Cfrgtkky

I read in the O'reilly Bash book (third edition, 2005) what I understood as that if I'll execute hash I'll see a "hash table" with a list of all possible Bash commands plus all commands common to most environments including Bash that are available in my system.

Yet in WSL-Ubuntu (16.04 - xenial) I executed hash and got just:

hits    command

   1    /usr/bin/mesg

Might this be unique to WSL (I don't have a non-WSL Ubuntu machine to test on right now)?

Update

I misunderstood the chapter - the hash table doesn't include all Bash commands but seemingly all recently used non-bash commands (with the path to the files containing their data);

it's just that the authors gave a list containing some very common commands which I mistakenly confused as "builtin" from a quick look on the list, although they are standalone utilities (and here's the list from the book - page 72):

cat

stat

less

man

apropos

more

ln

ls

ps

vi

Of course, these are not Bash builtin commands like cd, echo, set, shopt, source, bash, if, case, for, while, kill, trap, exit, etc, but standalone utilities very common in Bash based environments.

By now I've internalized the difference deep enough and humbly believe I won't confuse in this embarrasing way again. This is an important lesson for me; I never really dived deep enough on the differences between what very common program is a Bash builtin or a "very common" utility in Bash-based environments that might be grasped as a "builtin" but my stance is - "no more!".

As help hash shows, the hash function is used to determine and remember the full pathnames of programs:

hash: hash [-lr] [-p pathname] [-dt] [name ...]

    Remember or display program locations.



    Determine and remember the full pathname of each command NAME.  If

    no arguments are given, information about remembered commands is displayed.



    Options:

      -d        forget the remembered location of each NAME

      -l        display in a format that may be reused as input

      -p pathname       use PATHNAME as the full pathname of NAME

      -r        forget all remembered locations

      -t        print the remembered location of each NAME, preceding

                each location with the corresponding NAME if multiple

                NAMEs are given

    Arguments:

      NAME      Each NAME is searched for in $PATH and added to the list

                of remembered commands.



    Exit Status:

    Returns success unless NAME is not found or an invalid option is given.

In your case it returns /usr/bin/mesg because obviously mesg has been called before and hash now remembers its path.

To get help on bash builtins, functions etc. there’s the help function. Call it without an argument to get a list of shell commands defined internally. To get a list of bash builtins you can use compgen -b.