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+ + + ++cat 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. +In practice this is usually accomplished by printing the contents of each sequential file argument to standard output. +
++cat 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)”). +cat’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 cat include options that are technically redundant - see the often-included cat -e, -t, and -v that replace the ends of lines, tabs, and invisible characters respectively with printing portrayals (“cat(1p)”). +
++The POSIX standard as of 2003 requires only the option -u to be implemented, which prevents cat from buffering its output - on some systems, cat buffers its output in 512-byte blocks (McIlroy), similarly to dd’s default as defined by POSIX (“dd(1p)”), though most currently popular cat implementations do this by default and ignore the -u flag altogether (busybox, GNU coreutils). +POSIX doesn’t mandate buffering by default. +
+
+This is a POSIX-compliant implementation of UNIX cat with no additional features nor buffered output (it ignores cat -u) in C:
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define STDIN_NAME "<stdin>"
+#define STDOUT_NAME "<stdout>"
+
+/* these two errors will exit out of the program with an unsuccessful status,
+ * and print a diagnostic message to standard error */
+void
+file_access_error(char *argv0, char *file_name){
+ fprintf(stderr, "%s: %s: cannot open file\n", argv0, file_name);
+ exit(1);
+}
+
+void
+file_write_error(char *argv0, char *file_name){
+ fprintf(stderr, "%s: %s: error writing to file\n", argv0, file_name);
+ exit(1);
+}
+
+/* print input to output, returns 0 if successful and 1 if unsuccessful */
+int
+file_copy(FILE *input, FILE *output){
+ char c;
+ while((c = getc(input)) != EOF)
+ if(putc(c, output) == EOF)
+ return 1;
+ return 0;
+}
+
+int
+main(int argc, char *argv[]){
+ /* the name of the file being printed (for diagnostics) */
+ char *file_name;
+
+ /* allocate this ahead of time */
+ char *stdin_file_name = STDIN_NAME;
+
+ /* the file pointer of the file being printed */
+ FILE *input;
+
+ /* this will always be stdout */
+ FILE *output = stdout;
+
+ int i;
+
+ /* whether or not options are being parsed */
+ int parsing_opts = 1;
+
+ /* usage with 0 arguments - print standard input to standard output */
+ if(argc == 1 && file_copy(stdin, stdout))
+ file_write_error(argv[0], STDOUT_NAME);
+ else if(argc == 1)
+ return 0;
+
+ for(i = 1; i < argc; ++i){
+ /* parsing options */
+
+ /* after `--`, interpret `--`, `-`, and `-u` as literal
+ * filenames */
+ if(parsing_opts && !strcmp(argv[i], "--")){
+ parsing_opts = 0;
+ continue;
+ /* ignore `-u` if still parsing options */
+ }else if(parsing_opts && !strcmp(argv[i], "-u"))
+ continue;
+
+ /* take `-` to mean standard input if still parsing options */
+ else if(parsing_opts && !strcmp(argv[i], "-")){
+ file_name = stdin_file_name;
+ input = stdin;
+
+ /* non-option; open the file and make sure file_name points to
+ * the right string */
+ }else{
+ file_name = argv[i];
+ input = fopen(file_name, "r");
+ if(input == NULL)
+ file_access_error(argv[0], file_name);
+ }
+
+ /* print input to output */
+ if(file_copy(input, output))
+ file_write_error(argv[0], STDOUT_NAME);
+ }
+
+ /* exit successfully */
+ return 0;
+}
+It’s worth noting that this concept of cat as a utility that sequentially prints given files to standard output means cat can be replaced by a simple shell script that does the same using dd and printf; cat as defined by POSIX is actually totally redundant to other POSIX utilities. Here’s the shell script:
+
+#!/bin/sh
+
+# some older systems will use the former POSIX_ME_HARDER rather than
+# POSIXLY_CORRECT to request strict POSIX coherence
+[ -n "$POSIX_ME_HARDER" ] & [ -z "$POSIXLY_CORRECT" ] \
+ && POSIXLY_CORRECT=1
+ || true
+
+# for usage()
+argv0="$0"
+
+# dd_ is used so that dd can easily be re-defined to the unbuffered variant -
+# dd bs=1
+dd_() { dd "$@"; }
+
+# if the system wants strict POSIX coherence, don't permit usage of `cat -h`
+# because it's not defined by POSIX
+usage() {
+ [ -z "$POSIXLY_CORRECT" ] \
+ && printf "Usage: %s [-hu] [file...]\n" "$argv0" \
+ || printf "Usage: %s [-u] [file...]\n" "$argv0"
+ exit 1
+}
+
+# usage with 0 arguments - print standard input to standard output
+if [ -z "$1" ]; then
+ dd_ 2>/dev/null
+ exit $?
+fi
+
+while [ -n "$1" ]; do
+ # parsing options
+
+ # after `--`, interpret `--`, `-`, and `-u` as literal filenames
+ [ "$1" = "--" ] && [ -z "$DONT_PARSE_ARGS" ] \
+ && DONT_PARSE_ARGS=1 && shift 1 && continue \
+ || true
+
+ # if `-u` specified and still parsing options, enable unbuffered output
+ # this dd is probably super slow. TODO?: find better way
+ [ "$1" = "-u" ] && [ -z "$DONT_PARSE_ARGS" ] \
+ && dd_() { dd bs=1 "$@"; } && shift 1 && continue \
+ || true
+
+ # the `-h` flag isn't specified within POSIX, so ignore it if the
+ # environment is strictly conforming to POSIX
+ [ "$1" = "-h" ] && [ -z "$DONT_PARSE_ARGS" ] \
+ && [ -z "$POSIXLY_CORRECT" ] \
+ && usage \
+ || true
+
+ # take `-` to mean standard input if still parsing options
+ if [ "$1" = "-" ] && [ -z "$DONT_PARSE_ARGS" ]; then
+ dd_ </dev/stdin 2>/dev/null || exit $?
+ shift 1
+ continue
+ fi
+
+ # print input to output
+ dd_ <"$1" 2>/dev/null || exit $?
+
+ shift 1
+done
+
+# exit successfully
+exit 0
+cat still has a purpose though. This shell script is relatively slow for short files and very slow for very large files (though dd itself should probably be used to copy large files from one medium to another anyway). This is provided for educational purposes (though I personally use this shell script in my system PATH; the C implementation provided compiles to a much larger binary using gcc 11.1.0, so this saves a couple kilobytes).
+ +