Bash内部变量学习

$SECONDS变量---------脚本已经运行的秒数，脚本实例：

 1 #!/bin/bash -
 2 #===========================================================================================
 3 #  4 #  5 #                                    FILE:seconds.sh
 6 #                                    USAGE:./seconds.sh                              
 7 # DESCRIPTION:  8 #  9 #         OPTIONS:---
10 #    REQUIREMENTS:---
11 #            BUGS:---
12 #           NOTES:---
13 # AUTHOR:Yuan Shaoqian(Mark),yuanshaoqiancom@gmail.com 14 # ORGANIZATION: 15 #         CREATED:09/02/2016 21:19
16 #        REVISION:---
17 #===========================================================================================
18 TIME_LIMIT=10
19 INTERVAL=1
20 
21 echo
22 echo "Hit Control-C to exit before $TIME_LIMIT seconds."
23 echo 
24 
25 while [ "$SECONDS" -le "$TIME_LIMIT" ] 26 do # $SECONDS is an internal shell variable 27 if [ "$SECONDS" -eq 1 ] 28 then
29   units=second 30 else
31   units=seconds 32 fi
33 
34 echo "This script has been running $SECONDS $units."
35 #On a slow or overburdened machine,the script may skip a count every once in a while. 36 sleep $INTERVAL 37 done 
38 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），但能够被修改。

1 $ set x y z 2 $ IFS=“:;-” 3 $ echo "$*"
4 x:y:z

$OSTYPE变量-----操做系统的类型

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

 1 set -o nounset  2 TMOUT=3
 3 
 4 echo "Are you sure?(Y/N)"
 5 read input  6 
 7 if [ "$input" == "Y" ]  8 then
 9   echo "Continue..."
10 else
11   echo "Exit!"
12 fi 

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

1 root_id=0
2 if [ "$UID" -eq "$root_id" ] 3 then
4   echo "You are root."
5 else
6   echo "You are just an ordinary user."
7 fi
8 
9 exit 0