B.1 Implementation Differences From The SVR4.2 Shell

Since Bash is a completely new implementation, it does not suffer from many of the limitations of the SVR4.2 shell. For instance:

• Bash does not fork a subshell when redirecting into or out of a shell control structure such as an if or while statement.

• Bash does not allow unbalanced quotes. The SVR4.2 shell will silently insert a needed closing quote at EOF under certain circumstances. This can be the cause of some hard-to-find errors.

• The SVR4.2 shell uses a baroque memory management scheme based on trapping SIGSEGV. If the shell is started from a process with SIGSEGV blocked (e.g., by using the system() C library function call), it misbehaves badly.

• In a questionable attempt at security, the SVR4.2 shell, when invoked without the `-p' option, will alter its real and effective UID and GID if they are less than some magic threshold value, commonly 100. This can lead to unexpected results.

• The SVR4.2 shell does not allow users to trap SIGSEGV, SIGALRM, or SIGCHLD.

• The SVR4.2 shell does not allow the IFS, MAILCHECK, PATH, PS1, or PS2 variables to be unset.

• The SVR4.2 shell treats `^' as the undocumented equivalent of `|'.

• Bash allows multiple option arguments when it is invoked (-x -v); the SVR4.2 shell allows only one option argument (-xv). In fact, some versions of the shell dump core if the second argument begins with a `-'.

• The SVR4.2 shell exits a script if any builtin fails; Bash exits a script only if one of the POSIX 1003.2 special builtins fails, and only for certain failures, as enumerated in the POSIX 1003.2 standard.

• The SVR4.2 shell behaves differently when invoked as jsh (it turns on job control).