Bash内部变量学习

$SECONDS变量---------脚本已经运行的秒数，脚本实例：

#!/bin/bash -
#===========================================================================================
#
#
#                                    FILE:seconds.sh
#                                    USAGE:./seconds.sh                              
#    DESCRIPTION:
#    
#         OPTIONS:---
#    REQUIREMENTS:---
#            BUGS:---
#           NOTES:---
#          AUTHOR:Yuan Shaoqian(Mark),yuanshaoqiancom@gmail.com
#    ORGANIZATION:
#         CREATED:09/02/2016 21:19
#        REVISION:---
#===========================================================================================
TIME_LIMIT=10
INTERVAL=1

echo
echo "Hit Control-C to exit before $TIME_LIMIT seconds."
echo 

while [ "$SECONDS" -le "$TIME_LIMIT" ]
do # $SECONDS is an internal shell variable
if [ "$SECONDS" -eq 1 ]
then
  units=second
else
  units=seconds
fi

echo "This script has been running $SECONDS $units."
#On a slow or overburdened machine,the script may skip a count every once in a while.
sleep $INTERVAL
done 
exit 0

$HOME变量-------------用于引用Bash实例的全路径名。

$IFS变量--------IFS时内部字段分隔符的缩写。此变量决定当Bash解析字符串时将怎样识别字段，或单词分界线。变量$IFS的默认值是空格（空格/制表符/换行），但可以被修改。

$ set x y z
$ IFS=“:;-”
$ echo "$*"
x:y:z

$OSTYPE变量-----操作系统的类型

$TMOUT变量------如果$TMOUT被指定了一个非零的值，此值会被Bash的内部命令read作为默认的超时秒数。

set -o nounset
TMOUT=3

echo "Are you sure?(Y/N)"
read input

if [ "$input" == "Y" ]
then
  echo "Continue..."
else
  echo "Exit!"
fi 

$UID变量------当前用户的账号识别码（ID号），与/etc/passwd中记录的相同，$UID是一个只读变量，不支持修改。

root_id=0
if [ "$UID" -eq "$root_id" ]
then
  echo "You are root."
else
  echo "You are just an ordinary user."
fi

exit 0