bash is fully compatible with the 1992 POSIX standard. It has several of the most important Korn shell features and the C shell features that the Korn shell has appropriated, including aliases, functions, tilde notation, emacs and vi editing modes, arithmetic expressions, job control, etc.

The overlap of features between bash and the Korn shell has increased in recent years. It includes many ksh93 features. But it is not an exact ksh clone. The bash FAQ, published monthly by Chet Ramey, lists the following differences between bash and ksh93. Items enclosed in square brackets ([...]) are listed in this book, but not in the FAQ.

The following new things in ksh93 are not available in bash 2.05:

• Associative arrays

• Floating-point arithmetic and variables

• Math library functions

• ${!name[sub]} name of subscript for associative array

• "." is allowed in variable names to create a hierarchical namespace

• More extensive compound assignment syntax

• Discipline functions

• sleep and getconf built-ins (bash has loadable versions)

• typeset -n and nameref variables

• The KEYBD trap

• The variables: .sh.edchar, .sh.edmode, .sh.edcol, .sh.edtext, .sh.version, .sh.name, .sh.subscript, .sh.value, HISTEDIT [The .sh.match variable also]

• Backreferences in pattern matching (\N)

• The & operator in pattern lists for matching

• print -f (bash uses printf)

• fc has been renamed to hist

• The dot command (.) can execute shell functions

• Exit statuses between 0 and 255

• The set -o pipefail option

• The += variable assignment operator

• TMOUT is default timeout for read and select

• <&n- and >&n- redirections (combination dup and close) [Here-strings with <<<]

• FPATH and PATH mixing

• getopts -a

• The -R invocation option

• DEBUG trap now executed before each simple command, instead of after

• The printf %H, %P, %T modifiers, and an output base for %d [Also %Z.]

The following new things in ksh93 are present in bash 2.05:

• The for ((...;...;...)) ; do list; done arithmetic for command

• The ?:, ++, --, and comma (,) arithmetic operators

• The shell variable expansions: ${!param}, ${param:offset[:len]}, ${param/pat[/str]}, ${!param*}

• Compound array assignment

• The ! reserved word

• Loadable built-ins -- but ksh uses builtin while bash uses enable

• The command, builtin, and disown built-ins

• New $'...' and $"..." quoting

• FIGNORE (but bash uses GLOBIGNORE), HISTCMD

• set -o notify, set -C

• Changes to kill built-in

• read -A (bash uses read -a)

• read -t/read -d

• trap -p

• exec -a/exec -c

• The dot command (.) restores the positional parameters when it completes

• The test command conforms to POSIX.

• umask -S

• unalias -a

• Command and arithmetic substitution performed on PS1, PS4, and ENV

• Command name completion

• ENV processed only for interactive shells

bash has many features of its own that make it a very powerful and flexible environment. Here are some of the highlights: