Bash is the GNU Project's shell. Bash is the Bourne Again SHell. Bash is an sh-compatible shell that incorporates useful features from the Korn shell (ksh) and C shell (csh). It is intended to conform to the IEEE POSIX P1003.2/ISO 9945.2 Shell and Tools standard. It offers functional improvements over sh for both programming and interactive use. In addition, most sh scripts can be run by Bash without modification.
major feature: This is a terse description of the new features added to bash-4.4 since the release of bash-4.3. As always, the manual page (doc/bash.1) is the place to look for complete descriptions. 1. New Features in Bash a. There is now a settable configuration #define that will cause the shell to exit if the shell is running setuid without the -p option and setuid to the real uid fails. b. Command and process substitutions now turn off the `-v' option when executing, as other shells seem to do. c. The default value for the `checkhash' shell option may now be set at compile time with a #define. d. The `mapfile' builtin now has a -d option to use an arbitrary character as the record delimiter, and a -t option to strip the delimiter as supplied with -d. e. The maximum number of nested recursive calls to `eval' is now settable in config-top.h; the default is no limit. f. The `-p' option to declare and similar builtins will display attributes for named variables even when those variables have not been assigned values (which are technically unset). g. The maximum number of nested recursive calls to `source' is now settable in config-top.h; the default is no limit. h. All builtin commands recognize the `--help' option and print a usage summary. i. Bash does not allow function names containing `/' and `=' to be exported. j. The `ulimit' builtin has new -k (kqueues) and -P (pseudoterminals) options. k. The shell now allows `time ; othercommand' to time null commands. l. There is a new `--enable-function-import' configuration option to allow importing shell functions from the environment; import is enabled by default. m. `printf -v var ""' will now set `var' to the empty string, as if `var=""' had been executed. n. GLOBIGNORE, the pattern substitution word expansion, and programmable completion match filtering now honor the value of the `nocasematch' option. (...)