05-14-2010
Bash builtins really aren't that difficult code-wise. The parameter list is a simple linked list instead of an array. But you can't build them without building bash itself.
I've been experimenting with a bash builtin which creates arbitrary anonymous pipes... The behavior's interesting. Since the shell's holding it open, it doesn't block like a named pipe when you redirect into it. If it's not full, it doesn't block even when you write to it. This is of course the definition of a buffer, but it's interesting to have it happen when you haven't created your reader yet. Some useful concurrency could happen there. There might be some communication arrangements doable with this that'd be impossible with a named pipe since the shell can't open both ends at the same time.
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RBASH(1) General Commands Manual RBASH(1)
NAME
rbash - restricted bash, see bash(1)
RESTRICTED SHELL
If bash is started with the name rbash, or the -r option is supplied at invocation, the shell becomes restricted. A restricted shell is
used to set up an environment more controlled than the standard shell. It behaves identically to bash with the exception that the follow-
ing are disallowed or not performed:
o changing directories with cd
o setting or unsetting the values of SHELL, PATH, ENV, or BASH_ENV
o specifying command names containing /
o specifying a file name containing a / as an argument to the . builtin command
o specifying a filename containing a slash as an argument to the -p option to the hash builtin command
o importing function definitions from the shell environment at startup
o parsing the value of SHELLOPTS from the shell environment at startup
o redirecting output using the >, >|, <>, >&, &>, and >> redirection operators
o using the exec builtin command to replace the shell with another command
o adding or deleting builtin commands with the -f and -d options to the enable builtin command
o using the enable builtin command to enable disabled shell builtins
o specifying the -p option to the command builtin command
o turning off restricted mode with set +r or set +o restricted.
These restrictions are enforced after any startup files are read.
When a command that is found to be a shell script is executed, rbash turns off any restrictions in the shell spawned to execute the script.
SEE ALSO
bash(1)
GNU Bash-4.0 2004 Apr 20 RBASH(1)