bash - brace expansion not expanding?

Question

On Kubuntu 15.10

echo $BASH_VERSION
4.3.42(1)-release

I try

reps=1
threads={1}{10..60..10}

for((r=0;r<$reps;r++))
do
    tCount=0
    for t in $threads
    do
        echo "t=$t, tCount=${tCount}"
        #do something funny with it
        ((tCount++))
    done
done

and it produces a single line

t={1}{10..60..10}, tCount=0

How do I get this to work?

edit

I expect

t=1, tCount=0
t=10, tCount=1
t=20, tCount=2
t=30, tCount=3
t=40, tCount=4
t=50, tCount=5
t=60, tCount=6

update

note that threads=({1}{10..60..10})

and then for t in ${threads[@]}

will prefix the 10..60..10 range with the string {1}

(i.e. {1}10,{1}20,..,{1}60 )

@hek2mg This prefixes ``10,20,..,60`` with ``{1}`` (i.e. ``{1}10,{1}20,..,{1}60``. Which is also the reason why this isn't a duplicate question. — User1291, Oct 31 '16 at 08:30
The `{1}` is simply not required. What's the reason behind using it? — hek2mgl, Oct 31 '16 at 08:33

score 4 · Accepted Answer · answered Oct 31 '16 at 08:31

4

The {1} expression is just a string, since it doesn't conform to the brace expansion syntax:

A sequence expression takes the form {X..Y[..INCR]}, where X and Y are either integers or single characters, and INCR, an optional increment, is an integer.

The brace expansion is performed before variable expansion, so you can't expand braces by just referring to a variable:

The order of expansions is: brace expansion; tilde expansion, parameter and variable expansion, arithmetic expansion, and command substitution (done in a left-to-right fashion); word splitting; and filename expansion.

Whether write the brace expansion directly, or use eval (usually not recommended).

Example:

tCount=0
for t in {1,{10..60..10}}; do
  echo "t=$t tCount=$tCount"
  (( tCount++ ))
done

answered Oct 31 '16 at 08:31

Ruslan Osmanov

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Thank you! Could you elaborate on why ``eval`` is "usually not recommended"? – User1291 Oct 31 '16 at 08:35
Command substitution should be used instead of `eval`: `threads=$(echo {10..60..10})` – hek2mgl Oct 31 '16 at 08:35
@hek2mgl or in this case, ``threads=({1,{10..60..10}})``. Again, please elaborate on *why* this "should" be used. – User1291 Oct 31 '16 at 08:36
That would produce an array. – hek2mgl Oct 31 '16 at 08:38
2

@User1291, [much has been written](http://stackoverflow.com/questions/17529220/why-should-eval-be-avoided-in-bash-and-what-should-i-use-instead) regarding the bad `eval`, just read these posts. – Ruslan Osmanov Oct 31 '16 at 08:40

bash - brace expansion not expanding?

1 Answers1