bash - mean in the following bash shell code? It seems to be used to take the output the last code as the input. If so, can I just write it as
curl --silent --location https://rpm.nodesource.com/setup | bash -
When in doubt, read the source code. =)
/* A single `-' signals the end of options. From the 4.3 BSD sh. An option `--' means the same thing; this is the standard getopt(3) meaning. */ if (arg_string == '-' && (arg_string == '\0' || (arg_string == '-' && arg_string == '\0'))) return (next_arg);
In other words, the
- is saying that there are no more options. If there were any more words on the command line, they would be treated as a filename, even if the word started with a
In your example, of course, that
- is completely redundant, as there is nothing following it anyway. In other words,
bash - is exactly equivalent to
Bash takes its commands
It is a misconception that
bash - tells Bash to read its commands from its standard input. While it is true that in your example, Bash will read its commands from stdin, it would have done so regardless of whether there was a
- on the command line, because, as stated above,
bash - is identical to
To further illustrate that
- does not mean stdin, consider:
cat command is designed to interpret a
- as stdin. For example:
$ echo xxx | cat /etc/hosts - /etc/shells 127.0.0.1 localhost xxx # /etc/shells: valid login shells /bin/sh /bin/dash /bin/bash /bin/rbash /bin/zsh /usr/bin/zsh /usr/bin/screen /bin/tcsh /usr/bin/tcsh /usr/bin/tmux /bin/ksh93
In contrast, you can't get Bash to execute
/bin/hostname by trying this:
$ echo date | bash - hostname /bin/hostname: /bin/hostname: cannot execute binary file
Rather, it tries to interpret
/bin/hostname as a shell script file, which fails because it's a bunch of binary gobbledygook.
You can't execute
date +%s using
bash - either.
$ date +%s 1448696965 $ echo date | bash - Sat Nov 28 07:49:31 UTC 2015 $ echo date | bash - +%s bash: +%s: No such file or directory
Can you write
xargs bash instead? No.
curl | xargs bash would invoke bash with the contents of the script as command-line arguments. The first word of the contents would be the first argument, and it would likely be misinterpreted as a script filename.