Shell programming needs to know

Recently, shell has been used. It is very simple and has no logic. I wrote it here just to help myself remember it. It's just a messy note. I hope this will help you. Please advise me more.

1 If else

If [$ #-lt 5]; then

Help
Exit 1

Else

Do something...
Fi

Where-lt is equivalent to <

-GT is equivalent to>

2 while

The followingProgramExecute the loop 101 times. Note that-Ge is greater than or equal

X = 100

While [$ X-GE 0]

Do

Something...

(X = x-1) // note that the operation in Shell needs to be written like this

Done

 

3 built-in Variables

$0	Equivalent to C LanguageMainFunctionArgv [0]
$1,$2...	These are called positional parameter, which is equivalent to the C LanguageMainFunctionArgv [1],Argv [2]...
$ #	Equivalent to C LanguageMainFunctionArgc-1, Note that#Not comment
$ @	Parameter List"$1" "$2 "...For exampleForIn the loopIn.
$?	Exit status of the previous command
$	Process Number of the Current Shell

4. Date command usage

Common commands date-d "condition" Format

For example, date-d "yesterday" + % Y % m % d gets 20120201