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The line editor ed in UNIX/Linux has a "command mode" and an "insert mode" and there is no visual way to tell which mode you are in. However, there is a -p option that causes it to display a prompt when you are in command mode, which is very helpful.
Why is the default to have no prompt? Masochism?
Just when you think this question has been put to bed It raises its head yet again with greater authority. This time, from the horses mouth - so to speak - Bill Joy himself. It is an interview with the UNIX Review in the web masoleum called archive.org:
Excerpt from an interview of Bill Joy of Sun Microsystems by Jim Joyce of UNIX Review. Entitled: Interview with Bill Joy the text appeared in the August 1984 issue of Unix Review magazine.
https://web.archive.org/web/20120210184000/http://web.cecs.pdx.edu/~kirkenda/joy84.html. Bill is being asked about how things evolved around him and here they have arrived at the subject of usability in the context of limited hardware capability at that time. They start out by joking that 'cat' is the most efficient editor for the job at that time ... ...
REVIEW: Real programmers use cat as their editor. JOY: That's right! There you go! It is too much trouble to say ed, because cat's smaller and only needs two pages of memory - plus you're not likely to get swapped out. That's why ed didn't prompt, you know. The performance of the system was just horrible. It would swap things out randomly and do all sorts of things. In ed you might type "a", but have no idea how far behind the system was. And it didn't matter, and long as it didn't get more that a few hundred characters behind and start throwing lines away without telling you.
Typically it wasn't that bad. If it had been prompting, you would have hit carriage return and wait for the prompt, and it would have taken three seconds to comment. That's something we noticed when we put em up. We put in the prompt and suddenly realized it had to go through the operating system.
I think UNIX has lived with grace for years. We've had the grace of people not being able to tell when the system was doing a bad job of scheduling the CPU. Now we can't hide behind time-sharing.
...
There's plenty more before this point in the article but it would be off-topic. So they had noticed that a prompt which was in the 'em' editor they were also working on, was vulnerable to UNIX swapper stack trashing and the prompt response queue was also too narrow to timeously notify the user ...
So they did not implement an 'ed' prompt on account of performance under UNIX multi-user time share loads, and also over hardware limitations of that era.
Having used ed years ago on printing terminals (such as teletypes and DECwriters), I think the reason for having no prompt was that on pressing RETURN after one command, you didn't have to wait for a prompt to be printed before starting to type the next command.
Similar considerations made it better for ed to have ? as the only error message: you would have worked out what the mistake was long before a longer error message had finished printing.
When I was a lad, the only reason ed was included at all in our environment is because it was used by the standard system scripts.
Originally, having to clean the 'prompts' out of ed output to clean up the data would have just made everything more difficult -- much more difficult.
Later, having ed default to something different would have just broken all the existing scripts.
sed that didn't have a write back to input file in the manual (big fat book). It may well have been there, but it wasn't in the manual.
However, there is a -p option that causes it to display a prompt when you are in command mode
The -p option is meant to set a different prompt. When used, a side-effect is that ed starts in addition with prompt enabled.
Why is the default to have no prompt?
There is a default, * (asterisk); it's just not switched on. Prompt display can be toggled by applying a P command (*1).
Ed is meant as well for automated (scripted) use (*2). Having the prompt displayed for each command entered is at least annoying; on a real TTY it will waste a lot of paper.
To display no prompt by default seems sensible in an early environment, doesn't it?
In addition, it might be good to keep in mind that programming back then, especially when done online, was handled mostly in the head, not on a screen or alike. When using ed, one had to have a proper image present. Printing the whole file or just parts thereof would eat up a lot of paper and even more important time. Listing just a page would easily take a minute or two.
With that background it's rather trivial to keep aware what mode one is in. After all, it's not complicated, as default is command and input is only activated on request.
*1 - The prompt is a rather new addition as Stephen Kitt mentioned.
*2 - I always loved the feature to 'open' any shell command as an input 'file'.
In fact, ed even predates next to all other Unix tools as well as basic output direction itself (which was a shell feature until pipes were added in V3 (?)). So the standard way to capture a program output was to 'open' it as ed input file and save it as text file:
$ ed
r !ls -l
w directory_listing
q
ed don’t support prompts at all, they’re a somewhat recent addition.
Dec 5, 2019 at 20:15
P at all, just looked it up a few minutes ago. Do you know when it was added?
The -p option is a late addition. I did some checking and the Seventh Edition man page for ed doesn't mention the -p option, but it does appear in the Ultrix 4.2 man page. Fifth Edition Unix also doesn't mention a -p option, but it does say that if ed is invoked as a login shell (i.e. the program name provided in argv[0] is "-") that it will enable a * prompt. This was presumably done in support of the early use of Unix as a text editing system.
I am guessing that the -p option was added to provide an easier way to enable a prompt without jumping through the required hoops to invoke ed with arg[0] set to "-". But by that time, I suspect that very few people were still using ed for interactive editing: vi/ex and Emacs would have taken over for most. If you were still using ed, you were either familiar with it already due to using it for many years (in which case you wouldn't need the prompt), or you were using it non-interactively (giving it input from a file) and you wouldn't have wanted prompts anyway.
P command was first found in UNIX System III. I think the -p option was first added in UNIX System V.
Mar 21, 2020 at 19:23
Looking at the previous answers I see everyone has missed an essential feature most system run level 1 commands, and that is that they were amenable to piping. The non-interactive mode is therefore the default in ed, ex, sed, awk, grep etc
TFTP (Trivial FTP) is also another good example of a stripped down version of FTP used in disk-less boot-up processes to boot from a remote network boot image. No user prompts are emitted as is done in the full-blown FTP.
Because of the universality of printing terminals when ed was current. An editor prompt on each data line would have consumed space on the paper, which would have cut that much off of what you could type into each line, and see. Printing terminals didn't autowrap, and typing blindly off the end of the line was Unpleasant. Even if they had autowrap, that would have consumed more paper, and reduced the correspondence (on paper) between what you had typed the first time, versus what you'd see if you then printed it again (sans prompts). (Conservation of paper was a significant issue, even if it was just to prevent you from becoming mired in an overlarge pile of paper from your session.)
A bit late in the day, but here's a timeless broad-brush non-explanation that quashes the OP question. It's from Queen Mary's College Univ of London - of all places.
http://www.eecs.qmul.ac.uk/~gc/history/
Someone asked for a synopsis; so I'll do it in little bits as per reader reactions. First the one-page story. Two personalities appear in the Unix Editors story but if one goes back a little (during MULTICS predecessor phase) one discovers more central actors. In this case it is George Coulouris (GC), part of the UK side of the MULTICS. Folklore (and GC confirms here) has it Ken Thompson (KT) wrote ed and was quite happy not seeing the final / intermediate editing result as he merely kept it in his head. Input was teletype and 'seeing' it meant teletype printing the buffer which was a distraction (as would be the printing of a prompt). The UK crowd had a small Amiga dedicated-cpu terminal/console being developed that provided more capability which GC exploited at Queen Mary College at Univ of London, and named it "ED for Mortals" as a rejoinder and in deference to KT's standing and Editor (ED). So the quibbles over return prompt take on a different level of importance for that time. And this was during MULTICS and KT was the author of the ED being discussed here while QC of EM.
Secondly, GC and his collaborators across the British Computer Science fraternity had begun addressing the editing into rapid response one character at a time as an improvement on the one line at a time on hitting 'return'. So QC's EM did one response per char and when he installed it at U of C Berkeley the sysadmins there who had visual displays rejected it as too expensive on their multi-user UNIX CPU. It was Bill Joy (BJ) the Pascal Compiler BSD (Berkeley Software Distribution) guy who showed GC around and eventually extended GC's EM into EX and then into EX's 'open buffer command' Vi with the facility of 'raw mode' recently made available in DEC Virtual Terminal devices. Until then teletypes were truly Dumb Terminals as IBMers called them.
I'll stop here to not spoil it for those that might want to browse the QMCL.ac.uk link for themselves.
Side Note: During QC's stint at Berkeley the hot thing going then, he says, was the creation / development of Ingress. I think IBM had not long before released the definition of RDBMS into the public domain. Remember, Ingress was the mother of Postgress, the Auntie of Sybase, the God Mother of Informix and Grand Mother of MS SQL Server ...
The other minor historical is that BSD (Berkeley Software Distribution with the Regents U o C Copyright Notice and all) as a distribution of UNIX was pretty much predated in the Software Distribution by Bill Joy of his Pascal Compiler. UNIX (more or less mainly him again) came later under his 'Frienzied' PhD years there adding plenty core tools into BSD UNIX including building the TCP/IP stack from scratch (a first ever) after throwing out the 'IMPS' buggy and RFC-outdated variant that came with AT&T UNIX.
There's a lot more that was happening in frenzied fashion at Berkerley. Another big intersection was Bill Joy with the goings on at MIT that eventually got James Gosling to join them years later at SUN Microsystems (R.I.P), to eventually put together the ultimate byte code interpreter Mother of all interpreters we now call JVM Java. Despite all the Lore one hears, Emacs as we know it might have never come to being were it not for that same JG.
Last nugget (could'nt resist): That UK Amiga was also the basis of all the ARM architecture design that rules the embedded and SoC worlds today.
