How do ulimit settings impact Linux?

I have made my homework and (almost) found what each option does. Also, I've noted that there is more options in /etc/security/limits.conf than it appears with ulimit -a. Therefore, I've only documented the latter here. Of course, everyone is invited to enrich this answer!

  • core file size (blocks, -c)

    The maximum size of core files created. Core dump is a system snapshot (RAM + context switch + processor registers).

  • data seg size (kbytes, -d)

    The maximum size of a process's data segment. a data segment is a portion of an object file or the corresponding virtual address space of a program that contains initialized static variables.

  • scheduling priority (-e)

    The maximum scheduling priority ("nice") a process can be given.

  • file size (blocks, -f)

    The maximum size of files written by the shell and its children.

  • pending signals (-i)

    Set of signals that are pending for delivery to the calling thread.

  • max locked memory (kbytes, -l)

    The maximum size that may be locked into memory. Memory locking ensures the memory is always in RAM and never moved to the swap disk.

  • max memory size (kbytes, -m)

    How much memory a process currently has in main memory (RAM), opposed to how much virtual memory the process has in total.

  • open files (-n)

    The maximum number of open file descriptors. A file descriptor is an abstract indicator used to access a file or other input/output resource, such as a pipe or network socket.

    List file descriptors:

  • pipe size (512 bytes, -p)

    Pipe's internal buffer size. See "pipe capacity" section in

  • POSIX message queues (bytes, -q)

    The maximum number of bytes in POSIX message queues. POSIX message queues allow processes to exchange data in the form of messages.

    Message queues in general

  • real-time priority (-r)

    The maximum real-time scheduling priority. A realtime priority thread can never be pre-empted by timer interrupts and runs at a higher priority than any other thread in the system.

  • stack size (kbytes, -s)

    The maximum stack size. The stack size is a reserved region of memory that is used to store the location of function calls in order to allow return statements to return to the correct location.

  • cpu time (seconds, -t)

    The maximum amount of cpu time in seconds.

  • max user processes (-u)

    The maximum number of processes a user can start or fork.

    This command shows how much processes each user is currently using:

    ps h -Led -o user | sort | uniq -c | sort -n

  • virtual memory (kbytes, -v)

    The maximum amount of virtual memory available to the shell. Virtual memory maps memory addresses used by a program, called virtual addresses, into physical addresses in computer memory.

  • file locks (-x)

    File locking is a mechanism that restricts access to a computer file by allowing only one user or process access at any specific time.