/usr/bin/bash: Difference between revisions

Bash-4.0  introduced  the concept of a `shell compatibility level', specified as a set of options to the shopt builtin compat31, compat32, compat40, compat41, and so on).  There is only one  current compatibility  level  -- each option is mutually exclusive.  The compatibility level is intended to allow users to select behavior from previous versions that is incompatible  with  newer versions  while  they migrate scripts to use current features and behavior. It's intended to be a temporary solution.

This section does not mention behavior that is standard for a particular version  (e.g.,  setting compat32 means that quoting the rhs of the regexp matching operator quotes special regexp characters in the word, which is default behavior in bash-3.2 and above).

If a user enables, say, compat32, it may affect the behavior of other compatibility levels up  to and  including  the  current compatibility level.  The idea is that each compatibility level controls behavior that changed in that version of bash, but that behavior may have been  present  in earlier  versions.  For instance, the change to use locale-based comparisons with the [[ command came in bash-4.1, and earlier versions used ASCII-based comparisons, so  enabling  compat32  will enable ASCII-based comparisons as well.  That granularity may not be sufficient for all uses, and as a result users should employ compatibility levels carefully.  Read  the  documentation  for  a particular feature to find out the current behavior.

Bash-4.3  introduced  a  new shell variable: BASH_COMPAT.  The value assigned to this variable (a decimal version number like 4.2, or an integer corresponding to the compatNN option, like 42) determines the compatibility level.

Starting  with  bash-4.4, Bash has begun deprecating older compatibility levels.  Eventually, the options will be removed in favor of BASH_COMPAT.

Bash-5.0 is the final version for which there will be an individual shopt option for the previous version. Users should use BASH_COMPAT on bash-5.0 and later versions.

The  following  table  describes the behavior changes controlled by each compatibility level setting.  The compatNN tag is used as shorthand for setting the compatibility level to NN using  one of  the following mechanisms.  For versions prior to bash-5.0, the compatibility level may be set using the corresponding compatNN shopt option.  For bash-4.3 and later versions, the  BASH_COMPAT variable is preferred, and it is required for bash-5.1 and later versions.

;compat31

* quoting  the  rhs  of the [[ command's regexp matching operator (=~) has no special effect

;compat32

* interrupting a command list such as "a ; b ; c" causes the execution  of  the  next command  in  the  list (in bash-4.0 and later versions, the shell acts as if it received the interrupt, so interrupting one command in a list aborts the execution of the entire list)

;compat40

* the  <  and  >  operators to the [[ command do not consider the current locale when comparing strings; they use ASCII ordering.  Bash versions prior  to  bash-4.1  use ASCII  collation  and strcmp(3); bash-4.1 and later use the current locale's collation sequence and strcoll(3).

;compat41

* in posix mode, time may be followed by options and still be  recognized  as  a  reserved word (this is POSIX interpretation 267)

* in  posix  mode,  the parser requires that an even number of single quotes occur in the word portion of a double-quoted parameter expansion and treats them  specially, so  that  characters  within the single quotes are considered quoted (this is POSIX interpretation 221)

;compat42

* the replacement string in double-quoted pattern substitution does not undergo quote removal, as it does in versions after bash-4.2

* in posix mode, single quotes are considered special when expanding the word portion of a double-quoted parameter expansion and can be used to quote a closing brace  or other  special  character (this is part of POSIX interpretation 221); in later versions, single quotes are not special within double-quoted word expansions

;compat43

* the shell does not print a warning message if an attempt is made to  use  a  quoted compound  assignment as an argument to declare (declare -a foo='(1 2)'). Later versions warn that this usage is deprecated

* word expansion errors are considered non-fatal errors that cause the  current  command to fail, even in posix mode (the default behavior is to make them fatal errors that cause the shell to exit)

* when executing a shell function, the loop state (while/until/etc.)  is  not  reset, so  break  or continue in that function will break or continue loops in the calling context. Bash-4.4 and later reset the loop state to prevent this

;compat44

* the shell sets up the values used by BASH_ARGV and BASH_ARGC so they can expand  to the shell's positional parameters even if extended debugging mode is not enabled

* a  subshell inherits loops from its parent context, so break or continue will cause the subshell to exit.  Bash-5.0 and later reset the loop state to prevent the exit

*variable assignments preceding builtins like  export  and  readonly  that  set  attributes continue to affect variables with the same name in the calling environment even if the shell is not in posix mode

;compat50

* Bash-5.1 changed the way $RANDOM is generated to introduce  slightly  more  randomness.  If  the  shell  compatibility level is set to 50 or lower, it reverts to the method from bash-5.0 and previous versions, so seeding the random number  generator by assigning a value to RANDOM will produce the same sequence as in bash-5.0

* If  the command hash table is empty, bash versions prior to bash-5.1 printed an informational message to that effect, even when producing output that can  be  reused as input. Bash-5.1 suppresses that message when the -l option is supplied.