single user mode shell runs as in unix
Single user mode Unix-like operating systems provide single user mode functionality either through the System V-style runlevels, BSD-style boot-loader options, or other boot-time options.
expr expr evaluates integer or string expressions, including pattern matching regular expressions. Each symbol (operator, value, etc.) in the expression must be given as a separate parameter. Most of the challenge posed in writing expressions is preventing the invoking command line shell from acting on characters intended for expr to process.