concatenate files and print on the standard output

Concatenate FILE(s), or standard input, to standard output.

With no FILE, or when FILE is -, read standard input.

Pipelines
    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
    below.

    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).

build and execute command lines from standard input

-I replace-str
       Replace occurrences of replace-str in the initial-arguments with names read from  standard  input.
       Also,  unquoted  blanks  do  not  terminate  input  items;  instead  the  separator is the newline
       character.  Implies -x and -L 1.

--max-procs=max-procs
-P max-procs
       Run up to max-procs processes at a time; the default is 1.  If max-procs is 0, xargs will  run  as
       many  processes as possible at a time.  Use the -n option with -P; otherwise chances are that only
       one exec will be done.

transfer a URL

-H, --header <header>
       (HTTP) Extra header to use when getting a web page. You may specify any number of  extra  headers.
       Note  that  if  you  should add a custom header that has the same name as one of the internal ones
       curl would use, your externally set header will be used instead of the internal one.  This  allows
       you to make even trickier stuff than curl would normally do. You should not replace internally set
       headers without knowing perfectly well what you're doing. Remove an internal header  by  giving  a
       replacement without content on the right side of the colon, as in: -H "Host:".

       curl  will  make sure that each header you add/replace is sent with the proper end-of-line marker,
       you should thus not add that as a part of the header content: do  not  add  newlines  or  carriage
       returns, they will only mess things up for you.

       See also the -A, --user-agent and -e, --referer options.

       This option can be used multiple times to add/replace/remove multiple headers.

-o, --output <file>
       Write  output  to <file> instead of stdout. If you are using {} or [] to fetch multiple documents,
       you can use '#' followed by a number in the <file> specifier. That variable will be replaced  with
       the current string for the URL being fetched. Like in:

         curl http://{one,two}.site.com -o "file_#1.txt"

       or use several variables like:

         curl http://{site,host}.host[1-5].com -o "#1_#2"

       You may use this option as many times as the number of URLs you have.