The ulimit command can be used to limit the amount of memory a user may use. The ulimit command is a shell builtin that allows the user to view or modify the resource limits imposed by the operating system. The resource limits can affect the maximum size of files, the maximum number of processes, the maximum amount of CPU time, and the maximum amount of virtual memory a user can access.
To limit the amount of memory a user may use, the -v option can be used with the ulimit command. The -v option sets the maximum amount of virtual memory available to the current shell and its children in kilobytes. For example, the command ulimit -v 1000000 would limit the virtual memory to 1 GB. The -m option can also be used to set the maximum resident set size, which is the amount of physical memory used by a process, but this option is not supported by all systems.
The ulimit command can be used interactively in a shell session, or it can be placed in a shell initialization file, such as .bashrc or .profile, to apply the limits to all future shell sessions. The ulimit command can also be used in conjunction with the /etc/security/limits.conf file, which allows the system administrator to set global or per-user resource limits for all users and processes. The /etc/security/limits.conf file can specify hard and soft limits for each resource, as well as the scope of the limit, such as user, group, or domain.
The other commands listed are not related to limiting the amount of memory a user may use. The umask command sets the default file permissions for newly created files and directories. The usermod command modifies the user account information, such as the home directory, the login shell, or the password expiration date. The passwd command changes the user password. The chage command changes the password aging information, such as the minimum and maximum number of days between password changes, or the number of days before the password expires. Reference:
ulimit(1) - Linux manual page
How to limit user environment with ulimit Linux command
Control server access using hosts.allow and hosts.deny files