mirror of
https://github.com/flokoe/bash-hackers-wiki.git
synced 2024-11-01 14:53:06 +01:00
93 lines
4.2 KiB
Plaintext
93 lines
4.2 KiB
Plaintext
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]]
|
|
|