for key in ~/.ssh/id_*; do ssh-keygen(1) -l -f "${key}"; done | uniq(1)
for name [ [ in [ word ... ] ] ; ] do list ; done
       The  list of words following in is expanded, generating a list of items.  The variable name is set
       to each element of this list in turn, and list is executed each time.  If the in word is  omitted,
       the  for  command  executes  list  once  for each positional parameter that is set (see PARAMETERS
       below).  The return status is the exit status of the last command that executes.  If the expansion
       of  the  items  following  in  results  in an empty list, no commands are executed, and the return
       status is 0.
authentication key generation, management and conversion
-l      Show fingerprint of specified public key file.  Private RSA1 keys are also supported.  For RSA and
        DSA keys ssh-keygen tries to find the matching public key file and prints its fingerprint.  If
        combined with -v, an ASCII art representation of the key is supplied with the fingerprint.
-f filename
        Specifies the filename of the key file.
    A  pipeline is a sequence of one or more commands separated by one of the control operators | or |&.  The
    format for a pipeline is:

           [time [-p]] [ ! ] command [ [||&] command2 ... ]

    The standard output of command is connected  via  a  pipe  to  the  standard  input  of  command2.   This
    connection  is performed before any redirections specified by the command (see REDIRECTION below).  If |&
    is used, the standard error of command is connected to command2's standard input through the pipe; it  is
    shorthand  for  2>&1  |.   This  implicit  redirection  of  the  standard  error  is  performed after any
    redirections specified by the command.

    The return status of a pipeline is the exit status of the last command, unless  the  pipefail  option  is
    enabled.   If  pipefail  is  enabled,  the  pipeline's return status is the value of the last (rightmost)
    command to exit with a non-zero status, or zero if all commands exit successfully.  If the reserved  word
    !   precedes  a  pipeline, the exit status of that pipeline is the logical negation of the exit status as
    described above.  The shell waits for all commands in the pipeline to terminate before returning a value.

    If the time reserved word precedes a pipeline, the elapsed as well as user and system  time  consumed  by
    its execution are reported when the pipeline terminates.  The -p option changes the output format to that
    specified by POSIX.  When the shell is in posix mode, it does not recognize time as a  reserved  word  if
    the  next  token begins with a `-'.  The TIMEFORMAT variable may be set to a format string that specifies
    how the timing information should be displayed; see the description of TIMEFORMAT under  Shell  Variables

    When the shell is in posix mode, time may be followed by a newline.  In this case, the shell displays the
    total user and system time consumed by the shell and its children.  The TIMEFORMAT variable may  be  used
    to specify the format of the time information.

    Each command in a pipeline is executed as a separate process (i.e., in a subshell).
report or omit repeated lines
source manpages: ssh-keygenuniq