lnux常用的命令格式一般为commend [option] [file],具体格式可以通过man或者help来查看获得帮助，
例如：echo 等待标准输入，后面可以接字符串，也支持正则表达式

[root@centos ~]#echo 1
1
[root@centos ~]#echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin

例如：ifconfig默认什么都不接是显示网卡信息，后面也可以接其他不同的参数

[root@centos ~]#ifconfig -s
Iface MTU RX-OK RX-ERR RX-DRP RX-OVR TX-OK TX-ERR TX-DRP TX-OVR Flg
ens33 1500 40765 0 0 0 17575 0 0 0 BMRU
lo 65536 132 0 0 0 132 0 0 0 LRU
virbr0 1500 0 0 0 0 0 0 0 0 BMU
[root@centos ~]#ifconfig -v
ens33: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.38.105 netmask 255.255.255.0 broadcast 192.168.38.255
inet6 fe80::6df2:7e92:462a:8df3 prefixlen 64 scopeid 0x20
ether 00:0c:29:9b:94:7b txqueuelen 1000 (Ethernet)
RX packets 40773 bytes 40093948 (38.2 MiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 17580 bytes 2214634 (2.1 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10
loop txqueuelen 1000 (Local Loopback)
RX packets 132 bytes 14100 (13.7 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 132 bytes 14100 (13.7 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

virbr0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
inet 192.168.122.1 netmask 255.255.255.0 broadcast 192.168.122.255
ether 52:54:00:d5:ea:62 txqueuelen 1000 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

[root@centos ~]#ifconfig -a
ens33: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.38.105 netmask 255.255.255.0 broadcast 192.168.38.255
inet6 fe80::6df2:7e92:462a:8df3 prefixlen 64 scopeid 0x20
ether 00:0c:29:9b:94:7b txqueuelen 1000 (Ethernet)
RX packets 40780 bytes 40094496 (38.2 MiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 17586 bytes 2216520 (2.1 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10
loop txqueuelen 1000 (Local Loopback)
RX packets 132 bytes 14100 (13.7 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 132 bytes 14100 (13.7 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

virbr0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
inet 192.168.122.1 netmask 255.255.255.0 broadcast 192.168.122.255
ether 52:54:00:d5:ea:62 txqueuelen 1000 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

virbr0-nic: flags=4098<BROADCAST,MULTICAST> mtu 1500
ether 52:54:00:d5:ea:62 txqueuelen 1000 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

例如：date默认有固定的显示模式，后面接参数可以显示参数的信息

[root@centos ~]#date
Wed Feb 5 21:12:09 CST 2020
[root@centos ~]#date +%F
2020-02-05
[root@centos ~]#date +%T
21:12:22

例如：export可以显示当前bash的环境变量

[root@centos ~]#export
declare -x DISPLAY="localhost:10.0"
declare -x HISTCONTROL="ignoredups"
declare -x HISTSIZE="1000"
declare -x HOME="/root"
declare -x HOSTNAME="centos"
declare -x LANG="en_US.UTF-8"
declare -x LESSOPEN="||/usr/bin/lesspipe.sh %s"
declare -x LOGNAME="root"
declare -x LS_COLORS="rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:mi=01;05;37;41:su=37;41:sg=30;43:ca=30;41:tw=30;42:ow=34;42:st=37;44:ex=01;32:.tar=01;31:.tgz=01;31:.arc=01;31:.arj=01;31:.taz=01;31:.lha=01;31:.lz4=01;31:.lzh=01;31:.lzma=01;31:.tlz=01;31:.txz=01;31:.tzo=01;31:.t7z=01;31:.zip=01;31:.z=01;31:.Z=01;31:.dz=01;31:.gz=01;31:.lrz=01;31:.lz=01;31:.lzo=01;31:.xz=01;31:.bz2=01;31:.bz=01;31:.tbz=01;31:.tbz2=01;31:.tz=01;31:.deb=01;31:.rpm=01;31:.jar=01;31:.war=01;31:.ear=01;31:.sar=01;31:.rar=01;31:.alz=01;31:.ace=01;31:.zoo=01;31:.cpio=01;31:.7z=01;31:.rz=01;31:.cab=01;31:.jpg=01;35:.jpeg=01;35:.gif=01;35:.bmp=01;35:.pbm=01;35:.pgm=01;35:.ppm=01;35:.tga=01;35:.xbm=01;35:.xpm=01;35:.tif=01;35:.tiff=01;35:.png=01;35:.svg=01;35:.svgz=01;35:.mng=01;35:.pcx=01;35:.mov=01;35:.mpg=01;35:.mpeg=01;35:.m2v=01;35:.mkv=01;35:.webm=01;35:.ogm=01;35:.mp4=01;35:.m4v=01;35:.mp4v=01;35:.vob=01;35:.qt=01;35:.nuv=01;35:.wmv=01;35:.asf=01;35:.rm=01;35:.rmvb=01;35:.flc=01;35:.avi=01;35:.fli=01;35:.flv=01;35:.gl=01;35:.dl=01;35:.xcf=01;35:.xwd=01;35:.yuv=01;35:.cgm=01;35:.emf=01;35:.axv=01;35:.anx=01;35:.ogv=01;35:.ogx=01;35:.aac=01;36:.au=01;36:.flac=01;36:.mid=01;36:.midi=01;36:.mka=01;36:.mp3=01;36:.mpc=01;36:.ogg=01;36:.ra=01;36:.wav=01;36:.axa=01;36:.oga=01;36:.spx=01;36:*.xspf=01;36:"
declare -x MAIL="/var/spool/mail/root"
declare -x OLDPWD
declare -x PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin"
declare -x PWD="/root"
declare -x SELINUX_LEVEL_REQUESTED=""
declare -x SELINUX_ROLE_REQUESTED=""
declare -x SELINUX_USE_CURRENT_RANGE=""
declare -x SHELL="/bin/bash"
declare -x SHLVL="1"
declare -x SSH_CLIENT="192.168.38.1 49893 22"
declare -x SSH_CONNECTION="192.168.38.1 49893 192.168.38.105 22"
declare -x SSH_TTY="/dev/pts/0"
declare -x TERM="xterm"
declare -x USER="root"
declare -x XDG_DATA_DIRS="/root/.local/share/flatpak/exports/share:/var/lib/flatpak/exports/share:/usr/local/share:/usr/share"
declare -x XDG_RUNTIME_DIR="/run/user/0"
declare -x XDG_SESSION_ID="60"