Cameron Newham [Cameron Newham] - Learning the bash Shell, 3rd Edition

• The standard Bourne shell, as a kind of baseline

• The IEEE POSIX 1003.2 shell Standard, to which bash adheres and other shells will adhere in the future

• The Korn shell ( ksh ), a popular commercial shell provided with many UNIX systems

• pdksh , a widely used public domain Korn shell

• zsh , a popular alternative to bash and the Korn shell

bash is almost completely backward-compatible with the Bourne shell. The only significant feature of the latter that bash doesnt support is ^ (caret) as a synonym for the pipe (|) character. This is an archaic feature that the Bourne shell includes for its own backward compatibility with earlier shells. No modern UNIX version has any shell code that uses ^ as a pipe.

To describe the differences between the Bourne shell and bash , well go through each chapter of this book and enumerate the features discussed in the chapter that the Bourne shell does not support. Although some versions of the Bourne shell exist that include a few bash features,[] we refer to the standard Bourne shell that has been around for many years.

The cd - form of the cd command; tilde ( ~ ) expansion; the jobs command; the help built-in.

All. (That is, the Bourne shell doesnt support any of the readline , history, and editing features discussed in this chapter.)

Aliases; prompt string customization; set options. The Bourne shell supports only the following: -e , -k , -n , -t , -u , -v , -x , and - . It doesnt support option names ( -o ). The shopt built-in. Environment files arent supported. The following built-in variables arent supported:

Functions; the type command; the local command; the ${#parameter} operator; pattern-matching variable operators ( % , %% , # , ## ). Extended pattern matching. Command-substitution syntax is different: use the older `command` instead of $(command ). The built-in pushd and popd commands.

The ! keyword; the select construct isnt supported. The Bourne shell return doesnt exit a script when it is sourced with . (dot).

Use the external command getopt instead of getopts , but note that it doesnt really do the same thing. Integer arithmetic isnt supported: use the external command expr instead of the $((arithmetic-exp)) syntax. The arithmetic conditional ((arithmetic-exp)) isnt supported; use the old condition test syntax and the relational operators -lt , -eq , etc. Array variables are not supported. declare and let arent supported.

The command , builtin , and enable built-ins. The -e and -E options to echo are not supported. The I/O redirectors >| and <> are not supported. None of the options to read is supported. printf is usually available as an external command.

Job controlspecifically, the jobs , fg , and bg commands. Job number notation with % , i.e., the kill and wait commands only accept process IDs. The - option to trap (reset trap to the default for that signal). trap only accepts signal numbers, not logical names. The disown built-in.

The DEBUG, ERR, and RETURN fake signals are not supported. The EXIT fake signal is supported as signal 0.

The ulimit command and privileged mode arent supported. The -S option to umask is not supported. The Bourne shells restrictive counterpart, rsh , only inhibits assignment to PATH .

There have been many attempts to standardize UNIX. Hardware companies monolithic attempts at market domination, fragile industry coalitions, marketing failures, and other such efforts are the stuff of historyand the stuff of frustration.

Only one standardization effort has not been tied to commercial interests: the Portable Operating System Interface, known as POSIX. This effort started in 1981 with the /usr/group