自訂變數導出 export

1. 取得 shell 後自訂的變數，只限目前的 shell 有效，其他 shell 或其子程序 shell 都無法使用。

   [dywang@dywmac ~]$ myip=192.168.122.1
   [dywang@dywmac ~]$ echo $myip
   192.168.122.1
   [dywang@dywmac ~]$ bash
   [dywang@dywmac ~]$ echo $myip
   
   [dywang@dywmac ~]$ exit
   exit
   

2. 若要讓目前設定的變數可以讓子程序 shell 使用，必須使用 export 導出。export 不加參數列出所有導出 (export) 的變數。

   [dywang@dywmac ~]$ export
   declare -x CVS_RSH="ssh"
   declare -x G_BROKEN_FILENAMES="1"
   declare -x HISTCONTROL="ignoredups"
   declare -x HISTSIZE="1000"
   declare -x HOME="/home/dywang"
   declare -x HOSTNAME="dywmac"
   declare -x LANG="en_US.UTF-8"
   declare -x LESSOPEN="||/usr/bin/lesspipe.sh %s"
   declare -x LOGNAME="dywang"
   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:...
   declare -x MAIL="/var/spool/mail/dywang"
   declare -x OLDPWD
   declare -x /usr/local/bin:/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/sbin"
   declare -x PWD="/home/dywang"
   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_ASKPASS="/usr/libexec/openssh/gnome-ssh-askpass"
   declare -x SSH_CLIENT="::1 50346 22"
   declare -x SSH_CONNECTION="::1 50346 ::1 22"
   declare -x SSH_TTY="/dev/pts/11"
   declare -x TERM="xterm"
   declare -x USER="dywang"
   declare -x XMODIFIERS="@im=gcin"
   

3. 設定 myip 變數並以 export 來導出變數，進入子程式後 myip 變數仍然有效。

   [dywang@dywmac ~]$ export myip=192.168.122.1
   [dywang@dywmac ~]$ export | grep myip
   declare -x myip="192.168.122.1"
   [dywang@dywmac ~]$ bash
   [dywang@dywmac ~]$ echo $myip
   192.168.122.1
   [dywang@dywmac ~]$ exit
   exit