How can I strip first X characters from string using sed?

Here's a concise method to cut the first X characters using cut(1). This example removes the first 4 characters by cutting a substring starting with 5th character.

echo "$pid" | cut -c 5-

The following should work:

var="pid: 1234"
var=${var:5}

Are you sure bash is the shell executing your script?

Even the POSIX-compliant

var=${var#?????}

would be preferable to using an external process, although this requires you to hard-code the 5 in the form of a fixed-length pattern.