CAT(1POSIX)

💖 CAT(1POSIX) — or, bestie, let me introduce you to the icon herself 💖

okay so this one is from the POSIX Programmer's Manual (that's the 1POSIX up in the corner, iykyk). Heads up though, fr fr:

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

like. she's the standard, not necessarily what your lil Linux box is doing. check your local page too. 🫶

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✨ NAME ✨

cat — concatenate and print files

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🌸 SYNOPSIS 🌸

cat [-u] [file...]

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🦋 DESCRIPTION 🦋

cat reads files in sequence and writes their contents to standard output, in that same sequence. That's it. That's the whole vibe. Read, then write, in order. 💅

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💅 OPTIONS 💅

cat shall conform to the Base Definitions volume of POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines.

The following option shall be supported:

• -u — writes bytes from the input file to standard output without delay as each is read. no buffering, no waiting, just straight through. slay.

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🎀 OPERANDS 🎀

The following operand shall be supported:

• file — a pathname of an input file. If no file operands are specified, standard input shall be used. If a file is -, cat shall read from standard input at that point in the sequence. And here's the cute detail: cat shall not close and reopen standard input when it's referenced this way, but it shall accept multiple occurrences of - as a file operand. so you can be like cat - - - and she'll just keep sipping from stdin. queen behavior. 👑

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⭐ STDIN ⭐

Standard input shall be used only if no file operands are specified, or if a file operand is -. See the INPUT FILES section.

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🌈 INPUT FILES 🌈

The input files can be any file type. literally anything. she does not discriminate. 💜

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💖 ENVIRONMENT VARIABLES 💖

The following environment variables shall affect the execution of cat:

• LANG — provides a default value for the internationalization variables that are unset or null. (See the Base Definitions volume of POSIX.1‐2017, Section 8.2, Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.)
• LC_ALL — if set to a non-empty string value, override the values of all the other internationalization variables. she overrides. main character energy.
• LC_CTYPE — determines the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multi-byte characters in arguments).
• LC_MESSAGES — determines the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
• NLSPATH — determines the location of message catalogs for the processing of LC_MESSAGES.

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🧚 ASYNCHRONOUS EVENTS 🧚

Default. (yeah. default. nothing spicy here bestie.)

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🍓 STDOUT 🍓

Standard output shall contain the sequence of bytes read from the input files. Nothing else shall be written to standard output.

And uh, important lil caveat: if standard output is a regular file, and is the same file as any of the input file operands, the implementation may treat this as an error. so like. don't cat a file into itself and act surprised. 😭

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💀 STDERR 💀

Standard error shall be used only for diagnostic messages.

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🔥 OUTPUT FILES 🔥

None.

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✨ EXTENDED DESCRIPTION ✨

None.

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👑 EXIT STATUS 👑

The following exit values shall be returned:

1. 0 — all input files were output successfully.
2. >0 — an error occurred.

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🌸 CONSEQUENCES OF ERRORS 🌸

Default.

The following sections are informative.

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🎀 APPLICATION USAGE 🎀

The -u option has value in prototyping non-blocking reads from FIFOs. The intent is to support the following sequence:

mkfifo foo
cat -u foo > /dev/tty13 &
cat -u > foo

It is unspecified whether standard output is or is not buffered in the default case. This is sometimes of interest when standard output is associated with a terminal, since buffering may delay the output. The presence of the -u option guarantees that unbuffered I/O is available. It is implementation-defined whether the cat utility buffers output if the -u option is not specified. Traditionally, the -u option is implemented using the equivalent of the setvbuf() function defined in the System Interfaces volume of POSIX.1‐2017.

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🦋 EXAMPLES 🦋

The following command:

cat myfile

writes the contents of the file myfile to standard output.

The following command:

cat doc1 doc2 > doc.all

concatenates the files doc1 and doc2 and writes the result to doc.all.

Because of the shell language mechanism used to perform output redirection, a command such as this:

cat doc doc.end > doc

causes the original data in doc to be lost before cat even begins execution. This is true whether the cat command fails with an error or silently succeeds (the specification allows both behaviors). In order to append the contents of doc.end without losing the original contents of doc, this command should be used instead:

cat doc.end >> doc

okay this one is a real one bestie, > will absolutely eat your file. use >> if you're appending. no cap. 💀

The command:

cat start - middle - end > file

when standard input is a terminal, gets two arbitrary pieces of input from the terminal with a single invocation of cat. Note, however, that if standard input is a regular file, this would be equivalent to the command:

cat start - middle /dev/null end > file

because the entire contents of the file would be consumed by cat the first time - was used as a file operand and an end-of-file condition would be detected immediately when - was referenced the second time.

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💜 RATIONALE 💜

Historical versions of the cat utility include the -e, -t, and -v, options which permit the ends of lines, <tab> characters, and invisible characters, respectively, to be rendered visible in the output. The standard developers omitted these options because they provide too fine a degree of control over what is made visible, and similar output can be obtained using a command such as:

sed -n l pathname

The latter also has the advantage that its output is unambiguous, whereas the output of historical cat -etv is not.

The -s option was omitted because it corresponds to different functions in BSD and System V-based systems. The BSD -s option to squeeze blank lines can be accomplished by the shell script shown in the following example:

sed -n '
# Write non-empty lines.
/./   {
      p
      d
      }
# Write a single empty line, then look for more empty lines.
/^$/  p
# Get next line, discard the held <newline> (empty line),
# and look for more empty lines.
:Empty
/^$/  {
      N
      s/.//
      b Empty
      }
# Write the non-empty line before going back to search
# for the first in a set of empty lines.
      p
'

The System V -s option to silence error messages can be accomplished by redirecting the standard error. Note that the BSD documentation for cat uses the term blank line'' to mean the same as the POSIX empty line'': a line consisting only of a <newline>.

The BSD -n option was omitted because similar functionality can be obtained from the -n option of the pr utility.

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⭐ FUTURE DIRECTIONS ⭐

None.

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🌈 SEE ALSO 🌈

more

The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment Variables, Section 12.2, Utility Syntax Guidelines

The System Interfaces volume of POSIX.1‐2017, setvbuf()

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💖 COPYRIGHT 💖

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

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and that's the whole thing bestie 🥹 brought to you by IEEE/The Open Group, dated 2017, filed under CAT(1POSIX). she's been iconic since forever and she'll be iconic forever. go pipe something into something else, you got this. 💅✨🫶