Why Standard Input redirection command comes first?

  • Posted February 25, 2022

In the below command which prints a 32 character length key why < command is placed at the beginning?

< /dev/urandom tr -dc A-Za-z0-9 | head -c32


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The tr command can read directly from stdin. So the two following expressions are more or less going to work in the same way:

< /dev/urandom tr -dc A-Za-z0-9 | head -c32


tr -dc A-Za-z0-9 < /dev/urandom  | head -c32

Basically, the redirections can appear anywhere on a simple command.

An alternative approach is to use the cat command which is going to just add an extra step but produce the same result:

cat /dev/urandom | tr -dc A-Za-z0-9 | head -c32

Hope that this helps!