Linux Shell Script 保留/系統預設的變數 （轉貼）

 

保留/系統預設的變數

系統預設已經有相當多的變數定義了, 因此在你的shell script裡面 要去避免這些變數. 以下就是一些預設的系統變數. 

$BASH_ENV	absolute path of startup file
$CDPATH	directories searched by cd
$FCEDIT	absolute path of history editor
$HISTCMD	the history number of the current command
$HISFILE	absolute path of history file
$HISTSIZE	number of remembered commands
$HOME	login directory
$IFS	token delimiters
$LINENO	current line number in shell script
$LINES	terminal height
$MAIL	absolute path of mailbox
$MAILCHECK	number of seconds to check mail
$OLDPWD	absolute path of previous directory
$OPTARG	option set by getopt
$OPTIND	option's ordinal position set by getopt
$OSTYPE	the OS on which bash is executing
$PATH	command search path
$PPID	process ID of parent
$PS1	primary prompt
$PS2	secondary prompt
$PWD	absolute path of current directory
$RANDOM	random integer
$REPLY	default variable for read
$SECONDS	number of seconds since shell started
$SHELL	absolute pathname of preferred shell
$TMOUT	seconds to log out after lack of use
$UID	user ID of the current user
$$	process ID of current shell
$?	exit status of most recent statement
$#	參數的數目
$*	代表所有參數
$!	PID of the most recently started backgroup job

另外, 還有參數的傳遞也需要了解, 例如當我們編寫一個 script 內容如下:

#!/bin/bash
echo "\$0 ==> $0"
echo "\$1 ==> $1"
echo "\$2 ==> $2"

則執行以下命令的時候:

./myscript Hello World
$0 ==> ./myscript
$1 ==> Hello
$2 ==> World




Reusing output from last command in Bash




You can use $(!!) to recompute (not re-use) the output of the last command.

The !! on its own executes the last command.

$ echo pierre
pierre
$ echo my name is $(!!)
echo my name is $(echo pierre)
my name is pierre