56 lines
2.7 KiB
Plaintext
56 lines
2.7 KiB
Plaintext
SHELL DESIGN NOTES
|
|
------------------
|
|
|
|
The shell has few bells and whistles. It allows up to 128
|
|
backgrounded jobs (after this point you have to wait for some to exit,
|
|
because the table it uses to track these cannot be resized.)
|
|
|
|
The background jobs are tracked in an array of MAXBG pid_t's. If an
|
|
open slot is found, a background job's pid can be stashed there.
|
|
Background jobs can be collected using the "wait" built-in command,
|
|
which removes any pids whose exit status it collects from the
|
|
background jobs table.
|
|
|
|
The wait built-in command takes an optional argument, the process
|
|
id to wait for. The shell will attempt to wait for any process, not
|
|
just the ones it actually started as its own background jobs. However,
|
|
since no facility exists for looking up the pids of running processes,
|
|
this ability is not necessarily useful. If no argument is provided,
|
|
wait waits for all outstanding background jobs.
|
|
|
|
The shell uses WNOHANG if WNOHANG is defined, in which case
|
|
background jobs are polled after every command, like in Unix shells.
|
|
If WNOHANG is not defined, background jobs are polled only by user
|
|
request. In OS/161 2.0, WNOHANG is always defined in the kernel header
|
|
files, but the implementation is only suggested, not required. To make
|
|
the shell stop trying to use WNOHANG, patch it, or remove WNOHANG from
|
|
kern/wait.h.
|
|
|
|
There are two other built-in commands: chdir, which uses the chdir
|
|
system call to change directory, and can also be accessed as just cd,
|
|
and exit, which causes the shell to exit with a specified exit status
|
|
(0 if not supplied).
|
|
|
|
Note that all these built-in commands must be built into the shell
|
|
in order to work usefully.
|
|
|
|
The shell processes commands by reading lines and then splitting
|
|
them up into words using whitespace characters (space, tab, carriage
|
|
return, and newline) as separators. No punctuation characters are
|
|
interpreted, except for `&'. No variable substitution or argument
|
|
wildcard expansion ("globbing") is performed.
|
|
|
|
The `&' character, if present as the last word on a command line,
|
|
is treated as the "background" operator: the command is run as a
|
|
background job, that is, after starting it the shell immediately
|
|
prints another prompt and accepts more commands. Note that the `&'
|
|
must be preceded by whitespace to be recognized. The process id of the
|
|
background job is printed as it starts. Note that shell builtins
|
|
cannot be backgrounded; furthermore, because the OS/161 console does
|
|
not support job control, starting background jobs that perform
|
|
terminal input (or, to a lesser extent, terminal output) may produce
|
|
confusing and/or unwanted results.
|
|
|
|
The shell also supports the "sh -c COMMAND" syntax in the hopes
|
|
that it will be useful.
|