bash Cookbook

Looping with a Count

Problem

You need to loop a fixed number of times. You could use a while loop and do the counting and testing, but programming languages have for loops for such a common idiom. How does one do this in bash ?

Solution

Use a special case of the for syntax, one that looks a lot like C Language, but with double parentheses:

$ for (( i=0 ; i < 10 ; i++ )) ; do echo $i ; done

Discussion

In early versions of the shell, the original syntax for the for loop only included iterating over a fixed list of items. It was a neat innovation for such a word-oriented language as shell scripts, dealing with filenames and such. But when users needed to count, they sometimes found themselves writing:

for i in 1 2 3 4 5 6 7 8 9 10
do
    echo $i
done

Now that’s not too bad, especially for small loops, but let’s face it—that’s not going to work for 500 iterations. (Yes, you could nest loops 5 x 10, but come on!) What you really need is a for loop that can count.

The special case of the for loop with C-like syntax is a relatively recent addition to bash (appearing in version 2.04). Its more general form can be described as:

for (( expr1 ; expr2 ; expr3 )) ; do list ; done

The use of double parentheses is meant to indicate that these are arithmetic expressions. You don’t need to use the $ construct (as in $i, except for arguments like $1) when referring to variables inside the double parentheses (just like the other places where double parentheses are used in bash). The expressions are integer arithmetic ...