Bash script 常見語法整理

Contents

1. 1. Array
   1. 1.1. Dereferencing the variables in array
2. 2. mysterious ‘-‘

在Linux上長需要寫些Shell script解決繁瑣的問題，可是很多語法常常寫過就忘，這篇主要是紀錄自己常忘記的東西。

Array

比較常用 array=(one two three)這種宣告方法

Dereferencing the variables in array

假設array=(one two three)
則echo ${array{*}}和echo ${array{@}}一樣都會印出 one two three，只是${array{*}}會將”one two three”視為單一字串。

mysterious ‘-‘

cd -可以在當前目錄和上一個拜訪過得目錄之間做切換，curl -sL http://xxxx | sudo -E bash -可以讓bash執行curl下載的script，到底-是啥東西呢？

根據POSIX Utility Syntax Guidelines

Guideline 13:
For utilities that use operands to represent files to be opened for either reading or writing, the ‘-‘ operand should be used to mean only standard input (or standard output when it is clear from context that an output file is being specified) or a file named -.

也就是說，當utility後面接的operand是檔案（檔名）時，-會被視為stdin或stdout，端看使用情境，否則就單純只是代表名為-的檔案。
另外，POSIX對cd時後面接的-有特別的定義：

-
When a is used as the operand, this shall be equivalent to the command:
Whencd “$OLDPWD” && pwd
Whenwhich changes to the previous working directory and then writes its name.

理解以上之後，可以試試看 echo "echo echo"| bash -，這會印出echo。