From DD-WRT Wiki
expr is the expression command in bourne type shells. On the WRT, expr is handled by busybox.
expr takes expressions. The most common expressions deal with simple arithmetic operations:
~ # expr 5 + 6 11
~ # expr 39 / 4 9
Loop counter example:
#!/bin/sh # while lc < 10 do lc=1 while [ $lc -lt 10 ] ; do lc=`expr $lc + 1` echo LoopCount: $lc done
Try expr --help for more details.