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Bash Features and Versions

This document attempts to outline which versions of Bash introduced certain features.

There is no comprehensive, canonical table in the Bash documentation that outlines which features relate to which versions of Bash.

Compatibility

Compatibility is a huge concern when writing Bash scripts.

We tend to get into habits of using features that are available on the newest (or a newer) version of Bash. All of these features may not be available on older versions of Bash.

So, we need to be mindful of the syntax and features we're using if our scripts need to be run in different environments.

For example, if you're writing a script that needs to be run across a fleet of servers, and one of them is an old CentOS server that only has Bash v4.0, then there will be some features that are not available.

For example, if you're using mapfile to read the output of a command into an array: 

mapfile -t -d ':' < <(getent passwd)
This won't work on Bash 4.0, or even 4.2.

The mapfile builtin is introduced in Bash 4.0, but the -d DELIM option isn't introduced until Bash 4.3.

Bash Version Feature Matrix (Timeline)

Below is a list of versions and the features they introduced.

Bash 2.x

  • ** globstar (recursive globbing) -- introduced in 2.02, enabled with shopt -s globstar
  • source as synonym for . -- 2.0

Bash 3.0 (July 2004)

  • [[ string =~ regex ]] (regex matching)
  • += for arrays (arr+=(foo bar))
  • coproc (but only as keyword in 3.0, no coproc NAME yet)
  • declare -f -F (list functions by name)
  • ${var:offset:length} substring expansion (more POSIX-like)

Bash 3.1 (Dec 2005)

  • printf -v var fmt ...
  • += for string variables (var+=value)
  • COMP_WORDBREAKS and programmable completion improvements

Bash 3.2 (Oct 2006)

  • echo -e behavior changed (no longer expands \c by default)
  • PIPESTATUS array

Bash 4.0 (Feb 2009)

Major jump. Introduced a ton of features:

  • Associative arrays: declare -A
  • mapfile / readarray
  • coproc NAME (named coprocs)
  • ** globstar updated & stabilized

  • Case modification expansions:

    • ${var,,}: lowercase
    • ${var^^}: uppercase
    • ${var~}: toggle first char

  • ${parameter@Q} quoting operator

  • New case fall-through operators:

    • ;& → continue execution with next pattern
    • ;;& → test next pattern list

Bash 4.1 (May 2010)

  • assoc+=( [k]=v ) to append to associative arrays
  • declare -g (declare globals inside functions)
  • extglob enhancements (@(alt1|alt2) etc. more robust)

Bash 4.2 (Feb 2011)

  • lastpipe option: pipelines can run the last command in the current shell
  • **=() process substitution in array assignment
  • help -d summaries
  • read -d DELIM (delimiter) -- BUT careful: mapfile -d comes later
  • printf -v var extended with array indexing

Bash 4.3 (Feb 2014)

  • local -n (name references, like aliases for variables)
  • mapfile -d DELIM → custom delimiters
  • globasciiranges option
  • wait -n (wait for any job to finish)
  • compopt for programmable completion
  • ${var@a} / ${var@A} attribute expansion
  • New BASH_COMPAT variable to force older compatibility mode

Bash 4.4 (Sep 2016)

  • declare -n improvements (namerefs with attributes)
  • wait -n polished
  • PROMPT_DIRTRIM support
  • help -m (man-style help formatting)
  • printf -v supports indexed arrays
  • @Q quoting expansion extended

Bash 5.0 (Jan 2019)

  • shopt -s localvar_inherit → local vars inherit attributes
  • globstar now follows symlinks by default
  • New history -d syntax (history -d offset [count])
  • Changes in how associative arrays iterate
  • wait -n extended to take PIDs

Bash 5.1 (Dec 2020)

  • EPOCHSECONDS and EPOCHREALTIME variables
  • BASH_ARGV0 variable (script name override)
  • getopts enhanced: supports long options if getopts_long patch applied
  • ${var@E} expansion (expand to re-expandable text)

Bash 5.2 (Sep 2022)

  • New SRANDOM variable (secure random generator)
  • history -d -w fixes
  • New completion API hooks
  • Case modification supports Unicode properly
  • $PS0 prompt string

Feature Table

Below is a table that contains the feature and its corresponding version.

Feature Syntax / example First in Notes for detection in scripts
Extended regex match in [[ ... ]] [[ s =~ re ]] 3.0 Gate if you use =~ or BASH_REMATCH.
set -o pipefail set -o pipefail 3.0 Useful for robust pipelines.
Brace sequence expansion {1..5}, {a..z} 3.0 Zero-pad also in 4.0; but basic sequence is 3.0.
printf -v printf -v var "%s" val 3.1 Assign formatted output without subshells.
nocasematch (shopt) shopt -s nocasematch 3.1 Case-insensitive [[ ]] / case.
Associative arrays declare -A map; map[k]=v 4.0 Arrays keyed by strings.
coproc coproc NAME { cmd; } 4.0 Background co-processes (ksh-like).
mapfile / readarray mapfile -t a < file 4.0 Bulk read lines into arrays.
globstar (recursive **) shopt -s globstar; echo **/*.py 4.0 Off by default.
PROMPT_DIRTRIM PROMPT_DIRTRIM=2 4.0 Trims \w in prompt.
Case-modifying expansions ${v^^} ${var^}, ${var^^}, ${var,}, ${var,,} 4.0 Upper/lower-casing parameter expansions.
\|& Shorthand for 2>&1 \| -- make \|& tee log 4.0 Same as 2>&1 \|.
Append-both redirection cmd &>>file 4.0 Appends both stdout+stderr.
case fallthrough ;;& and ;& 4.0 zsh/ksh-style.
lastpipe (shopt) shopt -s lastpipe 4.2 Run last pipeline cmd in current shell (no subshell) if job control is off.
direxpand (shopt) shopt -s direxpand 4.2 Completes ~/vars to literal paths.
globasciiranges (shopt) shopt -s globasciiranges 4.3 [a-z] like C locale.
Namerefs local -n ref=NAME 4.3 “Pointers” to variables.
Negative array subscripts a[-1] 4.3 From the 4.3 cycle.
Mapfile record delimiter mapfile -d ':' -t a 4.4 -d (and -t pairs nicely).
New ${var@...} transforms ${v@Q}, ${v@L}, etc. 4.4 Family of parameter transformations.
EPOCHSECONDS / EPOCHREALTIME $EPOCHSECONDS 5.0 Monotonic epoch counters.
localvar_inherit (shopt) shopt -s localvar_inherit 5.0 Locals inherit previous scope.
wait -f wait -f %1 5.0 Wait until job/child terminates.
SRANDOM $SRANDOM 5.1 CSPRNG-backed 32-bit random.
wait -n improvements wait -n -p pidvar 5.1–5.3 -p added in 5.1; broader targets & behavior clarified in 5.2/5.3.
BASH_COMPAT tuning BASH_COMPAT=42 5.x Compatibility level toggles.
globskipdots (shopt) shopt -s globskipdots 5.2 Don't return ./.. unless explicit.
New command-sub form ${ command; } 5.3 Captures output without forking (implementation-dependent).

Sources