Exit status ($?), && and ||, and exit codes in scripts

Every command that finishes returns an exit status, a number from 0 to 255. By convention 0 means success and any non-zero value means some kind of failure; the specific value can identify the error, and a command's man page often documents its codes. The shell uses exit statuses to drive if, while, && and ||, so understanding them is the key to scripts that react correctly to failure.