Parsing and execution [Bash Hackers Wiki]

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FIXME work in progress...

====== Parsing and execution ======

{{keywords>bash shell scripting syntax language behaviour executing execution}}

Nearly everything in [[syntax:basicgrammar | Bash grammar]] 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**):
  - 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
  - The rest of the words are [[syntax:expansion:intro| expanded]]. 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**.
  - [[syntax:redirection | Redirections]] are performed.
  - The text after the ''='' in each variable assignment undergoes [[syntax:expansion:tilde | tilde expansion]], [[syntax:pe | parameter expansion]], [[syntax:expansion:cmdsubst | command substitution]], [[syntax:expansion:arith | arithmetic expansion]], 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__**
<code>
#!/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
</code>


**__This one terminates only the enclosing compound command (the ''{ ...; }''):__**
<code>
#!/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

</code>

===== 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: [[syntax:redirection | Redirection]]
  * Internal: [[syntax:expansion:intro | Introduction to expansions and substitutions]]