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Chet Ramey
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.SH Shell Compatibility Mode
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Bash-4.0 introduced the concept of a `shell compatibility level', specified
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as a set of options to the shopt builtin
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.BR compat31 ,
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.BR compat32 ,
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.BR compat40 ,
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.BR compat41 ,
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and so on).
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There is only one current
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compatibility level -- each option is mutually exclusive.
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The compatibility level is intended to allow users to select behavior
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from previous versions that is incompatible with newer versions
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while they migrate scripts to use current features and
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behavior. It's intended to be a temporary solution.
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.PP
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This section does not mention behavior that is standard for a particular
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version (e.g., setting \fBcompat32\fP means that quoting the rhs of the regexp
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matching operator quotes special regexp characters in the word, which is
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default behavior in bash-3.2 and above).
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.PP
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If a user enables, say, \fBcompat32\fP, it may affect the behavior of other
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compatibility levels up to and including the current compatibility level.
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The idea is that each compatibility level controls behavior that changed
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in that version of \fBbash\fP,
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but that behavior may have been present in earlier versions.
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For instance, the change to use locale-based comparisons with the \fB[[\fP
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command came in bash-4.1, and earlier versions used ASCII-based comparisons,
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so enabling \fBcompat32\fP will enable ASCII-based comparisons as well.
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That granularity may not be sufficient for
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all uses, and as a result users should employ compatibility levels carefully.
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Read the documentation for a particular feature to find out the
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current behavior.
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.PP
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Bash-4.3 introduced a new shell variable:
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.SM
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.BR BASH_COMPAT .
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The value assigned
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to this variable (a decimal version number like 4.2, or an integer
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corresponding to the \fBcompat\fP\fINN\fP option, like 42) determines the
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compatibility level.
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.PP
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Starting with bash-4.4, Bash has begun deprecating older compatibility
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levels.
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Eventually, the options will be removed in favor of
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.SM
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.BR BASH_COMPAT .
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.PP
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Bash-5.0 is the final version for which there will be an individual shopt
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option for the previous version. Users should use
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.SM
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.B BASH_COMPAT
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on bash-5.0 and later versions.
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.PP
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The following table describes the behavior changes controlled by each
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compatibility level setting.
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The \fBcompat\fP\fINN\fP tag is used as shorthand for setting the
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compatibility level
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to \fINN\fP using one of the following mechanisms.
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For versions prior to bash-5.0, the compatibility level may be set using
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the corresponding \fBcompat\fP\fINN\fP shopt option.
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For bash-4.3 and later versions, the
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.SM
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.B BASH_COMPAT
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variable is preferred,
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and it is required for bash-5.1 and later versions.
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.PP
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.TP
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\fBcompat31\fP
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.IP \(bu
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quoting the rhs of the \fB[[\fP command's regexp matching operator (=~)
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has no special effect
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.TP
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\fBcompat32\fP
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.IP \(bu
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interrupting a command list such as "a ; b ; c" causes the execution
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of the next command in the list (in bash-4.0 and later versions,
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the shell acts as if it received the interrupt, so
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interrupting one command in a list aborts the execution of the
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entire list)
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.TP
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\fBcompat40\fP
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.IP \(bu
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the \fB<\fP and \fB>\fP operators to the \fB[[\fP command do not
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consider the current locale when comparing strings; they use ASCII
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ordering.
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Bash versions prior to bash-4.1 use ASCII collation and
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.IR strcmp (3);
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bash-4.1 and later use the current locale's collation sequence and
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.IR strcoll (3).
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.TP
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\fBcompat41\fP
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.IP \(bu
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in \fIposix\fP mode, \fBtime\fP may be followed by options and still be
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recognized as a reserved word (this is POSIX interpretation 267)
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.IP \(bu
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in \fIposix\fP mode, the parser requires that an even number of single
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quotes occur in the \fIword\fP portion of a double-quoted
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parameter expansion and treats them specially, so that characters within
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the single quotes are considered quoted
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(this is POSIX interpretation 221)
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.TP
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\fBcompat42\fP
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.IP \(bu
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the replacement string in double-quoted pattern substitution does not
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undergo quote removal, as it does in versions after bash-4.2
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.IP \(bu
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in posix mode, single quotes are considered special when expanding
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the \fIword\fP portion of a double-quoted parameter expansion
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and can be used to quote a closing brace or other special character
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(this is part of POSIX interpretation 221);
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in later versions, single quotes
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are not special within double-quoted word expansions
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.TP
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\fBcompat43\fP
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the shell does not print a warning message if an attempt is made to
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use a quoted compound assignment as an argument to declare
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(declare -a foo='(1 2)'). Later versions warn that this usage is
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deprecated
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.IP \(bu
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word expansion errors are considered non-fatal errors that cause the
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current command to fail, even in posix mode
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(the default behavior is to make them fatal errors that cause the shell
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to exit)
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.IP \(bu
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when executing a shell function, the loop state (while/until/etc.)
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is not reset, so \fBbreak\fP or \fBcontinue\fP in that function will break
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or continue loops in the calling context. Bash-4.4 and later reset
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the loop state to prevent this
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.TP
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\fBcompat44\fP
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.IP \(bu
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the shell sets up the values used by
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.SM
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.B BASH_ARGV
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and
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.SM
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.B BASH_ARGC
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so they can expand to the shell's positional parameters even if extended
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debug mode is not enabled
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.IP \(bu
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a subshell inherits loops from its parent context, so \fBbreak\fP
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or \fBcontinue\fP will cause the subshell to exit.
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Bash-5.0 and later reset the loop state to prevent the exit
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.IP \(bu
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variable assignments preceding builtins like \fBexport\fP and \fBreadonly\fP
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that set attributes continue to affect variables with the same
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name in the calling environment even if the shell is not in posix
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mode
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.TP
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\fBcompat50\fP
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.IP \(bu
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Bash-5.1 changed the way
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.SM
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.B $RANDOM
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is generated to introduce slightly
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more randomness. If the shell compatibility level is set to 50 or
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lower, it reverts to the method from bash-5.0 and previous versions,
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so seeding the random number generator by assigning a value to
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.SM
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.B RANDOM
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will produce the same sequence as in bash-5.0
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