A.7 The Future of the Korn Shell

David Korn continues to enhance the Korn shell at AT&T Bell Labs. At this writing, a new release is in beta test-usually the final step before a piece of software is released. However, negotiations between AT&T and USL (now Novell UNIX Systems Group) over distribution rights could very well postpone the new shell's public release for a couple of years or more.

Nevertheless, the new Korn shell has significant enhancements that make it worth looking forward to. These features are subject to change between the time of this writing and the new shell's public release. Here are some highlights:

• The ability to customize key bindings, as in bash and pdksh but applicable to vi- as well as emacs-mode. This is implemented as another "fake signal" trap (on keystrokes), so it's extremely flexible.

• Many new customization variables and options.

• A greatly expanded set of string operators, providing substrings, substitutions, and other functionality.

• An enhanced array variable facility that provides for associative arrays, which can be addressed by their contents rather than by indices.

• Better prompt string customization capabilities (with command substitution and arithmetic expression evaluation).

• Floating point (real number) arithmetic.

• An arithmetic for loop in the style of the C programming language.

• A new option to print that allows C language printf()-style output formatting.

• The ability to add new built-in commands, on systems that support dynamic loading (the dlopen() system call).

• More user control over command lookup order, as in the POSIX standard and bash.

• The ability to set timed "alarms" (with the ALRM signal).

• Expanded debugging support through additional fake signal traps.

• Online help for built-in commands through a standard -? option.

The next release is expected to be incompatible with the 1988 Korn shell in a few ways, some of which are necessary for POSIX compliance (refer to the section on POSIX in this Appendix for more details):

• The alias command will never display tracked aliases unless you specify the option -t.

• Functions defined with fname() (the syntax not used in this book) will have weaker POSIX-compliant semantics. However, functions defined with function fname will remain the same.

• Tilde expansion can take place inside ${...}-style variable expressions, whereas in the 1988 Korn shell, tildes in these expressions are treated literally.

• ! will be a keyword, as it is in the POSIX standard.

• Command substitution and arithmetic expression evaluation (in addition to parameter expansion) will be performed on the variables PS1, PS3, and ENV when they are expanded. This will allow for more flexible customization of prompt strings and environment files. It also means that grave accents (`) and $( must be quoted to be taken literally.

• Output of the built-in commands set, typeset, and alias will change so that any words containing special characters are single-quoted-so that these commands' output can be used directly for input.

• A new expansion operator, $'...', is used to delimit strings that conform to the ANSI C language standard for character strings. In the 1988 Korn shell, $' causes the dollar sign to be treated literally; in future releases, it must be backslash-escaped.

• command will be a built-in command, as in POSIX.

• Command lookup order will change so that built-in commands will be treated as if they were installed in /bin, i.e., will be found at the same time as /bin is searched when the shell goes through your PATH. The rules will also change to comply with the POSIX standard that allows certain built-in commands to take precedence over functions.

• Signal traps will propagate to subshells (whether nested or shell scripts invoked from shells) until the subshell issues a trap command (on any signal). Currently, traps do not propagate to subshells; see Chapter 8.

• The built-in variable ERRNO will be dropped; exit statuses will reflect system call error numbers.

Finally, the following features are expected eventually to become obsolete:

• The command fc will be renamed hist; the built-in variable FCEDIT will be renamed HISTEDIT.

• The -t option to alias, set -h, and set -o trackall. Alias tracking will always be on.

• The -k or -o keyword option. This will always be off (see Chapter 3).

• Grave accents (`) for command substitution. Use $(...) instead.

• The -a (file existence) condition test will be renamed -e, to avoid clashing with the -a operator (logical "and") in the old test or [...] syntax.

• The = pattern-matching operator in [[...]] condition tests will be replaced by == for better syntactic alignment with the C language.

• The arithmetic comparisons -eq, -lt, etc. Use ((...)) instead.