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homepage/cat/index.html
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<!DOCTYPE html>
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<HTML LANG="en">
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<HEAD>
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<LINK HREF="http://www.trinity.moe/knowledge/cat" REL="canonical" />
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<LINK HREF="https://raw.githubusercontent.com/devenblake/homepage/main/favicon.ico" REL="shortcut icon" TYPE="image/x-icon" />
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<TITLE>cat(1)</TITLE>
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<P><A HREF="/">~ Return to the rest of the site</A></P>
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</SCRIPT>
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<H1>POSIX cat(1)</H1>
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<H3>updated 2021-08-06</H3>
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<HR ALIGN="left" SIZE="1" WIDTH="25%" />
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<P><CODE>cat</CODE> on a POSIX or otherwise UNIX-like system is a program that exists to concatenate files; to “join” one file at its end to another at its start, and output that resulting file to standard output.</P>
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<P>
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<CODE>cat</CODE> was introduced in UNIX v1 to supercede the program pr which printed the contents of a single file to the screen (McIlroy); its first-edition manual page described cat as "about the easiest way to print a file" ("cat(1)").
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<CODE>cat</CODE>’s modern, typical use is more or less the same; it’s often introduced to UNIX beginners as a method to print the contents of a file to the screen, which is why many implementations of <CODE>cat</CODE> include options that, while possibly useful, can be redundant - see the often-included <CODE>cat</CODE> <CODE>-e</CODE>, <CODE>-t</CODE>, and <CODE>-v</CODE> that replace the ends of lines, tabs, and invisible characters respectively with printing portrayals ("cat(1p)").
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</P>
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<P>
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The POSIX standard as of 2003 requires only the option <CODE>-u</CODE> to be implemented, which prevents <CODE>cat</CODE> from buffering its output - on some systems, <CODE>cat</CODE> buffers its output in 512-byte blocks (McIlroy), similarly to <CODE>dd</CODE>’s default as defined by POSIX (“dd(1p)”), though most currently popular <CODE>cat</CODE> implementations do this by default and ignore the <CODE>-u</CODE> flag altogether (busybox, GNU coreutils).
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POSIX doesn’t mandate buffering by default - specifically, <CODE>-u</CODE> <I>has</I> to guarantee that the output is unbuffered, but <CODE>cat</CODE> doesn't have to buffer it in the first place and can ignore <CODE>-u</CODE> in that case.
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</P>
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<P>This is a POSIX-compatible implementation of UNIX <CODE>cat</CODE> with no additional features nor buffered output in C:</P>
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<INPUT ID="c_toggle" ONCLICK="window.load_highlighting('c');" TYPE="button" VALUE="Press this button to enable syntax highlighting within this code." />
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<PRE><CODE CLASS="language-c" DATA-LANG="c">
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#define STDIN_NAME "<stdin>"
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#define STDOUT_NAME "<stdout>"
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/* these are the predicted errors that could occur */
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enum error_type{
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FILE_ACCESS,
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FILE_CLOSE,
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FILE_WRITE
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};
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/* this is an error function that will print to standard error the error that
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* occurred in the program and exit */
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void
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error(enum error_type type, char *argv0, char *file_name){
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switch(type){
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case FILE_ACCESS:
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fprintf(stderr, "%s: %s: cannot open file\n", argv0, file_name);
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break;
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case FILE_CLOSE:
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fprintf(stderr, "%s: %s: cannot close file\n", argv0, file_name);
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break;
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case FILE_WRITE:
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fprintf(stderr, "%s: %s: cannot write to file\n", argv0, file_name);
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break;
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}
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exit(1);
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}
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/* print input to output, returns 0 if successful and 1 if unsuccessful */
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int
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file_copy(FILE *input, FILE *output){
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char c;
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while((c = getc(input)) != EOF)
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if(putc(c, output) == EOF)
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return 1;
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return 0;
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}
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int
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main(int argc, char *argv[]){
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/* the name of the file being printed (for diagnostics) */
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char *file_name;
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/* allocate this ahead of time */
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char *stdin_file_name = STDIN_NAME;
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/* the file pointer of the file being printed */
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FILE *input;
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/* this will always be stdout */
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FILE *output = stdout;
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int i;
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/* whether or not options are being parsed */
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int parsing_opts = 1;
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/* usage with 0 arguments - print standard input to standard output */
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if(argc == 1 && file_copy(stdin, stdout))
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error(FILE_WRITE, argv[0], STDOUT_NAME);
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else if(argc == 1)
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return 0;
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for(i = 1; i < argc; ++i){
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/* parsing options */
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/* after `--`, interpret `--`, `-`, and `-u` as literal
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* filenames */
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if(parsing_opts && !strcmp(argv[i], "--")){
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parsing_opts = 0;
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continue;
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/* ignore `-u` if still parsing options */
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}else if(parsing_opts && !strcmp(argv[i], "-u"))
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continue;
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/* take `-` to mean standard input if still parsing options */
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else if(parsing_opts && !strcmp(argv[i], "-")){
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file_name = stdin_file_name;
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input = stdin;
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/* non-option; open the file and make sure file_name points to
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* the right string */
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}else{
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file_name = argv[i];
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input = fopen(file_name, "r");
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if(input == NULL)
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error(FILE_ACCESS, argv[0], file_name);
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}
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/* print input to output */
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if(file_copy(input, output))
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error(FILE_WRITE, argv[0], STDOUT_NAME);
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/* close input file if it's not stdin */
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if(input != stdin && fclose(input))
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error(FILE_CLOSE, argv[0], file_name);
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}
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/* exit successfully */
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return 0;
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}
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</CODE></PRE>
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<P>
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This is also available at <A HREF="/knowledge/cat/cat.c">/knowledge/cat/cat.c on this website</A> as a plain .c file with which you can toy.
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</P>
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<P>It’s worth noting that this concept of cat as a utility that sequentially prints given files to standard output means <CODE>cat</CODE> can be replaced by a simple shell script that does the same using <CODE>dd</CODE> and <CODE>printf</CODE>; <CODE>cat</CODE> as defined by POSIX is actually totally redundant to other POSIX utilities. Here’s the shell script:</P>
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<INPUT ID="shell_toggle" ONCLICK="window.load_highlighting('shell');" TYPE="button" VALUE="Press this button to enable syntax highlighting within this code." />
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<PRE><CODE CLASS="language-shell" DATA-LANG="shell">
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#!/bin/sh
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# dd_ is used so that dd can easily be re-defined
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dd_() { dd "$@"; }
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# usage with 0 arguments - print standard input to standard output
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if [ -z "$1" ]; then
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dd_ 2>/dev/null
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exit $?
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fi
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while [ -n "$1" ]; do
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# parsing options
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# after `--`, interpret `--`, `-`, and `-u` as literal filenames
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[ "$1" = "--" ] && [ -z "$DONT_PARSE_ARGS" ] \
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&& DONT_PARSE_ARGS=1 && shift 1 && continue \
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|| true
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# if `-u` specified and still parsing options, enable unbuffered output
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# this is kind of a hack and a bit slow. technically it is buffered,
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# just one byte at a time
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[ "$1" = "-u" ] && [ -z "$DONT_PARSE_ARGS" ] \
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&& dd_() { dd bs=1 "$@"; } && shift 1 && continue \
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|| true
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# take `-` to mean standard input if still parsing options
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if [ "$1" = "-" ] && [ -z "$DONT_PARSE_ARGS" ]; then
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dd_ </dev/stdin 2>/dev/null || exit $?
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shift 1
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continue
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fi
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# print input to output
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dd_ <"$1" 2>/dev/null || exit $?
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shift 1
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done
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# exit successfully
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exit 0
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</CODE></PRE>
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<P>
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It's worth noting that the <CODE>dd_</CODE> shell function in the above sample that allows for re-aliasing of <CODE>dd</CODE> to <CODE>dd bs=1</CODE> could be replaced with a shell variable <CODE>$DD</CODE> with the initial value <CODE>dd</CODE> and a changed value according to <CODE>-u</CODE> of <CODE>dd bs=1</CODE>.
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However, <CODE>alias dd="dd bs=1"</CODE> would not work due to how shell aliases are parsed (ShellCheck).
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</P>
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<P>
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<CODE>cat</CODE> doesn't work well as a shell script though.
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The script is relatively slow for short files and very slow for very large files (though <CODE>dd</CODE> itself should probably be used to copy large files from one medium to another anyway).
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This is provided for educational purposes.
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</P>
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<H2>Cited media and further reading</H2><UL>
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<LI>Articles<UL>
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<LI><A HREF="https://www.cs.dartmouth.edu/~doug/reader.pdf">McIlroy, M. Douglas - “A Research Unix Reader”</A></LI>
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<LI><A HREF="https://github.com/koalaman/shellcheck/wiki/SC2262">ShellCheck wiki page SC2262 ("This alias can't be defined and used in the same parsing unit. Use a function instead.")</A></LI>
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</UL></LI>
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<LI>Manual pages<UL>
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<LI><A HREF="http://man.cat-v.org/unix-1st/1/cat">cat(1)</A> (UNIX v1)</LI>
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<LI><A HREF="https://www.unix.com/man-page/posix/1posix/cat/">cat(1p)</A> (The Open Group, 2003)</LI>
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<LI><A HREF="https://www.unix.com/man-page/posix/1posix/dd/">dd(1p)</A> (The Open Group, 2003)</LI>
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<LI><A HREF="https://www.unix.com/man-page/POSIX/1posix/printf/">printf(1p)</A> (The Open Group, 2003)</LI>
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</UL></LI>
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<LI>Notable cat implementations<UL>
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<LI>1979 - <A HREF="http://www.bell-labs.com/">Bell Labs</A>/<A HREF="https://en.wikipedia.org/wiki/Unix">UNIX v7</A> - <A HREF="https://www.tuhs.org/cgi-bin/utree.pl?file=V7/usr/src/cmd/cat.c">/usr/src/cmd/cat.c</A></LI>
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<LI>1992 - <A HREF="http://www.bell-labs.com/">Bell Labs</A>/<A HREF="https://9p.io/plan9/">Plan 9 from Bell Labs Fourth Edition</A> - <A HREF="https://github.com/plan9foundation/plan9/blob/main/sys/src/cmd/cat.c">/sys/src/cmd/cat.c</A></LI>
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<LI><B>ongoing</B> started 1992 - <A HREF="https://www.gnu.org/">GNU</A>/<A HREF="https://www.gnu.org/software/coreutils/">coreutils</A> - <A HREF="https://git.savannah.gnu.org/cgit/coreutils.git/tree/src/cat.c">src/cat.c</A></LI>
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<LI>1995 - <A HREF="https://www.berkeley.edu/">University of California, Berkeley</A>/<A HREF="https://en.wikipedia.org/wiki/Berkeley_Software_Distribution">4.4BSD-Lite2</A> - <A HREF="https://github.com/sergev/4.4BSD-Lite2/blob/master/usr/src/bin/cat/cat.c">/usr/src/bin/cat/cat.c</A></LI>
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<LI><B>ongoing</B> forked from 4.4BSD-Lite2 - <A HREF="https://www.netbsd.org/">NetBSD</A>/<A HREF="https://github.com/NetBSD/src">src</A> - <A HREF="https://github.com/NetBSD/src/blob/trunk/bin/cat/cat.c">bin/cat/cat.c</A></LI>
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<LI><B>ongoing</B> started 1995 - <A HREF="https://www.busybox.net/">busybox</A>/<A HREF="https://git.busybox.net/busybox/">busybox</A> - <A HREF="https://git.busybox.net/busybox/tree/coreutils/cat.c">coreutils/cat.c</A></LI>
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</UL></LI>
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</UL>
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<H2>Acknowledgements</H2><UL>
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<LI>Content help<UL>
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<LI>Miles</LI>
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<LI>WeedSmokingJew</LI>
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</UL></LI>
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<LI>JavaScript help<UL>
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<LI>adamz01h</H1>
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<LI>wiresToGround</LI>
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</UL></LI>
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<LI>JavaScript libraries used<UL>
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<LI><A HREF="https://highlightjs.org/">highlight.js</A></LI>
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</UL></LI>
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<LI>Sample code help<UL>
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<LI>Ando_Bando</LI>
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<LI>Miles</LI>
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<LI>u/oh5nxo</LI>
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<LI>WeedSmokingJew</LI>
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</UL></LI>
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</UL>
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</BODY>
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</HTML>
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