An Operating Systems Vade Mecum

242 The User Interface Chapter 7Process manipulation gains a new dimension when processes invoked from thecommand interpreter can sprout trees. What does it mean to ‘‘stop’’ a process? Sincethe command interpreter might not even know that the tree exists, it is likely that the onlyprocesses that can be directly mentioned to the command interpreter are the ‘‘top-level’’ones, the ones that it started. If one of those is stopped, it is reasonable for the kernel tostop its entire tree. Alternatively, commands might allow the user to direct attention toany process in the tree and to manipulate just that process.Interactive users often have two distinct modes of computer usage.Highly interactive, as is exhibited during text editingBatch-like, as is exhibited during compilation or while running long programs.While a batch-like program is working, the user might well want to engage in someother, highly interactive work in order not to waste human time just because the computeris busy on the last command. To serve this need, many operating systems provide abackground facility that allows a process to continue execution but to have the commandinterpreter resume without waiting for it to end. The command interpreter can nowbe used to start another process, perhaps a more interactive one.Some operating systems, like RT-11 for the DEC PDP-11, allow at most one backgroundprocess and one foreground (higher-priority, interactive) process. Other operatingsystems allow any number of background processes. (In VMS, the administrator maylimit the number of background processes each user may have; in Unix, the limit is builtinto the installation.) It is even possible with some command interpreters to change thestatus of a process back and forth from background to foreground. (BSD4.2 Unix hasthis ability.) Background processes are usually not affected by the ‘‘cancel’’ or ‘‘stop’’keys that the user might type. Instead, the command interpreter often provides explicitcommands that have the same effect.1.4 Advanced featuresThe list of command-language features supported under various operating systems isquite long; we cannot hope to cover all of them. However, a few words of introductionto some of the novel features appearing in relatively recent operating systems may beuseful.Wild cards.We mentioned in the chapter on files that some programs accepta naming convention for files that allows several files to be specified with one name byhaving ‘‘don’t care’’ or ‘‘wild card’’ entries in the name. The Transfer utility, for example,might understand * in a file-name component to stand for anything. Thus,transfer #list traffic.* might generate a list of all files that have ‘‘traffic’’in their first component.The wild-card facility can be embedded in various places in the operating system.