Bash內部變量學習

$SECONDS變量---------腳本已經運行的秒數，腳本實例：

 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變量-------------用於引用Bash實例的全路徑名。

$IFS變量--------IFS時內部字段分隔符的縮寫。此變量決定當Bash解析字符串時將怎樣識別字段，或單詞分界線。變量$IFS的默認值是空格（空格/製表符/換行），但能夠被修改。

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

$OSTYPE變量-----操做系統的類型

$TMOUT變量------若是$TMOUT被指定了一個非零的值，此值會被Bash的內部命令read做爲默認的超時秒數。

 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 

$UID變量------當前用戶的帳號識別碼（ID號），與/etc/passwd中記錄的相同，$UID是一個只讀變量，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