What does 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 bash
or xargs bash
?
curl --silent --location https://rpm.nodesource.com/setup | bash -
When in doubt, read the source code. =)
Bash 4.3, shell.c
line 830, in function parse_shell_options()
:
/* 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[0] == '-' &&
(arg_string[1] == '\0' ||
(arg_string[1] == '-' && arg_string[2] == '\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
.
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 bash
.
To further illustrate that -
does not mean stdin, consider:
The 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/date
then /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.
An argument of - is equivalent to --.
Nov 28, 2015 at 14:12
xargs
instead, it would work (in the limited scenario of a small enough input script) with | xargs bash -c
; but really, this is not a useful or idiomatic use of xargs
.
The intention of the command is to pipe the output from the curl command to bash so it immediately executes commands returned by curl. Basically it tells bash to get its input from stdin. See https://superuser.com/a/1494190/234355
However, there is some discussion about whether this actually works or not in that same answer.
If you just execute the first part of the command in the question curl --silent --location https://rpm.nodesource.com/setup
, it will output a valid bash script. I didn't try it, not wanting to execute some unknown script from the internet.