UNIX for Dummies Questions & Answers

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.

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

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})

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.

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.