What Are You Doing Right Now? (411 Viewers)
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icεmαή;2722014 said:
I can't believe its been 5 weeks since I quit work. Time sure does fly.
Martin, whats the easiest way to check whether a variable is a number using shell commands?
$ echo 43 | egrep "^[0-9]+$"
Egrep is grep in full regular expression mode. Here we say "find the beginning of the string with ^". Then "I want a character between 0 and 9 (ie. all the digits)". [] is used to say "between x and y". Then comes + which means "one or more such characters (ie. one or more digits)". Then we say "find the end of the string with $".
On success, grep returns 0, which you can test.
I googled and I couldn't find anything that would be more direct, here's stackoverflow:
http://stackoverflow.com/questions/806906/how-do-i-test-if-a-variable-is-a-number-in-bash
The most upvoted answer there does the same thing but in the shell itself. I prefer to use grep because I know it better.
The shell and most of the userland unix tools are basically string oriented, there is no strong notion of a type. This is both an advantage and a problem, because most of the time you mess with strings anyway, and it's a shortcut. If you want to do something with numbers it's inconvenient.
Another method is to use an integer test on the variable. If it's not a number the test will fail with an error message. I don't like this method as much, because it's relying on a side effect (that the test will produce the error message). So it's more obscure.
$ test $num -eq $num
bash: test: s: integer expression expected
Another method is to use an integer test on the variable. If it's not a number the test will fail with an error message. I don't like this method as much, because it's relying on a side effect (that the test will produce the error message). So it's more obscure.
$ test $num -eq $num
bash: test: s: integer expression expected
One would think that a script such as finding the greatest of three numbers would be quite straight forward because its one of the basic things you'd do when you are learning a new language. I've used shell scripting for quite some time now but it was always manipulation of strings in one way or the other. Never noticed that manipulating numbers can be a little complex. I've to say it brought a smile to my face when I finished
icεmαή;2722035 said:
One would think that a script such as finding the greatest of three numbers would be quite straight forward because its one of the basic things you'd do when you are learning a new language. I've used shell scripting for quite some time now but it was always manipulation of strings in one way or the other. Never noticed that manipulating numbers can be a little complex. I've to say it brought a smile to my face when I finished
Still, I love the shell for how portable it is and I have written stuff that would be much better in a better language. Biggest was probably this:
http://www.matusiak.eu/numerodix/blog/index.php/2009/06/14/networktest-improved-network-detection/
But this is pretty painful in the shell.
