[SYMBOL] [A] [B] [C] [D] [E] [F] [G] [H] [I] [J] [K] [L] [M] [N] [O] [P] [Q] [R] [S] [T] [U] [V] [W] [X] [Y] [Z] UDP (User Datagram Protocol)  uid variable (tcsh)  ul command  ulimit command (bash)  umask command     bash      ftp      tcsh  umount command  unabbreviate command (ex)  unalias command     bash      tcsh  uname command  uncomplete command (tcsh)  uncompress command  uncompressing and paging (zmore)  undo command (ex)  unedit command (CVS)  unexpand command  unhash command (tcsh)  unhide command     ex      GRUB  unicode_start command  unicode_stop command  uniq command  Unix shell command (sh)  Unix, compared to Linux  unlimit command (tcsh)  unmap command (ex)  unset command     bash      tcsh  unsetenv command (tcsh)  unshar command  unstr command  [See strfile command] until command (bash)  update command      CVS  uppermem command (GRUB)  uptime command  Usenet newsgroups  user command (ftp)  USER environment variable (tcsh)  user login times, tracking (ruptime)  user variable (tcsh)  useradd command  userdel command  usermod command  users     finding login names for      management commands  users command  usleep command  uudecode command  uuencode command  uuidgen command  UUIDs (Universal Unique Identifiers), creating (uuidgen)