2. Miscellaneous
2.1. Check version
$ cat /etc/centos-release
CentOS Linux release 8.5.2111
2.2. Disable SELinux
See current status:
$ getenforce
Temporarily disable it:
$ sudo setenforce 0
To permanently disable it, you can edit file /etc/selinux/config:
# enforcing - SELinux security policy is enforced.
# permissive - SELinux prints warnings instead of enforcing.
# disabled - No SELinux policy is loaded.
-SELINUX=enforcing
+SELINUX=permissive
# SELINUXTYPE= can take one of these three values:
# targeted - Targeted processes are protected,
# minimum - Modification of targeted policy. Only selected processes are protected.
2.3. Set swap off
Show devices with swap enabled:
$ cat /proc/swaps
Filename Type Size Used Priority
/dev/dm-1 partition 8269820 0 -2
Turn off swap for all devices:
$ sudo swapoff -av
swapoff /dev/dm-1
To permanently disable swap, edit /etc/fstab and comment out the line that mounts the swap partition.
2.4. ulimit
Show current limits:
$ ulimit -a
core file size (blocks, -c) 0
data seg size (kbytes, -d) unlimited
scheduling priority (-e) 0
file size (blocks, -f) unlimited
pending signals (-i) 63176
max locked memory (kbytes, -l) 64
max memory size (kbytes, -m) unlimited
open files (-n) 1024
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) 63176
virtual memory (kbytes, -v) unlimited
file locks (-x) unlimited
Set the maximum number of open files:
$ ulimit -n 1048576
This is effective only in current shell and any child processes. To make it permanent, you need to edit file /etc/security/limits:
# /etc/security/limits.conf
#
#This file sets the resource limits for the users logged in via PAM.
#It does not affect resource limits of the system services.
#
#Also note that configuration files in /etc/security/limits.d directory,
#which are read in alphabetical order, override the settings in this
#file in case the domain is the same or more specific.
#That means for example that setting a limit for wildcard domain here
#can be overriden with a wildcard setting in a config file in the
#subdirectory, but a user specific setting here can be overriden only
#with a user specific setting in the subdirectory.
#
#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
#
#<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 file descriptors
# - 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
#
#<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
The modification will take effect after a re-login.
Note
If there are files in /etc/security/limits.d, the values may be overridden by those files.
The limits are applied by a per-process manner. You can check the limits for process 1 (i.e. the init process):
$ cat /proc/1/limits
Limit Soft Limit Hard Limit Units
Max cpu time unlimited unlimited seconds
Max file size unlimited unlimited bytes
Max data size unlimited unlimited bytes
Max stack size 8388608 unlimited bytes
Max core file size unlimited unlimited bytes
Max resident set unlimited unlimited bytes
Max processes 63176 63176 processes
Max open files 1048576 1048576 files
Max locked memory 67108864 67108864 bytes
Max address space unlimited unlimited bytes
Max file locks unlimited unlimited locks
Max pending signals 63176 63176 signals
Max msgqueue size 819200 819200 bytes
Max nice priority 0 0
Max realtime priority 0 0
Max realtime timeout unlimited unlimited us
2.5. alternatives
$ alternatives --version
alternatives（备用）版本 1.19.1
List all software managed by alternatives:
$ alternatives --list
Config a software:
$ sudo alternatives --config modules.sh