Can I use a variable with brace expansion?


 
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# 1  
Old 08-02-2011
Can I use a variable with brace expansion?

So, I was bored on the train today, and was thinking of ways to loop through elements of an array. I came up with the following simple script, but it doesn't work as brace expansion doesn't seem to work with variables. Is there something I'm missing, or does the shell just not work like this?

Code:
(18:33:24\[D@DeCoWork15)
[~]$ cat prac.sh
#!/bin/bash

x=(aa bb cc dd ee ff gg hh ii jj kk ll mm nn oo pp qq rr ss tt uu vv ww xx  yy zz)
CAP=${#x[*]}
echo $CAP
for i in {1..$CAP};do echo ${x[${i}]};done

(18:35:19\[D@DeCoWork15)
[~]$ bash prac.sh
26
prac.sh: line 6: {1..26}: syntax error: operand expected (error token is "{1..26}")

So, clearly it is recognizing the variable, but it's not expanding the 1..26. Any thoughts?

Also, I'm aware that this will leave me with the issue that I'm expanding 1-26 when what I really want is 0-25...but I'll burn that bridge later.
# 2  
Old 08-02-2011
Quote:
Originally Posted by bash man page
A sequence expression takes the form {x..y}, where x and y are either integers or single characters. When integers are supplied, the expression expands to each number between x and y, inclusive. When characters are supplied, the expression expands to each character lexicographically between x and y, inclusive. Note that both x and y must be of the same type.

Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual.
There is no 26 there when the brace expansion is attempted. The 1 and the literal string "$CAP" are an invalid sequence expression at that stage.

---------- Post updated at 07:20 PM ---------- Previous update was at 07:15 PM ----------

Brace expansion is an extension to the POSIX sh standard. Different posix-like shells perform it at different times. bash performs it before all other expansions while ksh leaves it for much later (after parameter expansion and command substitution and field splitting).

---------- Post updated at 07:46 PM ---------- Previous update was at 07:20 PM ----------

I forgot to mention, if you want to loop over the elements of an array, there's a much simpler way to do it:
Code:
for i in "${x[@]}"; do whatever; done

The * and @ subscripts are analogous to the $* and $@ special parameters used by the shell to handle script arguments.

Regards,
Alister
This User Gave Thanks to alister For This Post:
# 3  
Old 08-03-2011
Yes, Alister showed the right way to loop over an array. To expand the brace expression with variables, you can use eval:
Code:
for i in $(eval echo {1..$CAP})

# 4  
Old 08-03-2011
Quote:
Brace expansion is an extension to the POSIX sh standard. Different posix-like shells perform it at different times. bash performs it before all other expansions while ksh leaves it for much later (after parameter expansion and command substitution and field splitting).
I think it would be more accurate to say that the POSIX.1 standard is silent on the issue of sequence expansion in the shell command langauge.
# 5  
Old 08-03-2011
Quote:
Originally Posted by alister
There is no 26 there when the brace expansion is attempted. The 1 and the literal string "$CAP" are an invalid sequence expression at that stage.

---------- Post updated at 07:20 PM ---------- Previous update was at 07:15 PM ----------

Brace expansion is an extension to the POSIX sh standard. Different posix-like shells perform it at different times. bash performs it before all other expansions while ksh leaves it for much later (after parameter expansion and command substitution and field splitting).

---------- Post updated at 07:46 PM ---------- Previous update was at 07:20 PM ----------

I forgot to mention, if you want to loop over the elements of an array, there's a much simpler way to do it:
Code:
for i in "${x[@]}"; do whatever; done

The * and @ subscripts are analogous to the $* and $@ special parameters used by the shell to handle script arguments.

Regards,
Alister
Bah, you're right on both counts. I read the man page, and that part about the expansion being done first, but it just didn't register. I think what was throwing me off was that it **did** show the number 26 when printing the error. But if I'd just done bash -x on the script, I would have seen the issue. Many thanks.
 
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