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93 lines
4.2 KiB
Plaintext
93 lines
4.2 KiB
Plaintext
FIXME work in progress...
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====== Parsing and execution ======
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{{keywords>bash shell scripting syntax language behaviour executing execution}}
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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.
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===== Simple command expansion =====
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<div center round info 60%>
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* http://lists.gnu.org/archive/html/bug-bash/2013-01/msg00040.html
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* http://lists.research.att.com/pipermail/ast-developers/2013q2/002456.html
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</div>
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This step happens after the initial command line splitting.
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The expansion of a simple command is done in four steps (interpreting the simple command **from left to right**):
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- The words the parser has marked as **variable assignments** and **redirections** are saved for later processing.
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* variable assignments precede the command name and have the form ''WORD=WORD''
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* redirections can appear anywhere in the simple command
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- 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**.
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- [[syntax:redirection | Redirections]] are performed.
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- 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.
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If **no command name** results after expansion:
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* The variable assignments affect the **current shell** environment.
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* This is what happens when you enter only a variable assignment at the command prompt.
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* Assignment to readonly variables causes an error and the command exits non-zero.
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* Redirections are performed, but do not affect the current shell environment.
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* that means, a ''> FILE'' without any command **will** be performed: the ''FILE'' will be created!
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* The command exits
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* with an exit code indicating the redirection error, if any
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* with the exit code of the last command-substitution parsed, if any
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* with exit code 0 (zero) if no redirection error happened and no command substitution was done
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Otherwise, if a command name results:
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* The variables saved and parsed are added to the environment of the executed command (and thus do not affect the current environment)
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* Assignment to readonly variables causes an error and the command exits with a non-zero error code.
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* **Assignment errors** in non-POSIX modes cause the //enclosing commands (e.g. loops) to completely terminate//
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* **Assignment errors** in (non-interactive) POSIX mode cause //the entire script to terminate//
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The behavior regarding the variable assignment errors can be tested:
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<div center round info 60%>http://lists.gnu.org/archive/html/bug-bash/2013-01/msg00054.html</div>
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**__This one exits the script completely__**
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<code>
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#!/bin/sh
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# This shell runs in POSIX mode!
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echo PRE
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# The following is an assignment error, since there is no digit '9'
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# for a base eight number!
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foo=$((8#9))
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echo POST
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</code>
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**__This one terminates only the enclosing compound command (the ''{ ...; }''):__**
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<code>
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#!/bin/bash
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# This shell runs in native Bash-mode!
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echo PRE
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# The following is an assignment error!
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# The "echo TEST" won't be executed, since the { ...; } is terminated
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{ foo=$((8#9)); echo TEST; }
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echo POST
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</code>
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===== Simple command execution =====
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If a parsed simple command contains no slashes, the shell attempts to locate and execute it:
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* shell functions
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* shell builtin commands
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* check own hash table
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* search along ''PATH''
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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).
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FIXME to be continued
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===== See also ====
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* Internal: [[syntax:redirection | Redirection]]
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* Internal: [[syntax:expansion:intro | Introduction to expansions and substitutions]]
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