bash-hackers-wiki/docs/syntax/grammar/parser_exec.md

FIXME work in progress\...

# Parsing and execution

![](keywords>bash shell scripting syntax language behaviour executing execution)

Nearly everything in [Bash grammar](../../syntax/basicgrammar.md) can be broken
down to a \"simple command\". The only thing Bash has to expand,
evaluate and execute is the simple command.

## Simple command expansion

\<div center round info 60%\>

-   <http://lists.gnu.org/archive/html/bug-bash/2013-01/msg00040.html>
-   <http://lists.research.att.com/pipermail/ast-developers/2013q2/002456.html>

\</div\>

This step happens after the initial command line splitting.

The expansion of a simple command is done in four steps (interpreting
the simple command **from left to right**):

1.  The words the parser has marked as **variable assignments** and
    **redirections** are saved for later processing.
    -   variable assignments precede the command name and have the form
        `WORD=WORD`
    -   redirections can appear anywhere in the simple command
2.  The rest of the words are [expanded](../../syntax/expansion/intro.md). If
    any words remain after expansion, the first word is taken to be the
    **name of the command** and the remaining words are the
    **arguments**.
3.  [Redirections](../../syntax/redirection.md) are performed.
4.  The text after the `=` in each variable assignment undergoes [tilde
    expansion](../../syntax/expansion/tilde.md), [parameter
    expansion](../../syntax/pe.md), [command
    substitution](../../syntax/expansion/cmdsubst.md), [arithmetic
    expansion](../../syntax/expansion/arith.md), and quote removal before being
    assigned to the variable.

If **no command name** results after expansion:

-   The variable assignments affect the **current shell** environment.
    -   This is what happens when you enter only a variable assignment
        at the command prompt.
    -   Assignment to readonly variables causes an error and the command
        exits non-zero.
-   Redirections are performed, but do not affect the current shell
    environment.
    -   that means, a `> FILE` without any command **will** be
        performed: the `FILE` will be created!
-   The command exits
    -   with an exit code indicating the redirection error, if any
    -   with the exit code of the last command-substitution parsed, if
        any
    -   with exit code 0 (zero) if no redirection error happened and no
        command substitution was done

Otherwise, if a command name results:

-   The variables saved and parsed are added to the environment of the
    executed command (and thus do not affect the current environment)
    -   Assignment to readonly variables causes an error and the command
        exits with a non-zero error code.
    -   **Assignment errors** in non-POSIX modes cause the *enclosing
        commands (e.g. loops) to completely terminate*
    -   **Assignment errors** in (non-interactive) POSIX mode cause *the
        entire script to terminate*

The behavior regarding the variable assignment errors can be tested:
\<div center round info
60%\><http://lists.gnu.org/archive/html/bug-bash/2013-01/msg00054.html>\</div\>

**[This one exits the script completely]{.underline}**

    #!/bin/sh
    # This shell runs in POSIX mode!

    echo PRE

    # The following is an assignment error, since there is no digit '9'
    # for a base eight number!
    foo=$((8#9))

    echo POST

**[This one terminates only the enclosing compound command (the
`{ ...; }`):]{.underline}**

    #!/bin/bash
    # This shell runs in native Bash-mode!

    echo PRE

    # The following is an assignment error!
    # The "echo TEST" won't be executed, since the { ...; } is terminated
    { foo=$((8#9)); echo TEST; }

    echo POST

## Simple command execution

If a parsed simple command contains no slashes, the shell attempts to
locate and execute it:

-   shell functions
-   shell builtin commands
-   check own hash table
-   search along `PATH`

As of Bash Version 4, when a command search fails, the shell executes a
shell function named `command_not_found_handle()` using the failed
command as arguments. This can be used to provide user friendly messages
or install software packages etc. Since this function runs in a separate
execution environment, you can't really influence the main shell with
it (changing directory, setting variables).

FIXME to be continued

## See also

-   Internal: [Redirection](../../syntax/redirection.md)
-   Internal: [Introduction to expansions and
    substitutions](../../syntax/expansion/intro.md)