What does `bash -` do?-CodePudding

I recently saw the following line:

curl -fsSL https://deb.nodesource.com/setup_17.x | bash -

What is the difference between bash and bash -? I tried running both variants, and they seem to do the same thing.

man bash didn't supply an answer either.

CodePudding user response：

The commands are identical.

The author probably believes that passing - as an argument will make bash read commands from stdin but bash will do that anyway.

In fact, the bash man-page explains that - is equivalent to -- and terminates option processing:

--    A  --  signals the end of options and disables further option
      processing.  Any arguments after the -- are treated as  file‐
      names and arguments.  An argument of - is equivalent to --.

Note that if there was something after the -, then behaviour might be different. For example:

$ echo "echo foo" > ./-c
$ chmod  x ./-c
$ export PATH=.:"$PATH"
$ bash   -c "echo bar"
bar
$ bash - -c "echo bar"
foo
$

Note also, it is not the case that using bash - makes it a login shell. This can be demonstrated by adding something printable to ~/.bash_profile:

$ export HOME=$(mktemp -d)
$ cd
$ echo "echo login shell" > .bash_profile
$ echo "echo hello" | bash
hello
$ echo "echo hello" | bash -
hello
$ echo "echo hello" | bash -l
login shell
hello
$

The meaning in the manpage of "A login shell is one whose first character of argument zero is a -" is that the command name starts with a hyphen:

$ cat .bash_profile
echo login shell
$ ln -s /bin/bash ./notLoginShell
$ ln -s /bin/bash ./-isLoginShell
$ export PATH=~:"$PATH"
$ echo "echo hello" | notLoginShell
hello
$ echo "echo hello" | -isLoginShell
login shell
hello
$

CodePudding user response：

It actually supplied an answer in man bash, but tricky.

For bash -, it is a login shell:

A login shell is one whose first character of argument zero is a -,...

For bash, it is an interactive shell:

An interactive shell is one started without non-option arguments and without the -c option whose standard input and error are both connected to terminals (as determined by isatty(3)), or one started with the -i option.

So the difference is the shell "type".