bash Cookbook

Writing Sequences

Problem

You need to generate a sequence of numbers, possibly with other text, for testing or some other purpose.

Solution

Use awk because it should work everywhere no matter what:

$ awk 'END { for (i=1; i <= 5; i++) print i, "text"}' /dev/null
1 text
2 text
3 text
4 text
5 text

$ awk 'BEGIN { for (i=1; i <= 5; i+=.5) print i}' /dev/null
1
1.5
2
2.5
3
3.5
4
4.5
5

Discussion

On some systems, notably Solaris, awk will hang waiting for a file unless you give it one, such as /dev/null. This has no effect on other systems, so it’s fine to use everywhere.

Note that the variable in the print statement is i, not $i. If you accidentally use $i it will be interpolated as a field from the current line being processed. Since we’re processing nothing, that’s what you’ll get if you use $i by accident (i.e., nothing).

The BEGIN or END patterns allow for startup or cleanup operations when actually processing files. Since we’re not processing a file, we need to use one of them so that awk knows to actually do something even though it has no normal input. In this case, it doesn’t matter which we use.

There is a GNU utility called seq that does exactly what this recipe calls for, but it does not exist by default on many systems, for example BSD, Solaris, and Mac OS X. It offers some useful formatting options and is numeric only.

Thankfully, as of bash 2.04 and later, you can do arithmetic integer for loops:

# Bash 2.04+ only, integer only $ for ((i=1; i<=5; i++)); do echo "$i text"; done 1 text ...