Using a variable in brace expansion range fed to a for loop

Question

Here is myscript.sh

#!/bin/bash
for i in {1..$1};
do
    echo $1 $i;
done

If I run myscript.sh 3 the output is

3 {1..3}

instead of

3 1
3 2
3 3

Clearly $3 contains the right value, so why doesn't for i in {1..$1} behave the same as if I had written for i in {1..3} directly?

Mingmingche

Your example script will work in both ksh93 and zsh. – Phio

Minor thing: You wrote $3 ("Clearly $3 contains ...") but are using $1 in your script. Why not just remove the $1? It doesn't have that much relevance except to prove that variable expansion works. – Diachronic

Answer 1

You should use a C-style for loop to accomplish this:

for ((i=1; i<=$1; i++)); do
   echo $i
done

This avoids external commands and nasty eval statements.

Smiga

Not working for me test.sh: line 1: ((: i<=: syntax error: operand expected (error token is "<=") – Goda

@Goda are you using bash? Is your shebang #!/bin/bash or #!/bin/sh? – Smiga

@jordann, file contains only these 3 lines. I'm using GNU bash, version 4.3.11(1)-release (x86_64-pc-linux-gnu). I run script bash test.sh – Goda

@Goda you need to run it like this: bash test.sh 12. The $1 is a positional arg. – Smiga

Yes, I haven't noticed. Sorrry:) – Goda

Answer 2

Because brace expansion occurs before expansion of variables. http://www.gnu.org/software/bash/manual/bashref.html#Brace-Expansion.

If you want to use braces, you could so something grim like this:

for i in `eval echo {1..$1}`;
do
    echo $1 $i;
done

Summary: Bash is vile.

Unfriended

Heh, +1 for the answer, -0.5 for the vile. – Ewart

@glennjackman: Wow, you're saying that Bash's myriad expansion and escaping rules aren't vile? – Unfriended

I feel dirty for saying +1 on an answer involving eval. Re: vile: bash isn't vile, but it's a bit like drinking grain alcohol straight. Some people seem to like it but it's hard not to choke at first and the more you do it the less you're bothered by it. – Tremayne

@Sorpigal: It must just be me then. I use it pretty much every day, and it still bothers me... (but then I'm not good with straight alchohol either) – Unfriended

This is yet working and solving the issue. Thank you! – Merciful

Answer 3

You can use seq command:

for i in `seq 1 $1`

Or you can use the C-style for...loop:

for((i=1;i<=$1;i++))

Hypochlorite

A C-style for loop is preferred over the external seq command. – Smiga

Answer 4

Here is a way to expand variables inside braces without eval:

end=3
declare -a 'range=({'"1..$end"'})'

We now have a nice array of numbers:

for i in ${range[@]};do echo $i;done
1
2
3

Metzger

Answer 5

I know you've mentioned bash in the heading, but I would add that 'for i in {$1..$2}' works as intended in zsh. If your system has zsh installed, you can just change your shebang to zsh.

Using zsh with the example 'for i in {$1..$2}' also has the added benefit that $1 can be less than $2 and it still works, something that would require quite a bit of messing about if you wanted that kind of flexibility with a C-style for loop.

Closure