s the core that DEFINES all the limits that’ll be applicable to the users.

As shown in the file, each entry has to follow the following structure.

The “domain” part can be any of these values.

• A specific user
• A group
• Wildcard (* and %)

The “type” part allows the following values.

• “soft” (for implementing soft limits)
• “hard” (for implementing hard limits)

Next up, the “item” part. The list of available options is quite long! Here are some of the interesting ones.

• core: Core file size (in KB).
• data: Max data size (in KB)
• fsize: Max file size (in KB).
• memlock: Max locked-in-memory address space (in KB).
• nofile: Max number of file descriptors
• stack: Max size of the stack (in KB).
• cpu: Maximum CPU time (in MIN).
• maxlogins: Maximum number of logins for the current user/group
• priority: Set the priority of the user’s processes
• rtprio: Max priority in real-time.

As of the “value” field, put an integer value. Note that the unit of each field is associated with the value of “item” part.

The changes will take into effect after exiting and re-logging into the associated user account.

For the in-depth info on the “limits.conf” configuration file, check out its man page.

Final thoughts

While the usage of the “ulimit” command isn’t so complex, it plays an important role in determining the available system resources and ultimately, determining the performance. To know more about this command, check out its man and info pages.

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Control System Resource Usage With Ulimit Command

In this beginner tutorial, you’ll learn about the ulimit command in Linux. You’ll how to use it to control system resource allocations.

What is ulimit in Linux?

Ulimit is a built-in shell command designed to display, allocate, and limit resources. It is essential for any system to regulate these types of controls.

This type of control can be enforced at the global, group, and user levels. In addition to ensuring smooth processing of tasks, it prevents unwanted processes from being able to devour system resources like RAM, and CPU power.

Ulimit is linked to a security configuration file. Your exact location may vary but it is typically something like /etc/security/limits.conf . Ulimit allows us to edit that configuration quickly.

Soft vs Hard Limits

As a user, you can actually adjust your ulimit settings.

You may be wondering why even set a limit if a user can adjust it. This is where soft and hard limits come into play.

So from the admin perspective, you may prefer your user to hover around a certain value. This would be your soft limit (let’s say 25).

Then, you could establish a hard limit that could not be exceeded by that user (50). The user would be authorized to increase their limit from 25 up to 50.

Using ulimit in Linux

Here’s the syntax for ulimit command:

Display all limits for any user

You can display all kind of limits for a specified user in this manner:

The -a flag will display all options and their configuration for your specific user name.

If you omit the user_name, it shows limits for you. Let me show you the limits set up for me by default:

Your default values may be different than mine, of course. This view displays a description, the assigned flag (that can be used for changing the limits), and the configuration.

Display Hard and Soft Limits

It is also possible to see either of these respective limits with a flag.

To display soft limits, use option -S:

To display hard limits, use option -H:

It is more useful to combine these with specific flags from above. So if you want to check the hard limit on the maximum number of user processes, you would type:

Change the limits (temporarily)

Now, let’s change that value to 31500 for demonstration purposes and check the hard limit again.

It is worth noting that any changes your priviliege allows you will only be temporarily written and affect your current shell.

To confirm this, I exited my shell and created a new terminal and got the original default value.

The next section shows how to make the changes permanent.

Making permanent changes to ulimit

As I mentioned in the beginning of the article, ulimit is governed by /etc/security/limits.conf. If you want to make any permanent changes to the ulimits of any user, you will have to make changes to the security file as root.

When editing you need to include these four elements:

Here is the text I appended to the file to set the hard limit on the number of processes for user christoper (i.e. my own account):

Keep in mind that it is a best practice not to enable the root account unless you are fully aware of the potential consequences. I’ve done this on a virtual machine so you don’t have to do it yourself.

As you can see, the limit for ‘christopher’ was changed to 20000.

How do you know what keyword to use for editing the /etc/security/limits.conf file? Well, here is a table that includes possible item keywords and their descriptions:

Item Keyword	Description
core	limits the core file size (KB)
data	max data size (KB)
fsize	maximum filesize (KB)
memlock	max locked-in-memory address space (KB)
nofile	max number of open file descriptors
rss	max resident set size (KB)
stack	max stack size (KB)
cpu	max CPU time (MIN)
nproc	max number of processes
as	address space limit (KB)
maxlogins	max number of logins for this user
maxsyslogins	max number of logins on the system
priority	the priority to run user process with
locks	max number of file locks the user can hold
sigpending	max number of pending signals
msgqueue	– max memory used by POSIX message queues (bytes)
nice	max nice priority allowed to raise to values: [-20, 19]
rtprio	max realtime priority
chroot	change root to directory (Debian-specific)

Limit Type	Description
hard	hard limit
soft	soft limit
–	both hard and soft limit

Change ulimit for groups

Changing a group policy is very similar to what you saw in the previous section but you will include an @ symbol before the name of the group.

Here’s an example where I set the maximum number of logins for the group student is 4 by editing the /etc/security/limits.conf file:

Conclusion

I hope you liked this quick primer on ulimit Linux command.

If you have any comments or questions, leave them below. If you have any suggestions for topics you’d like to see covered, feel free to leave those as well.

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How to Increase Number of Open Files Limit in Linux

In Linux, you can change the maximum amount of open files. You may modify this number by using the ulimit command. It grants you the ability to control the resources available for the shell or process started by it.

In this short tutorial we will show you how to check your current limit of open files and files descriptions, but to do so, you will need to have root access to your system.

First, Lets see how we can find out the maximum number of opened file descriptors on your Linux system.

Find Linux Open File Limit

The value is stored in:

The number you will see, shows the number of files that a user can have opened per login session. The result might be different depending on your system.

For example on a CentOS server of mine, the limit was set to 818354, while on Ubuntu server that I run at home the default limit was set to 176772.

If you want to see the hard and soft limits, you can use the following commands:

Check Hard Limit in Linux

Check Soft Limits in Linux

To see the hard and soft values for different users, you can simply switch user with “su” to the user which limits you want to check.

How to Check System wide File Descriptors Limits in Linux

If you are running a server, some of your applications may require higher limits for opened file descriptors. A good example for such are MySQL/MariaDB services or Apache web server.

You can increase the limit of opened files in Linux by editing the kernel directive fs.file-max . For that purpose, you can use the sysctl utility.

For example, to increase open file limit to 500000, you can use the following command as root:

You can check the current value for opened files with the following command:

With the above command the changes you have made will only remain active until the next reboot. If you wish to apply them permanently, you will have to edit the following file:

Add the following line:

Of course, you can change the number per your needs. To verify the changes again use:

Users will need to logout and login again for the changes to take effect. If you want to apply the limit immediately, you can use the following command:

Set User Level Open File limits in Linux

The above examples, showed how to set global limits, but you may want to apply limits per user basis. For that purpose, as user root, you will need to edit the following file:

If you are a Linux administrator, I suggest you that you become very familiar with that file and what you can do to it. Read all of the comments in it as it provides great flexibility in terms of managing system resources by limiting users/groups on different levels.

The lines that you should add take the following parameters:

Here is an example of setting a soft and hard limits for user marin:

Final thoughts

This brief article showed you a basic example of how you can check and configure global and user level limits for maximum number of opened files.

While we just scratched the surface, I highly encourage you to have a more detailed look and read regarding /etc/sysctl.conf and /etc/security/limits.conf and learn how to use them. They will be of great help for you one day.

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