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From Make bash use external `time` command rather than shell built-in, Stéphane Chazelas wrote:

There is no time bash builtin. time is a keyword so you can do for instance time { foo; bar; }

We can verify it:

$ type -a time
time is a shell keyword
time is /usr/bin/time

It doesn't show that time can be a builtin command.

  1. What is the definition of a "keyword"?
  2. is "keyword" the same concept as "reserved word" in Bash Reference Manual?

    reserved word

    A word that has a special meaning to the shell. Most reserved words introduce shell fl ow control constructs, such as for and while.

  3. Is a keyword necessarily not a command (or not a builtin command)?

    As a keyword, is time not a command (or not a builtin command)?

    Based on the definitions of keyword and of builtin, why is time not a builtin but a keyword?

  4. Why "you can do for instance time { foo; bar; }" because "time is a keyword"?

8

Keyword, reserved word, and builtin are all the "first word" of a Simple command. Could be placed in two groups: Keyword and Builtin. The two are mutually exclusive. A word (token) can be either a Keyword or a builtin, but not both.

Why the "first word"

From the POSIX definition of "Simple command"(emphasis mine):

A "simple command" is a sequence of optional variable assignments and redirections, in any sequence, optionally followed by words and redirections, terminated by a control operator.

2.- The words that are not variable assignments or redirections shall be expanded. If any fields remain following their expansion, the first field shall be considered the command name and remaining fields are the arguments for the command.

After that "first word" has been identified, and after is has been expanded (by an alias, for example) the final word is "the command", there could be only one command in each line. That command word could be a Built-in or a keyword.

Keyword

Yes, a keyword is a "reserved word". Load "man bash" and search for keyword. (or just execute this command: LESS=+/'keyword' man bash. The first hit on search say this:

keyword Shell reserved words.

It happens in the completion section, but is quite clear.

Reserved Words

In POSIX, there is this definition of "Reserved Words" and some description of what Reserved Words do.

But the bash manual has a better working definition.
Search for "RESERVED WORDS" (LESS=+/'RESERVED WORDS' man bash) and find this:

RESERVED WORDS Reserved words are words that have a special meaning to the shell.
The following words are recognized as reserved when unquoted and either the first word of a simple command or the third word of a case or for command:

! case  do done elif else esac fi for function if
in select then until while { } time [[ ]]

Builtin

It is not defined in the bash manual, but it is quite simple:

It is a command that has been implemented inside the shell for UN-avoidable needs of the shell (cd, pwd, eval), or speed in general or to avoid conflicting interpretations of external utilities in some cases.

Time is a keyword.

why is time not a builtin but a keyword?

To allow the existence of a command as the second word.

It is similar as how an if ... then .... fi allow the inclusion of commands (even compound commands) after the first key-word if. Or while or case, etc.

  • Thanks. Why is time a keyword, because it is in order "To allow the existence of a command as the second word"? Can't a builtin command precede another command? If I use the external command explicitly as /usr/bin/time date, it still work, so an external command can precede another command. Then why can't a builtin command precede another command? – Tim Mar 9 '16 at 9:53
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    @Tim In the /usr/bin/time date command line time is the (only) command (as the shell understand it), date is an argument to time which happens to be a command for the user. Not for the shell, which only see one command: time and one or several arguments, and date is one of them. – user79743 Mar 9 '16 at 17:05
  • So what you meant is in time date, time is a keyword not a command, and date is a command and not an argument to time, while in /usr/bin/time date, /usr/bin/time is a command and date is an argument to it? Why in time date, is time not a builtin command and date is an arugment to it? Can a builtin command take another command as its argument? – Tim Mar 9 '16 at 17:37
  • @Tim Because the developer of bash decided to implement it that way. – user79743 Mar 9 '16 at 20:11
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    @Tim A builtin command is the command of the line, period. If the builtin decides to promote an argument to a command is the decision of the builtin (like the builtin command or eval for example). – user79743 Mar 9 '16 at 20:13
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Q1, Q2

What's the definition of a keyword?

Is "keyword" the same concept as "reserved word" in Bash Reference Manual?

Basically, special words important to the syntactic structure. In C there are goto, if, while, enum and so on; in bash you have if, while (these sound familiar..), time, etc.

And yes, they are the same.

I am taking some liberty in interpreting the question, since the basic syntactic elements of POSIX shell and bash are similar. So let's look at the definition in POSIX.1:2013 Shell Command Language:

2.4 Reserved Words

Reserved words are words that have special meaning to the shell; see Shell Commands. The following words shall be recognized as reserved words:

...

This recognition shall only occur when none of the characters is quoted and when the word is used as:

...

See the grammar in Shell Grammar.

Let's take a look at the POSIX grammar to see how the Special Words—now syntatic tokens after lexing—in action:

for_clause       : For name linebreak                            do_group
                 | For name linebreak in          sequential_sep do_group
                 | For name linebreak in wordlist sequential_sep do_group
/* ... */
do_group         : Do compound_list Done           /* Apply rule 6 */

That looks familiar, right? Note that For, Do and Done are really tokens that are supposed to be mapped and recognized by the lexer:

%token  If    Then    Else    Elif    Fi    Do    Done
/*      'if'  'then'  'else'  'elif'  'fi'  'do'  'done'   */


%token  Case    Esac    While    Until    For
/*      'case'  'esac'  'while'  'until'  'for'   */

/* and 'In' too -- They made a mistake! */
%token  In    /*      'in'   */

If you have heard about Yacc or Bison (or well, jison), that's how people might use the grammar. With these parsers generators, they can generate something that figures out what parts of the grammar a given flow of input 'tokens' are spoken in.

Q3

Is a keyword necessarily not a command (or not a builtin command)?

As a keyword, is time not a command (or not a builtin command)?

None of the keywords are treated like commands. But sure, you can have commands/functions with the same name, e.g.:

# make a command to avoid command-not-found for `FOO=BAR time cmd`
time(){ time "$@"; }

Based on the definitions of keyword and of builtin, why is time not a builtin but a keyword?

Because that's how people decided to make it:

// Licensed under GPLv2, bash:parse.y
// Copyright (C) 1989-2012 Free Software Foundation, Inc.
pipeline_command: pipeline
                | BANG pipeline_command
                | timespec pipeline_command
                | timespec list_terminator
                | BANG list_terminator;
pipeline        : pipeline '|' newline_list pipeline
                | pipeline BAR_AND newline_list pipeline
                | command;
timespec        : TIME | TIME TIMEOPT | TIME TIMEOPT TIMEIGN;

And they get extra power from this (see next question).

Q4

Why "you can do for instance time { foo; bar; }" because "time is a keyword"?

As a part of the grammar, people can naturally let the parser handle everything, and make decisions like allowing time before compound commands. If time was implemented as just a command, you will get a syntax error for constructs like such (try echo { foo; bar; }), since it's really parsed with the 'usual' rules.

Also think about [[. If [[ have not been a keyword, constructs like [[ ($a == 2) || ($b != 3) ]] will make the shell find stray parentheses and complain. (Replace [[ with [ and see).

P.S. time is a utility instead of a keyword in POSIX, although the latter is still considered acceptable. The whole time-a-trunk-of-commands thing is a ksh and bash extension.

  • @BinaryZebra Not possible for the same word, but time means the command or function time when it doesn't meet the requirements of being recognized as a keyword… – Arthur2e5 Mar 6 '16 at 1:56
  • @BinaryZebra Edited (43 mins ago). – Arthur2e5 Mar 6 '16 at 2:40
  • Thanks. From the view of the parser or interpreter, why in time { foo; bar; }, must time be a keyword, why can't `time be a builtin command? – Tim Mar 14 '16 at 9:36
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    @tim "Replace time with echo". It's just something that normal commands don't get interpreted as... – Arthur2e5 Mar 15 '16 at 2:13
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    @Tim, the grammar doesn't allow it. (Just like in English "I ate" is a sentence but "I kitchen" isn't, because there has to be a verb; that's just how the grammar works.) There's two "layers" to the question you can ask: (1) What are the rules by which echo { foo; bar;} was disallowed in the design of shell grammar, and (2) Why were the rules designed that way. They are really different questions; one is "how" and one is "why". – Wildcard Mar 17 '16 at 0:43

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