bash.builtins.rme

#2018#02#01 #bash #shell #builtin-commands
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                             Bash shell - builtins

   What are shell builtins? These are reserved words (or commands) that are
   contained within the Bash toolset, meaning they are literally built in
   to the shell. When executing these builtins, the shell performs no
   lookup for a command (in your PATH) and doesn't fork off a subshell to
   execute the command. The builtin is executed right then and there.

      There are a few builtins that MUST be implemented as a builtin
      (cd, exit, pwd, exec, pushd, popd, dirs, and others), and those that
      are implemented as a builtin either for performance issues or because
      it makes things more convenient.
        (why do you think 'kill' is convenient as a builtin? what if your 
          system is out of resources and cannot create another process? it
          would be difficult to spawn a /usr/bin/kill process, wouldn't it?)

      In the bash shell, certain words are both a reserved word and a command.
      Like echo:
        echo is both a shell builtin which is different than /bin/echo.

      Try it yourself.

         [sciro@dionysus ~]$ type echo
         echo is a shell builtin
         echo is /usr/bin/echo
         [sciro@dionysus ~]$ 


    ----------------------------
    2016-09-29 @ 22:55, Thursday
    - From the POSIX Programmer's Manual
        - IEEE Std 1003.1, 2013 Edition, Standard for Information Technology 
          -- Portable Operating System Interface (POSIX), the Open Group Base 
          Specifications Issue 7, copyright (C) 2013 by the Institute of 
          Electrical and Electronics engineers, Inc and The Open group.
        - This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.
        - (URL) http://www.unix.org/online.html
    
    ::Environment::
    - Arch Linux 4.7.4-1-ARCH x86_64
      - Thu Sep 15 15:24:29 CEST 2016 GNU/Linux
      (URL) https://www.archlinux.org/
    

    Since type must be aware of the contents of the current shell execution
    environment  (such  as  the lists of commands, functions, and built-ins
    processed by hash), it is always provided as a shell regular  built-in.
    If  it  is  called in a separate utility execution environment, such as
    one of the following:

    $ nohup type writer
    $ find . −type f | xargs type

    it might not produce accurate results.


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      Some shell builtins:
      
        - kill
        - cd, pwd
        - pushd, popd, dirs
        - eval, exec
        - true, false
        - : (colon)
        - . (a period)
        - break, continue, return
        - exit
        - getopts, shift
        - hash
        - readonly
        - test
        - times
        - trap
        - umask
        - unset


      Some Bash builtins (all previous builtins included):

        - echo
        - printf
        - alias
        - bind
        - builtin
        - caller
        - command
        - declare
        - enable
        - help
        - let
        - local
        - logout
        - mapfile
        - read
        - readarray
        - source
        - type
        - typeset
        - ulimit
        - unalias

   *-*-* According to Bash *-*-*                                      2017.12.01
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   According to GNU Bash v4.4.12(1)-release (x86_64-unknown-linux-gnu),
   the enable builtin has a POSIX flag which displays the set of POSIX shell
   builtins only. Running a diff on both outputs yields the following:

      $ diff -ywBW80 <(enable -a) <(enable -as)

            enable .                enable .
            enable :                enable :
            enable [              <
            enable alias          <
            enable bg             <
            enable bind           <
            enable break            enable break
            enable builtin        <
            enable caller         <
            enable cd             <
            enable command        <
            enable compgen        <
            enable complete       <
            enable compopt        <
            enable continue         enable continue
            enable declare        <
            enable dirs           <
            enable disown         <
            enable echo           <
            enable enable         <
            enable eval             enable eval
            enable exec             enable exec
            enable exit             enable exit
            enable export           enable export
            enable false          <
            enable fc             <
            enable fg             <
            enable getopts        <
            enable hash           <
            enable help           <
            enable history        <
            enable jobs           <
            enable kill           <
            enable let            <
            enable local          <
            enable logout         <
            enable mapfile        <
            enable popd           <
            enable printf         <
            enable pushd          <
            enable pwd            <
            enable read           <
            enable readarray      <
            enable readonly         enable readonly
            enable return           enable return
            enable set              enable set
            enable shift            enable shift
            enable shopt          <
            enable source           enable source
            enable suspend        <
            enable test           <
            enable times            enable times
            enable trap             enable trap
            enable true           <
            enable type           <
            enable typeset        <
            enable ulimit         <
            enable umask          <
            enable unalias        <
            enable unset            enable unset
            enable wait           <
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

    *-*-* SPECIAL BUILTINS *-*-*                                      2018.02.01
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   For historical reasons, the POSIX standard has classified several builtin
   commands as _special_. When Bash is executing in POSIX mode, the special
   builtins differ from other builtin commands in three respects:

 1. Special builtins are found before shell functions during command lookup.
 2. If a special builtin returns an error status, a non-interactive shell exits.
 3. Assignment statements preceding the command stay in effect in the shell
    environment after the command completes.

   When Bash is not executing in POSIX mode, these builtins behave exactly the
   same as the rest of the builtin commands. There are fourteen special builtin
   commands -- they are:
         - :         the null command (typed as colon), always returns true
         - .         the source command (typed as dot), same as 'source'
         - break     exit for, while, or until loops
         - continue  resume for, while, or until loops
         - eval      combine and execute arguments as a shell command
         - exec      replace shell with the given command
         - exit      exit the shell
         - export    set export attribute for shell variables/functions
         - readonly  mark shell variables/functions as read-only (unchangeable)
         - return    return from a shell function
         - set       set or unset values of shell options and positional params
         - shift     shift positional parameters
         - trap      trap signals and other events using handlers
         - unset     unset values and attributes of shell variables/functions

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