fish-shell/doc_src/cmds/printf.rst
Johannes Altmanninger 414d9a1eb1 Reference more non-fish shell builtins that have relevant differences
When writing scripts for other shells, it can be confusing and annoying
that our `man` function shadows other manual pages, for example `exec(1p)`
from [Linux man-pages]. I almost never want to see the fish variant for such
contended cases (which obviuosly don't include fish-specific commands like
`string`, only widely-known shell builtins).

For the contented cases like `exec`, the POSIX documentation is more
substantial and useful, since it describes a (sub)set of languages widely
used for scripting.

Because of this I think we should stop overriding the system's man pages.
Nowadays we offer `exec -h` as intuitive way to show the documentation for
the fish-specific command (note that `help` is not a good replacement because
it uses a web browser).

Looking through the contended commands, it seems like for most of them,
the fish version is not substantially different from the system version.
A notable exception is `read` but I don't think it's a very important one.

So I think we should can sacrifice a bit of the native fish-scripting
experience in exchange for playing nicer with other shells. I think the
latter is more important because scripting is not our focus, the way I see it.
So maybe put our manpath at the end.

In lieu of that, let's at least have `exec.rst` reference the system variant.

[Linux man-pages]: https://www.kernel.org/doc/man-pages/

Closes 
2024-04-20 13:34:08 +02:00

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4.0 KiB
ReStructuredText

.. _cmd-printf:
printf - display text according to a format string
==================================================
Synopsis
--------
.. synopsis::
printf FORMAT [ARGUMENT ...]
Description
-----------
.. only:: builder_man
NOTE: This page documents the fish builtin ``printf``.
To see the documentation on any non-fish versions, use ``command man printf``.
``printf`` uses the format string *FORMAT* to print the *ARGUMENT* arguments. This means that it takes format specifiers in the format string and replaces each with an argument.
The *FORMAT* argument is re-used as many times as necessary to convert all of the given arguments. So ``printf %s\n flounder catfish clownfish shark`` will print four lines.
Unlike :doc:`echo <echo>`, ``printf`` does not append a new line unless it is specified as part of the string.
It doesn't support any options, so there is no need for a ``--`` separator, which makes it easier to use for arbitrary input than ``echo``. [#]_
Format Specifiers
-----------------
Valid format specifiers are taken from the C library function ``printf(3)``:
- ``%d`` or ``%i``: Argument will be used as decimal integer (signed or unsigned)
- ``%o``: An octal unsigned integer
- ``%u``: An unsigned decimal integer - this means negative numbers will wrap around
- ``%x`` or ``%X``: An unsigned hexadecimal integer
- ``%f``, ``%g`` or ``%G``: A floating-point number. ``%f`` defaults to 6 places after the decimal point (which is locale-dependent - e.g. in de_DE it will be a ``,``). ``%g`` and ``%G`` will trim trailing zeroes and switch to scientific notation (like ``%e``) if the numbers get small or large enough.
- ``%e`` or ``%E``: A floating-point number in scientific (XXXeYY) notation
- ``%s``: A string
- ``%b``: As a string, interpreting backslash escapes, except that octal escapes are of the form \0 or \0ooo.
``%%`` signifies a literal "%".
Conversion can fail, e.g. "102.234" can't losslessly convert to an integer, causing printf to print an error. If you are okay with losing information, silence errors with ``2>/dev/null``.
A number between the ``%`` and the format letter specifies the width. The result will be left-padded with spaces.
Backslash Escapes
-----------------
printf also knows a number of backslash escapes:
- ``\"`` double quote
- ``\\`` backslash
- ``\a`` alert (bell)
- ``\b`` backspace
- ``\c`` produce no further output
- ``\e`` escape
- ``\f`` form feed
- ``\n`` new line
- ``\r`` carriage return
- ``\t`` horizontal tab
- ``\v`` vertical tab
- ``\ooo`` octal number (ooo is 1 to 3 digits)
- ``\xhh`` hexadecimal number (hhh is 1 to 2 digits)
- ``\uhhhh`` 16-bit Unicode character (hhhh is 4 digits)
- ``\Uhhhhhhhh`` 32-bit Unicode character (hhhhhhhh is 8 digits)
Errors and Return Status
------------------------
If the given argument doesn't work for the given format (like when you try to convert a number like 3.141592 to an integer), printf prints an error, to stderr. printf will then also return non-zero, but will still try to print as much as it can.
It will also return non-zero if no argument at all was given, in which case it will print nothing.
This printf has been imported from the printf in GNU Coreutils version 6.9. If you would like to use a newer version of printf, for example the one shipped with your OS, try ``command printf``.
Example
-------
::
printf '%s\t%s\n' flounder fish
Will print "flounder fish" (separated with a tab character), followed by a newline character. This is useful for writing completions, as fish expects completion scripts to output the option followed by the description, separated with a tab character.
::
printf '%s: %d' "Number of bananas in my pocket" 42
Will print "Number of bananas in my pocket: 42", `without` a newline.
See Also
--------
- the :doc:`echo <echo>` command, for simpler output
Footnotes
---------
.. [#] In fact, while fish's ``echo`` supports ``--``, POSIX forbids it, so other implementations can't be used if the input contains anything starting with ``-``.