我使用过的Linux命令之ulimit - 在shell进程中限制系统资源

本文链接：http://codingstandards.iteye.com/blog/967718 （转载请注明出处）

用途说明

ulimit是一个shell内建命令，用于控制由shell启动的进程的可用资源（Provides control over the resources available to the shell and to processes started by it, on systems that allow such control.）。对资源的限制分为两种，一种是硬性限制，一种是软性限制。硬性限制一旦设定就不能增加（A hard limit cannot be increased once it is set），而软性限制可以增加到硬性控制为止（a soft limit may be increased up to the value of the hard limit）。Linux系统可以对多种资源的使用进行限制，比如允许创建的文件数、允许打开的文件数、是否允许生成core等。要注意的是，该设置只对当前shell进程的子进程产生作用，并不会影响其他shell进程。

最初接触到这个命令，是在从事C/C++开发时经常会出现段错误、断言错误之类的，但是Linux操作系统中默认是不生成core文件的，所以无法确切的知道问题出现的位置。因此，通常的做法就是修改/etc/profile中的那个ulimit设置，如下所示：

vi +/ulimit /etc/profile 写道

# No core files by default
ulimit -S -c 0 > /dev/null 2>&1

通常把允许生成的core文件的大小改成100M之类的，比如：

ulimit -S -c 100000000 > /dev/null 2>&1

PS: 由于本人理解的错误，-c的数值单位其实是blocks，一般是1K。具体block size是多少，与具体的文件系统有关，大伙可以参考本文后面列出的参考资料。

常用参数

使用示例

示例一与ulimit有关的帮助信息

[root@node34 root]# type -a ulimit
ulimit is a shell builtin

[root@node34 root]# help ulimit
ulimit: ulimit [-SHacdflmnpstuv] [limit]
    Ulimit provides control over the resources available to processes
    started by the shell, on systems that allow such control. If an
    option is given, it is interpreted as follows:

        -S      use the `soft' resource limit
        -H      use the `hard' resource limit
        -a      all current limits are reported
        -c      the maximum size of core files created
        -d      the maximum size of a process's data segment
        -f      the maximum size of files created by the shell
        -l      the maximum size a process may lock into memory
        -m      the maximum resident set size
        -n      the maximum number of open file descriptors
        -p      the pipe buffer size
        -s      the maximum stack size
        -t      the maximum amount of cpu time in seconds
        -u      the maximum number of user processes
        -v      the size of virtual memory

    If LIMIT is given, it is the new value of the specified resource;
    the special LIMIT values `soft', `hard', and `unlimited' stand for
    the current soft limit, the current hard limit, and no limit, respectively.
    Otherwise, the current value of the specified resource is printed.
    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
    increments of 512 bytes, and -u, which is an unscaled number of
    processes.
[root@node34 root]# cat /etc/security/limits.conf
# /etc/security/limits.conf
#
#Each line describes a limit for a user in the form:
#
#<domain>        <type> <item> <value>
#
#Where:
#<domain> can be:
#        - an user name
#        - a group name, with @group syntax
#        - the wildcard *, for default entry
#
#<type> can have the two values:
#        - "soft" for enforcing the soft limits
#        - "hard" for enforcing hard limits
#
#<item> can be one of the following:
#        - 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 files
#        - 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
#        - maxlogins - max number of logins for this user
#        - priority - the priority to run user process with
#        - locks - max number of file locks the user can hold
#
#<domain>      <type> <item>         <value>
#

#*               soft    core            0
#*               hard    rss             10000
#@student        hard    nproc           20
#@faculty        soft    nproc           20
#@faculty        hard    nproc           50
#ftp             hard    nproc           0
#@student        -       maxlogins       4

# End of file
[root@node34 root]#

示例二打印当前的资源限制

[root@node34 root]# ulimit -H -a
core file size        (blocks, -c) unlimited
data seg size         (kbytes, -d) unlimited
file size             (blocks, -f) unlimited
max locked memory     (kbytes, -l) 4
max memory size       (kbytes, -m) unlimited
open files                    (-n) 1024
pipe size          (512 bytes, -p) 8
stack size            (kbytes, -s) unlimited
cpu time             (seconds, -t) unlimited
max user processes            (-u) 2048
virtual memory        (kbytes, -v) unlimited
[root@node34 root]# ulimit -S -a
core file size        (blocks, -c) 0
data seg size         (kbytes, -d) unlimited
file size             (blocks, -f) unlimited
max locked memory     (kbytes, -l) 4
max memory size       (kbytes, -m) unlimited
open files                    (-n) 1024
pipe size          (512 bytes, -p) 8
stack size            (kbytes, -s) 10240
cpu time             (seconds, -t) unlimited
max user processes            (-u) 2048
virtual memory        (kbytes, -v) unlimited
[root@node34 root]#

示例三修改允许打开的文件数

[root@node34 root]# ulimit -n
1024
[root@node34 root]# ulimit -n 4096
[root@node34 root]# ulimit -n
4096
[root@node34 root]#

要永久修改，请参见后面的资料。