10

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

Kusalananda
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syss
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  • 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. – jimmij Feb 11 '19 at 12:53
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    You can also reference arguments in next command https://stackoverflow.com/a/3371299 – eckes Feb 11 '19 at 17:30

3 Answers3

12

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
Kusalananda
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  • Is it possible to achieve the effect with quoting, or would that still not work? – Time4Tea Feb 11 '19 at 11:20
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    Oh no, because you'd still have two commands on one line. – Time4Tea Feb 11 '19 at 11:21
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    @Time4Tea One command, in fact, with the wrong arguments. – Kusalananda Feb 11 '19 at 11:23
  • When you say "should", how would one be able to force multiple words? Or is it a "must"? – syss Feb 11 '19 at 13:11
  • @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. – Kusalananda Feb 11 '19 at 13:12
  • Another example similar to the "mkdir"-example, but with an empty expression and some whitespace is mkdir test{,2," three",\ four} (creates directories "test", "test2", "test three", "test four") – Freddy Feb 11 '19 at 13:40
  • If the brace expansion were performed at command line level instead of word level, that command would've made perfect sense; it just isn't and that's all that can be said about it. Simulating such a behavior with an eval: eval {'chown http','chmod 0666'}' /tmp/foo;'. –  Feb 11 '19 at 15:13
  • @mosvy Yes, with eval and with very careful quoting, it would be possible to do what the user wanted. – Kusalananda Feb 11 '19 at 15:22
  • @Kusalananda and wrong command name, as it'd be searching for an executable called "chown httpd" in PATH. – JoL Feb 11 '19 at 16:29
  • @JoL I don't think so actually. You would have to quote it en extra time for that. eval {'"chown http"', ...} – Kusalananda Feb 11 '19 at 16:31
  • @Kusalananda I just tried {"chown httpd","chmod 700"} /dir/test1 and got bash: chown httpd: command not found. – JoL Feb 11 '19 at 16:33
  • @JoL You left the eval out. – Kusalananda Feb 11 '19 at 16:33
  • @Kusalananda Ah. I was initially responding to your "One command [...] with the wrong arguments" comment, which responded to a discussion on simply using quotes. I hadn't seen the later discussion on eval. – JoL Feb 11 '19 at 16:36
  • @JoL :-) that explains it. In that case, you are absolutely right. – Kusalananda Feb 11 '19 at 16:37
  • Worth noting, that if the goal is to shorten the command OP has to type, they could reuse bash's history expansion to use the last command line argument as in !#:1. https://askubuntu.com/a/1109025/295286 – Sergiy Kolodyazhnyy Feb 11 '19 at 22:12
  • @Kusalananda I wasn't suggesting to use eval in this case, I was trying to illustrate what would happen in a hypothetical shell where the brace expansion is performed at command line level -- a command line would be turned into 2 or more command lines and that usage would make perfect sense. My point is that your claim that the command would've been nonsensical in any case is begging the question by already assuming a bash-like shell in which that simply doesn't work, whether sensical or nonsensical. –  Feb 12 '19 at 08:16
6

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}
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    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 – syss Feb 11 '19 at 13:10
  • 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. –  Feb 11 '19 at 13:39
  • Why then the line VAR={1..10} doesn't perform the brace expansion? – LRDPRDX May 31 '20 at 14:45
  • 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. – LRDPRDX Jun 04 '20 at 13:06
1

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
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