There's a strong difference between a builtin and a keyword, in the way Bash parses your code. Before we talk about the difference, let's list all keywords and builtins:

Builtins:

$ compgen -b

.         :         [         alias     bg        bind      break     

builtin   caller    cd        command   compgen   complete  compopt   

continue  declare   dirs      disown    echo      enable    eval      

exec      exit      export    false     fc        fg        getopts   

hash      help      history   jobs      kill      let       local     

logout    mapfile   popd      printf    pushd     pwd       read      

readarray readonly  return    set       shift     shopt     source    

suspend   test      times     trap      true      type      typeset   

ulimit    umask     unalias   unset     wait                          

Keywords:

$ compgen -k

if        then      else      elif      fi        case      

esac      for       select    while     until     do        

done      in        function  time      {         }         

!         [[        ]]        coproc              

Notice that, for example [ is a builtin and that [[ is a keyword. I'll use these two to illustrate the difference below, since they are well-known operators: everybody knows them and uses them regularly (or should).

A keyword is scanned and understood by Bash very early in its parsing. This allows for example the following:

string_with_spaces='some spaces here'

if [[ -n $string_with_spaces ]]; then

    echo "The string is non-empty"

fi

This works fine, and Bash will happily output

The string is non-empty

Note that I didn't quote $string_with_spaces. Whereas the following:

string_with_spaces='some spaces here'

if [ -n $string_with_spaces ]; then

    echo "The string is non-empty"

fi

shows that Bash isn't happy:

bash: [: too many arguments

Why does it work with keywords and not with builtins? because when Bash parses the code, it sees [[ which is a keyword, and understands very early that it's special. So it will look for the closing ]] and will treat the inside in a special way. A builtin (or command) is treated as an actual command that is going to be called with arguments. In this last example, bash understands that it should run the command [ with arguments (shown one per line):

-n

some

spaces

here

]

since variable expansion, quote removal, pathname expansion and word splitting occurs. The command [ turns out to be built in the shell, so it executes it with these arguments, which results in an error, hence the complaint.

In practice, you see that this distinction allows for sophisticated behavior, that wouldn't be possible with builtins (or commands).

Still in practice, how can you possibly distinguish a builtin from a keyword? this is a fun experiment to perform:

$ a='['

$ $a -d . ]

$ echo $?

0

When Bash parses the line $a -d . ], it sees nothing special (i.e., no aliases, no redirections, no keywords), so it just performs variable expansion. After variable expansions, it sees:

[ -d . ]

so executes the command (builtin) [ with arguments -d, . and ], which, of course is true (this only tests whether . is a directory).

Now look:

$ a='[['

$ $a -d . ]]

bash: [[: command not found

Oh. That's because when Bash sees this line, it sees nothing special, and hence expands all variables, and eventually sees:

[[ -d . ]]

At this time, alias expansions and keyword scanning has long been performed and is not going to be performed anymore, so Bash tries to find the command called [[, doesn't find it, and complains.

Along the same lines:

$ '[' -d . ]

$ echo $?

0

$ '[[' -d . ]]

bash: [[: command not found

and

$ [ -d . ]

$ echo $?

0

$ [[ -d . ]]

bash: [[: command not found

---

Alias expansion is something rather special too. You've all done the following at least once:

$ alias ll='ls -l'

$ ll

.... <list of files in long format> ....

$ ll

bash: ll: command not found

$ 'll'

bash: ll: command not found

The reasoning is the same: alias expansion occurs long before variable expansion and quote removal.

---

Keyword v.s. Alias

Now what do you think happens if we define an alias to be a keyword?

$ alias mytest='[['

$ mytest -d . ]]

$ echo $?

0

Oh, it works! so aliases can be used to alias keywords! nice to know.

---

Conclusion: builtins really behave like commands: they correspond to an action being executed with arguments that undergo direct variable expansion and word splitting and globbing. It's really just like having an external command somewhere in /bin or /usr/bin that is called with the arguments given after variable expansion, etc. Note that when I say it's really just like having an external command I only mean with respect to arguments, word splitting, globbing, variable expansion, etc. A builtin can modify the shell's internal state!

Keywords, on the other hand, are scanned and understood very early, and allow for sophisticated shell behavior: the shell will be able to forbid word splitting or pathname expansion, etc.

Now look at the list of builtins and keywords and try to figure out why some need to be keywords.

---

! is a keyword. It seems it would be possible to mimic its behavior with a function:

not() {

    if "$@"; then

        return 1

    else

        return 0

    fi

}

but this would forbid constructs like:

$ ! ! true

$ echo $?

0

(in that case, I mean not ! true which doesn't work) or

$ ! { true; }

echo $?

1

Same for time: it's more powerful to have it a keyword so that it can time complex compound commands and pipelines with redirections:

$ time grep '^#' ~/.bashrc | { i=0; while read -r; do printf '%4d %s

' "$((++i))" "$REPLY"; done; } > bashrc_numbered 2>/dev/null

If time where a mere command (even builtin), it would only see the arguments grep, ^# and /home/gniourf/.bashrc, time this, and then its output would go through the remaining parts of the pipeline. But with a keyword, Bash can handle everything! it can time the complete pipeline, including the redirections! If time were a mere command, we couldn't do:

$ time { printf 'hello '; echo world; }

Try it:

$ 	ime { printf 'hello '; echo world; }

bash: syntax error near unexpected token `}'

Try to fix (?) it:

$ 	ime { printf 'hello '; echo world;

time: cannot run {: No such file or directory

Hopeless.

---

Keyword vs alias?

$ alias mytime=time

$ alias myls=ls

$ mytime myls

What do you think happens?

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Really, a builtin is like a command, except that it's built in the shell, whereas a keyword is something that allows for sophisticated behavior! we can say it's part of the shell's grammar.