Bash internal variable learning and Bash variable learning

Bash internal variable learning and Bash variable learning

$ SECONDS variable --------- number of SECONDS after the script has been run. Script instance:

 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.com14 #    ORGANIZATION:15 #         CREATED:09/02/2016 21:1916 #        REVISION:---17 #===========================================================================================18 TIME_LIMIT=1019 INTERVAL=120 21 echo22 echo "Hit Control-C to exit before $TIME_LIMIT seconds."23 echo 24 25 while [ "$SECONDS" -le "$TIME_LIMIT" ]26 do # $SECONDS is an internal shell variable27 if [ "$SECONDS" -eq 1 ]28 then29   units=second30 else31   units=seconds32 fi33 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 $INTERVAL37 done 38 exit 0

The $ HOME variable ----------- is used to reference the full path name of a Bash instance.

$ IFS Variable -------- the abbreviation of the internal field separator when IFS is used. This variable determines how the field or word line is recognized when Bash parses a string. The default value of variable $ IFS is space (space/TAB/line feed), but can be modified.

1 $ set x y z2 $ IFS=“:;-”3 $ echo "$*"4 x:y:z

$ OSTYPE variable ----- operating system type

$ TMOUT variable ------ if $ TMOUT is specified with a non-zero value, this value will be used by the Bash Internal Command read as the default timeout seconds.

 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 else11   echo "Exit!"12 fi 

$ UID variable ------ the account ID (ID) of the current user, which is the same as the one recorded in/etc/passwd. $ UID is a read-only variable and cannot be modified.

1 root_id=02 if [ "$UID" -eq "$root_id" ]3 then4   echo "You are root."5 else6   echo "You are just an ordinary user."7 fi8 9 exit 0