I have a makefile where I'm stopping a service before removing a file. When it couldn't stop the service, it would break on error. This is clearly unwanted so I thought I'd add || true but missed a |. Making it:
stop service foo | true rm /etc/init/foo.conf I'm confused as to why this is working and what is happening. Does this mean that true is an application and not merely a keyword? Are they the same? Is there a good reason to use | true?
true and false are coreutils (also typically shell built-ins) that just return 0 and non-0, for situations where you happen to need that behavior. From the man pages:
true - do nothing, successfully
false - do nothing, unsuccessfully
So you're piping the output from stop service foo into true, which ignores it and returns 0. Technically it works, but you should probably use || true so it's obvious what your intention was; there's really no reason to pipe output into a program that's not using it
| true is that if the command produced enough output to fill up the pipe buffer, it would block waiting for true to read it. foo || true and foo | true won't do the same thing: foo || true will show the output from foo, whereas foo | true will discard everything foo writes to its standard output (and foo is likely to die with SIGPIPE or block, as already indicated).
trueis being ignored. The only side-effect is thatstdoutis redirected to nowhere in an unreliable way.