I can't print -- (two endashes) characters using bash printf function. When I execute the following code
printf "--"
I get the error message saying
printf: usage: printf [-v var] format [arguments]
Why is this happening? And how can I print them?
Use printf -- "--" (or printf -- --; the quotes are not needed in this case).
printf is a built-in command in bash, so a plain printf executes the built-in command rather than the system /usr/bin/printf. The bash man page (for GNU bash 3.2.57 on macOS 15.2) says “Unless otherwise noted, each builtin command documented in this section as accepting options preceded by - accepts -- to signify the end of the options.” Since printf accepts an option starting with - (it accepts a -v option), it accepts -- to signify the end of the options.
Thus the first -- in printf -- "--" tells printf to stop interpreting arguments as possible options, so the next argument must be the format string.
The bash man page says the syntax for printf is:
printf[-vvar]format[arguments]
where […] denotes something optional. If this were the true syntax for printf, then clearly the -- in printf -- must be the format, because it is not the -v. So the above is not the actual syntax for printf. It is modified by the earlier text that says commands accept -- to signify the end of options.
Similarly, printf -x would take -x as the format string, but instead it yields an error, “-x: invalid option”. In this case, I do not see any text in the bash man page that says this can happen. (This is a deficiency in the bash documentation; it fails to specify the actual behavior of the printf command, and likely others.)
What is happening here is that - is a ubiquitous character for introducing “switches”1 in Unix commands (whether standalone or built into a shell). So, while it is not documented, the true syntax of many commands is that arguments beginning with a - are interpreted as a user attempt to specify a switch. Then, if the switch is not recognized by the command, an error is issued. However, if we execute printf -- -x, then -- says not to consider following arguments as possible options, so -x is taken as the format string, and “-x” is printed.
1 Switches are arguments that specify settings or introduce supplementary information to a command, rather than being the main things the command operates on.