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I ran into a problem I find rather strange and I hope some can explain it. I set up a simple bash for loop that iterates a few numbers and copies files that has each number two times in the file name. This fails when not using ${var_name} but succeeds when doing so. Here is a short example explaining what I mean:

$ touch 123.foo.123bar
$ touch 456.foo.456bar
$ mkdir out
$ for i in {123,456}; do cp $i.foo.$ibar out; done
cp: cannot stat `123.foo.': No such file or directory
cp: cannot stat `456.foo.': No such file or directory
$ for i in {123,456}; do cp ${i}.foo.${i}bar out; done
$ ls out
123.foo.123bar  456.foo.456bar

What is the reason that the first foor lopp fails while the second does not?

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3 Answers 3

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If you write $ibar, bash understands that as ${ibar}. Indeed, where should it end the variable name? (A dot is not allowed in a variable name, so $i. doesn't cause a problem.)

Another way to prevent the problem is to use backslashes:

cp $i.foo.$i\bar out
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5 Comments

But why does it work fine at the first use $i and not at the second? Does the first . make any difference? In my experience it is not required to use {} when iterating in a for loop like this (when using the variable once).
@Krøllebølle: . cannot be a part of a variable name. "b" can.
Yeah, that makes sense. for i in {123,456}; do cp $ifoo.$ibar out; done fails in the same manner. I guess I have been lucky so far. Thanks!
Quoting anything that is not a variable also works. $i.foo.$i"bar"
@Jidder: Or quoting the variable $i.foo."$i"bar :-)
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Your first example references the variable $ibar, which is not defined. The second one explicitly uses $i, which is what you want.

The variable name is expanded by bash until the first word separator, therefore $i.foo will be parsed as ${i}.foo, since . terminates the word.

However, $ibar only ends at the whitespace and bash will therefore look for ${ibar} in the scope.

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It is always a good practice to quote your arguments with variables on it around doublequotes ("") in a normal command to prevent unexpected word splitting and pathname expansion:

cp "$i.foo.$ibar" out

And if you find variables getting themselves adjacent to valid variable characters, the preferred solution would always be to use the ${} format to keep it isolated. Quoting (like cp "$i.foo.$i\bar" out) may give confusion depending on implementation:

cp "${i}.foo.${i}bar" out

Using ${} most of the time is not bad as well.

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4 Comments

Which one do you refer to exactly? If you mean the first surely it won't, yet.
cp "$i.foo.$ibar" out
Of course. I haven't mentioned anything about adjacent variable characters there yet. Refer to the second one which is the true answer.
Ahh, okay , it was just a bit misleading.

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