Asked  7 Months ago    Answers:  5   Viewed   36 times

I just can't figure out how do I make sure an argument passed to my script is a number or not.

All I want to do is something like this:

test *isnumber* $1 && VAR=$1 || echo "need a number"

Any help?



One approach is to use a regular expression, like so:

if ! [[ $yournumber =~ $re ]] ; then
   echo "error: Not a number" >&2; exit 1

If the value is not necessarily an integer, consider amending the regex appropriately; for instance:


...or, to handle numbers with a sign:

Tuesday, June 1, 2021
answered 7 Months ago

There are several ways of checking if an variable is an array or not. The best solution is the one you have chosen.

variable.constructor === Array

This is the fastest method on Chrome, and most likely all other browsers. All arrays are objects, so checking the constructor property is a fast process for JavaScript engines.

If you are having issues with finding out if an objects property is an array, you must first check if the property is there.

variable.prop && variable.prop.constructor === Array

Some other ways are:


Update May 23, 2019 using Chrome 75, shout out to @AnduAndrici for having me revisit this with his question This last one is, in my opinion the ugliest, and it is one of the slowest fastest. Running about 1/5 the speed as the first example. This guy is about 2-5% slower, but it's pretty hard to tell. Solid to use! Quite impressed by the outcome. Array.prototype, is actually an array. you can read more about it here

variable instanceof Array

This method runs about 1/3 the speed as the first example. Still pretty solid, looks cleaner, if you're all about pretty code and not so much on performance. Note that checking for numbers does not work as variable instanceof Number always returns false. Update: instanceof now goes 2/3 the speed!

So yet another update === '[object Array]';

This guy is the slowest for trying to check for an Array. However, this is a one stop shop for any type you're looking for. However, since you're looking for an array, just use the fastest method above.

Also, I ran some test: So have some fun and check it out.

Note: @EscapeNetscape has created another test as is down. I wanted to make sure the original link stay for whenever jsperf comes back online.

Tuesday, June 1, 2021
answered 7 Months ago

Think of ! (negation operator) as "not", || (boolean-or operator) as "or" and && (boolean-and operator) as "and". See Operators and Operator Precedence.


if(!(a || b)) {
  // means neither a nor b

However, using De Morgan's Law, it could be written as:

if(!a && !b) {
  // is not a and is not b

a and b above can be any expression (such as test == 'B' or whatever it needs to be).

Once again, if test == 'A' and test == 'B', are the expressions, note the expansion of the 1st form:

// if(!(a || b)) 
if(!((test == 'A') || (test == 'B')))
// or more simply, removing the inner parenthesis as
// || and && have a lower precedence than comparison and negation operators
if(!(test == 'A' || test == 'B'))
// and using DeMorgan's, we can turn this into
// this is the same as substituting into if(!a && !b)
if(!(test == 'A') && !(test == 'B'))
// and this can be simplified as !(x == y) is the same as (x != y)
if(test != 'A' && test != 'B')
Saturday, June 5, 2021
answered 7 Months ago

Since this happens in dash and dash is simpler, I looked there first.

Seems like exec.c is the place to look, and the relevant functionis are tryexec, which is called from shellexec which is called whenever the shell things a command needs to be executed. And (a simplified version of) the tryexec function is as follows:

tryexec(char *cmd, char **argv, char **envp)
        char *const path_bshell = _PATH_BSHELL;


        execve(cmd, argv, envp);

        if (cmd != path_bshell && errno == ENOEXEC) {
                *argv-- = cmd;
                *argv = cmd = path_bshell;
                goto repeat;

So, it simply always replaces the command to execute with the path to itself (_PATH_BSHELL defaults to "/bin/sh") if ENOEXEC occurs. There's really no magic here.

I find that FreeBSD exhibits identical behavior in bash and in its own sh.

The way bash handles this is similar but much more complicated. If you want to look in to it further I recommend reading bash's execute_command.c and looking specifically at execute_shell_script and then shell_execve. The comments are quite descriptive.

Friday, June 18, 2021
answered 6 Months ago

You can use this script:

IFS=' ' read -ra arr -p "Enter numbers: "
Enter numbers: 4 -1 2 66 10

sort -n <(printf "%sn" "${arr[@]}")
  • IFS=' ' to make read all number delimited by space
  • 'read -ra` to read all numbers in an array
  • sort -n to sort numbers numerically
  • printf "%sn" "${arr[@]}" to print each element of array in separate line
  • <(printf "%sn" "${arr[@]}") is process substitution that make it printf command behave like a file for sort -n command.
Thursday, November 18, 2021
Peter Thomas
answered 3 Weeks ago
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