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D.Zou
Asked: 2015-09-14 16:58:37 +0800 CST

how does ulimit work in linux?

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I am learning about ulimit right now and this is a bit confusing.

I start with this as my default ulimit.

test@ubuntu:~$ ulimit -n
1024

I can see my hard and soft limit:

test@ubuntu:~$ ulimit -Sn
1024
test@ubuntu:~$ ulimit -Hn
65536

If I do the below I set the ulimit to my hard limit.

test@ubuntu:~$ ulimit -n hard
test@ubuntu:~$ ulimit -n
65536

But if I try to change it to my soft limit it doesn't work. It is still hard.

test@ubuntu:~$ ulimit -n soft
test@ubuntu:~$ ulimit -n
65536

I have two questions:

  1. Why can't I lower it back to the soft limit.

  2. What is the difference between these two below? One shows me unlimited the other shows 1024. (I started a new shell to be sure.)

    test@ubuntu:~$ ulimit
    unlimited
    test@ubuntu:~$ ulimit -n 1024

Running Ubuntu15 Desktop edition with the default bash environment.

Thanks

bash

1 Answers

  1. maybe useful? maybe because you are non-root?

    "man bash"

    gives this amongst other:

     ulimit [-HSTabcdefilmnpqrstuvx [limit]]
          Provides  control  over the resources available to the shell and to processes started by it, on systems that allow such control.  The -H and -S options
          specify that the hard or soft limit is set for the given resource.  A hard limit cannot be increased by a non-root user once it is set;  a  soft  limit
          may  be increased up to the value of the hard limit.  If neither -H nor -S is specified, both the soft and hard limits are set.  The value of limit can
          be a number in the unit specified for the resource or one of the special values hard, soft, or unlimited, which stand for the current hard  limit,  the
          current  soft  limit,  and  no limit, respectively.  If limit is omitted, the current value of the soft limit of the resource is printed, unless the -H
          option is given.  When more than one resource is specified, the limit name and unit are printed before the value.  Other  options  are  interpreted  as
          follows:
          -a     All current limits are reported
          -b     The maximum socket buffer size
          -c     The maximum size of core files created
          -d     The maximum size of a process's data segment
          -e     The maximum scheduling priority ("nice")
          -f     The maximum size of files written by the shell and its children
          -i     The maximum number of pending signals
          -l     The maximum size that may be locked into memory
          -m     The maximum resident set size (many systems do not honor this limit)
          -n     The maximum number of open file descriptors (most systems do not allow this value to be set)
          -p     The pipe size in 512-byte blocks (this may not be set)
        -q     The maximum number of bytes in POSIX message queues
          -r     The maximum real-time scheduling priority
          -s     The maximum stack size
          -t     The maximum amount of cpu time in seconds
          -u     The maximum number of processes available to a single user
          -v     The maximum amount of virtual memory available to the shell and, on some systems, to its children
          -x     The maximum number of file locks
          -T     The maximum number of threads

          If limit is given, and the -a option is not used, limit is the new value of the specified resource.  If no option is given, then -f is assumed.  Values are in 1024-byte increments, except for -t, which is in seconds; -p, which is in units of 512-byte blocks; and -T, -b, -n, and -u, which  are  unscaled values.  The return status is 0 unless an invalid option or argument is supplied, or an error occurs while setting a new limit.
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