FIXME work in progress\...
# Parsing and 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
-
-
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:
**[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)