Why are bash brace expansions not working for commands?

Question

Why is this working:

mkdir /dir/test{1,2,3}

and this not?

{chown httpd,chmod 700} /dir/test1
-bash: {chown: command not found

My Bash Version is: GNU bash, version 4.2.46(2)-release

• jimmij: Perhaps tee + xargs can help if you have a very long path which you don't want to repeat each time: tee >(xargs chown httpd) >(xargs chmod 700) <<< /dir/test1.
• eckes: You can also reference arguments in next command stackoverflow.com/a/3371299

Answer 1

Your brace expansion is not valid. A brace expansion must be one word in the shell.

A word is a string delimited by unquoted spaces (or tabs or newlines, by default), and the string {chown httpd,chmod 700} consists of the three separate words {chmod, http,chmod and 700} and would not be recognised as a brace expansion.

Instead, the shell would interpret the line as a {chown command, executed with the arguments http,chmod, 700} and /dir/test1.

The simplest way to test this is with echo:

$ echo {chown httpd,chmod 700} /dir/test1
{chown httpd,chmod 700} /dir/test1
$ echo {"chown httpd","chmod 700"} /dir/test1
chown httpd chmod 700 /dir/test1

Note that even if your brace expansion had worked, the command would have been nonsensical.

Just write two commands,

chown http /dir/test1
chmod 700 /dir/test1

• Time4Tea: Is it possible to achieve the effect with quoting, or would that still not work?
• Time4Tea: Oh no, because you'd still have two commands on one line.
• Kusalananda: @Time4Tea One command, in fact, with the wrong arguments.
• syss: When you say "should", how would one be able to force multiple words? Or is it a "must"?
• Kusalananda: @syss It's "must" (I will change it in the text). A brace expansion expands to a set of words. If the brace expansion itself contains spaces, then these have to be quoted, as I have shown.

Answer 2

because, as mentioned in the man page, bash will perform the brace expansion on each word after splitting a command line into words.

So, that command line will be first split into {chown, httpd,chmod and 700}, and then, since {chown is not a valid brace expansion pattern, it will be left as is and bash will try to run a command with that name.

This is the quote from the manpage:

Expansion is performed on the command line after it has been split into words. There are seven kinds of expansion performed: brace expansion, tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, word splitting, and pathname expansion.

Notice the order, which is different from other shells (in zsh, the brace expansion will be performed after the arithmetic expansion, and the extra word splitting won't be performed at all).

The following will print 1 2 in zsh or ksh, and x y in bash:

f=; f1=x; f2=y; echo $f{1,2}

• syss: when you say "Notice the order" on the expansions performed, brace expansion comes before word splitting. And in your first line you say that brace expansion comes after word splitting. I am confused by that
• user313992: that's an extra word splitting which is done just before path expansion (globbing). if the a variable contains the string x y, then a command line like echo $a will be 1st split into echo and $a, then $a will be expanded into x y, and then split again into x and y, giving echo, x and y as separate arguments. The latter step will be done using the value of IFS ( not necessarily containing spaces) and does not happen in zsh.
• LRDPRDX: Why then the line VAR={1..10} doesn't perform the brace expansion?
• LRDPRDX: The answer of my question above is: The text after the ‘=’ in each variable assignment undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before being assigned to the variable from bash reference guide.

Answer 3

Other answers have explained why the brace expansion doesn't work. Ignoring that question for a moment, you probably want to avoid repeating the filename, and there are other ways to do that. Either assign the file name to a variable, or use the $_ special variable (it contains the last shell word of the previous command):

f="some long and ugly filename"
chown httpd "$f"
chmod 700 "$f"

or

chown httpd "some long and ugly filename"
chmod 700 "$_"

• ilkkachu: history expansion would also be an option, I just can't remember the correct expansion