Linux shell command line process(命令行处理流程)

Splits the command into tokens that are separated by the fixed set of metacharacters: SPACE, TAB, NEWLINE,;,(,),<,>,|, and&. Types of tokens include words, keywords, I/O redirectors, and semicolons.

Checks the first token of each command to see if it is a keyword with no quotes or backslashes. If it's an opening keyword, such asifand other control-structure openers,function,{, or(, then the command is actually a compound command. The shell sets things up internally for the compound command, reads the next command, and starts the process again. If the keyword isn't a compound command opener (e.g., is a control-structure "middle" likethen,else, ordo, an "end" likefiordone, or a logical operator), the shell signals a syntax error.

Checks the first word of each command against the list of aliases. If a match is found, it substitutes the alias's definition and goes back to Step 1; otherwise, it goes on to Step 4. This scheme allows recursive aliases (seeChapter 3). It also allows aliases for keywords to be defined, e.g.,alias aslongas=whileoralias procedure=function.

Performs brace expansion. For example,a{b,c}becomesab ac.

Substitutes the user's home directory ($HOME) for tilde if it is at the beginning of a word. Substitutes user's home directory for~user.^[7]

^[7]Two obscure variations on this: the shell substitutes the current directory ($PWD) for~+and the previous directory ($OLDPWD) for~-. Inbash2.0 there are two more: ~N+ and ~N-. These are replaced by the corresponding element in the directory stack as given by thedirscommand.

Performs parameter (variable) substitution for any expression that starts with a dollar sign ($).

Does command substitution for any expression of the form$(string).

Evaluates arithmetic expressions of the form$((string)).

Takes the parts of the line that resulted from parameter, command, and arithmetic substitution and splits them into words again. This time it uses the characters in$IFSas delimiters instead of the set of metacharacters in Step 1.

Performs pathname expansion, a.k.a. wildcard expansion, for any occurrences of*,?, and[/]pairs.

Uses the first word as a command by looking up its source according to the rest of the list inChapter 4, i.e., as a function command, then as a built-in, then as a file in any of the directories in$PATH.