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125 lines
4.3 KiB
Markdown
125 lines
4.3 KiB
Markdown
FIXME work in progress...
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# Parsing and execution
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Nearly everything in [Bash grammar](/syntax/basicgrammar) can be broken
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down to a "simple command". The only thing Bash has to expand, evaluate
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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
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the simple command **from left to right**):
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1. The words the parser has marked as **variable assignments** and
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**redirections** are saved for later processing.
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- variable assignments precede the command name and have the form
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`WORD=WORD`
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- redirections can appear anywhere in the simple command
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2. The rest of the words are [expanded](/syntax/expansion/intro). If
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any words remain after expansion, the first word is taken to be the
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**name of the command** and the remaining words are the
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**arguments**.
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3. [Redirections](/syntax/redirection) are performed.
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4. The text after the `=` in each variable assignment undergoes [tilde
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expansion](/syntax/expansion/tilde), [parameter
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expansion](/syntax/pe), [command
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substitution](/syntax/expansion/cmdsubst), [arithmetic
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expansion](/syntax/expansion/arith), and quote removal before being
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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
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the command prompt.
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- Assignment to readonly variables causes an error and the command
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exits non-zero.
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- Redirections are performed, but do not affect the current shell
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environment.
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- that means, a `> FILE` without any command **will** be performed:
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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
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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
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executed command (and thus do not affect the current environment)
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- Assignment to readonly variables causes an error and the command
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exits with a non-zero error code.
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- **Assignment errors** in non-POSIX modes cause the *enclosing
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commands (e.g. loops) to completely terminate*
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- **Assignment errors** in (non-interactive) POSIX mode cause *the
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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
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60%\><http://lists.gnu.org/archive/html/bug-bash/2013-01/msg00054.html>\</div\>
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**<u>This one exits the script completely</u>**
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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=$((8#9))
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echo POST
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**<u>This one terminates only the enclosing compound command (the
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`{ ...; }`):</u>**
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#!/bin/bash
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# This shell runs in native Bash-mode!
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echo PRE
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# The following is an assignment error!
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# The "echo TEST" won't be executed, since the { ...; } is terminated
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{ foo=$((8#9)); echo TEST; }
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echo POST
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## Simple command execution
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If a parsed simple command contains no slashes, the shell attempts to
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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
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shell function named `command_not_found_handle()` using the failed
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command as arguments. This can be used to provide user friendly messages
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or install software packages etc. Since this function runs in a separate
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execution environment, you can't really influence the main shell with it
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(changing directory, setting variables).
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FIXME to be continued
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## See also
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- Internal: [Redirection](/syntax/redirection)
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- Internal: [Introduction to expansions and
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substitutions](/syntax/expansion/intro)
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