Environment Variables
Environment variable is to run in the "environment" context, in this context can be referenced. For example, a common cd, ls command, etc. Strictly speaking, you should use absolute paths such as / bin / ls to perform, due to the / bin directory to the PATH environment variable, the system will look for themselves whether there is a path in the PATH command.
Environment variables commonly used capital letters. Common environmental variables HOSTNAME, SHELL, HISTSIZE, USER, PATH, PWD, LANG, HOME, LOGNAME. Represent the current host name, SHELL path that is bash's type, history save how many records, the current user name, automatic search path, the current directory, language used (to change the variables to temporarily modify language), the current user's home directory, the current logged-on user.
Use env can view or export the current user's environment variables.
[root@localhost ~]# env HOSTNAME=localhost.localdomain TERM=vt100 SHELL=/bin/bash HISTSIZE=1000 SSH_CLIENT=192.168.1.5 50651 22 QTDIR=/usr/lib64/qt-3.3 QTINC=/usr/lib64/qt-3.3/include SSH_TTY=/dev/pts/0 USER=root 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 : = 30 ; 41 : tw = 30 ; 42 : ow = 34 ; 42 : st = 37 ; 44 : ex = 01 ; 32 : *. Tar = 01 ; 31 : *. Tgz = 01 ; 31 : *. Arj = 01 ; 31 : *. Taz = 01 ; 31 : *. Lzh = 01 ; 31 : *. Lzma = 01 ; 31 : *. Tlz = 01 ; 31 : *. Txz =01 ; 31 *. Zip = 01 ; 31 *. Z = 01 ; 31 *. Z = 01 ; 31 *. Dz = 01 ; 31 :. * G = 01 ; 31 *. L = 01 ; 31 *. X = 01 ; 31 *. Bz2 = 01 ; 31 *. Tbz = 01 ; 31 *. Tbz2 = 01 ; 31 *. Bz = 01 ; 31 *. T = 01 ; 31 *. Deb = 01 ;31:*.rpm=01;31:*.jar=01;31:*.rar=01;31:*.ace=01;31:*.zoo=01;31:*.cpio=01;31:*.7z=01;31:*.rz=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:*.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: MAIL=/var/spool/mail/root PATH=/usr/lib64/qt-3.3/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin PWD=/root LANG=zh_CN.UTF-8 HISTCONTROL=ignoredups SHLVL=1 HOME=/root LOGNAME=root QTLIB=/usr/lib64/qt-3.3/lib CVS_RSH=ssh SSH_CONNECTION=192.168.1.5 50651 192.168.1.213 22 LESSOPEN=||/usr/bin/lesspipe.sh %s G_BROKEN_FILENAMES=1 _=/bin/env
Using the echo value of the variable may be output.
[root@localhost ~]# echo $PATH /usr/lib64/qt-3.3/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin
For example, in the PATH environment variable to add a new directory / usr / local / mysql / bin.
[root@localhost ~]# PATH=/usr/local/mysql/bin:$PATH [root@localhost ~]# echo $PATH /usr/local/mysql/bin:/usr/lib64/qt-3.3/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin
Here can be seen the first two did not use $ PATH, the second use. When the variable operation itself, not using the $, when the operating variable values, use $ .
Ordinary variables