Bash中几个重要的顺序

1. 命令执行顺序：

1. Keywords (such as if, while, until)
2. Aliases (see typeset –f)
3. Built-in commands
4. Functions
5. Scripts and executables 

2. 变量扩展顺序×order of expansion):

1. Brace expansion
2. Tilde expansion
3. Parameter expansion
4. Variable substitution
5. Command substitution
6. Arithmetic expansion
7. Word splitting
8. Pathname expansion 

3. 命令行分解顺序:

1. History substitution is performed (if applicable).
2. Command line is broken up into tokens, or words.
3. History is updated (if applicable).
4. Quotes are processed.
5. Alias substitution and functions are defined (if applicable).
6. Redirection, background, and pipes are set up.
7. Variable substitution ($user, $name, etc.) is performed.
8. Command substitution (echo "Today is `date`") is performed.
9. Filename substitution, called globbing (cat abc.??, rm *.c, etc.) is performed.
10. Command is executed.