On Fri, 26 Aug 2022 at 18:02, <to...@tuxteam.de> wrote: > On Fri, Aug 26, 2022 at 08:17:10AM +0100, Tim Woodall wrote:
> > $ cat Makefile.test > > SHELL := /bin/bash -u > > MAKEFLAGS += --no-builtin-rules > > .SUFFIXES: > > > > all: > > echo done > > $ make -f Makefile.test > > echo done > > done > > $ make -f Makefile.test | cat > > echo done > > /etc/bash.bashrc: line 7: PS1: unbound variable > ^^^^^^^^^^^ > > Why do I get that unbound variable error? (I know how to fix it, I just > > don't understand why it only happens when I pipe the output) Hi, I'm no expert, but I would explain this as follows. > > $ make -f Makefile.test In the above command, make runs an interactive shell, because its standard output is connected to a terminal. > > $ make -f Makefile.test | cat Whereas in the above command, make runs a non-interactive shell, because it is in a pipeline so its standard output is not connected to a terminal, but rather to the standard input of the 'cat' command. PS1 is unbound in the second case because when the shell starts in non-interactive mode, it does not set PS1. The behaviour of interactive shells is explained in the three pages linked here: https://www.gnu.org/software/bash/manual/html_node/Interactive-Shells.html You can find expert responses on this topic by asking the mailing list: https://lists.gnu.org/mailman/listinfo/help-bash > [ -z "${PS1:-}" ] && return > > But I'll defer to more shell-savvy people for a proper fix. Regarding the side discussion of how to test whether a bash shell is interactive, both bash and dash shells provide the 'i' flag inside the special variable $- (hyphen) for precisely this purpose. So I would be inclinded to use that, rather than the old way of examining $PS1. Unlikely, but possible, that some bizarre user has unset their $PS1 variable. Some bash project documentation on this topic is here: https://www.gnu.org/software/bash/manual/html_node/Is-this-Shell-Interactive_003f.html
