Let's say the following is being returned when attempting to do something on a Linux system.

ulimit: open files: cannot modify limit: Operation not permitted

This can occur when attempting to set a ulimit that exceeds the hard ulimit set in one of these files:

• /etc/security/limits.d/90-nproc.conf
• /etc/security/limits.conf

For example, let's say john.doe hard ulimit for nofile (open files) is 2048.

john.doe  hard  nofile  2048

If john.doe attempts to use the ulimit -n command with a value that exceeds 2048, this can cause "-bash: ulimit: open files: cannot modify limit: Operation not permitted". Typically, root or sudo is needed to make this change.

~]# ulimit -n 4096

Or, if the ulimit is set in /home/john.doe/.bash_profile, "-bash: ulimit: open files: cannot modify limit: Operation not permitted" may be displayed when john.doe makes an SSH connection.