<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>
<P><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)”). <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 are technically 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)”).</P>
<P>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). 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.</P>
<P>
<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)").
<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>
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).
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>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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