系统环境：Ubunut 16.04 LTS
1, 查看当前限制参数：ulimit -a
guli@guli-Ubuntu1:~$ ulimit -a
core file size (blocks, -c) 0 (core dump 文件大小)
data seg size (kbytes, -d) unlimited
scheduling priority (-e) 0
file size (blocks, -f) unlimited
pending signals (-i) 7685
max locked memory (kbytes, -l) 64
max memory size (kbytes, -m) unlimited
open files (-n) 1024 (当前用户所能打开的最大文件描述符)
pipe size (512 bytes, -p) 8
POSIX message queues (bytes, -q) 819200
real-time priority (-r) 0
stack size (kbytes, -s) 8192
cpu time (seconds, -t) unlimited
max user processes (-u) 7685
virtual memory (kbytes, -v) unlimited
file locks (-x) unlimited
2，临时、立即生效的修改：ulimit -n 65535
3，永久生效的修改：修改 /etc/security/limits.conf 文件
guli@guli-Ubuntu1:~$ 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:
# - a user name
# - a group name, with @group syntax
# - the wildcard *, for default entry
# - the wildcard %, can be also used with %group syntax,
# for maxlogin limit
# - NOTE: group and wildcard limits are not applied to root.
# To apply a limit to the root user, <domain> must be
# the literal username root.
#
#<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 (KB)
# - maxlogins - max number of logins for this user
# - maxsyslogins - max number of logins on the system
# - priority - the priority to run user process with
# - locks - max number of file locks the user can hold
# - sigpending - max number of pending signals
# - msgqueue - max memory used by POSIX message queues (bytes)
# - nice - max nice priority allowed to raise to values: [-20, 19]
# - rtprio - max realtime priority
# - chroot - change root to directory (Debian-specific)
#
#<domain> <type> <item> <value>
#
#* soft core 0
#root hard core 100000
#* hard rss 10000
#@student hard nproc 20
#@faculty soft nproc 20
#@faculty hard nproc 50
#ftp hard nproc 0
#ftp - chroot /ftp
#@student - maxlogins 4
# End of file
配置示例：
* soft nofile 65535
* hard nofile 65535
注：
soft: 软限制
hard: 硬限制
注，* 不对 root 用户生效，如果要对 root 用户加限制，使用：
root soft nofile 65535
root hard nofile 65535
修改后保存，退出登录，重新登录，执行 ulimit -a 查看修改。
查看 ulimit 命令的使用帮助：man bash，然后搜索 ulimit：
ulimit [-HSTabcdefilmnpqrstuvx [limit]]
ulimit 用于资源限制，对于 shell 或 进程所有使用的资源进行限制。
限制方法：针对用户，或用户组进行设置。
-H: 硬限制
-S: 软限制
不加 -H,-S，默认都进行设置。
LIMIT 的值，除了可以是数字，也可以是
hard， soft， unlimited
hard: 当前 hard 限制值
soft: 当前 soft 限制值
unlimited: 无限制
不加 LIMIT，表示打印对应选项的资源（有多个选项时，会显示资源名和单位）：
guli@guli-Ubuntu1:~$ ulimit -f
unlimited
guli@guli-Ubuntu1:~$ ulimit -c
0
guli@guli-Ubuntu1:~$ ulimit -c -f
core file size (blocks, -c) 0
file size (blocks, -f) unlimited
-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 sup‐
plied, or an error occurs while setting a new limit.
使用方法:
ulimit -a 显示当前所有参数
ulimit -n LIMIT 设置最大文件描述符个数
ulimit LIMIT = ulimit -f LIMIT 默认使用 -f 选项，设置可创建文件的最大值
注，
一般选项单位： 1024-byte 为一个单位
-s 秒
-p 512-byte 为一个单位
-T -b -n -u 无单位
