читать из стандартного ввода в переменные оболочки (read from standard input into shell variables)
Пролог (Prolog)
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.
Имя (Name)
read — read from standard input into shell variables
Синопсис (Synopsis)
read [-r] var...
Описание (Description)
The read utility shall read a single logical line from standard
input into one or more shell variables.
By default, unless the -r option is specified, <backslash> shall
act as an escape character. An unescaped <backslash> shall
preserve the literal value of the following character, with the
exception of a <newline>. If a <newline> follows the
<backslash>, the read utility shall interpret this as line
continuation. The <backslash> and <newline> shall be removed
before splitting the input into fields. All other unescaped
<backslash> characters shall be removed after splitting the input
into fields.
If standard input is a terminal device and the invoking shell is
interactive, read shall prompt for a continuation line when it
reads an input line ending with a <backslash> <newline>, unless
the -r option is specified.
The terminating <newline> (if any) shall be removed from the
input and the results shall be split into fields as in the shell
for the results of parameter expansion (see Section 2.6.5, Field
Splitting); the first field shall be assigned to the first
variable var, the second field to the second variable var, and so
on. If there are fewer fields than there are var operands, the
remaining vars shall be set to empty strings. If there are fewer
vars than fields, the last var shall be set to a value comprising
the following elements:
* The field that corresponds to the last var in the normal
assignment sequence described above
* The delimiter(s) that follow the field corresponding to the
last var
* The remaining fields and their delimiters, with trailing IFS
white space ignored
The setting of variables specified by the var operands shall
affect the current shell execution environment; see Section 2.12,
Shell Execution Environment. If it is called in a subshell or
separate utility execution environment, such as one of the
following:
(read foo)
nohup read ...
find . -exec read ... \;
it shall not affect the shell variables in the caller's
environment.
Параметры (Options)
The read utility shall conform to the Base Definitions volume of
POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines.
The following option is supported:
-r Do not treat a <backslash> character in any special
way. Consider each <backslash> to be part of the input
line.
Операнды (Operands)
The following operand shall be supported:
var The name of an existing or nonexisting shell variable.
Стандартный ввод (Stdin)
The standard input shall be a text file.
Входные файлы (Input files)
None.
Переменные окружения (Environment variables)
The following environment variables shall affect the execution of
read:
IFS Determine the internal field separators used to delimit
fields; see Section 2.5.3, Shell Variables.
LANG Provide 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.
LC_CTYPE Determine 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
Determine the locale that should be used to affect the
format and contents of diagnostic messages written to
standard error.
NLSPATH Determine the location of message catalogs for the
processing of LC_MESSAGES.
PS2 Provide the prompt string that an interactive shell
shall write to standard error when a line ending with a
<backslash> <newline> is read and the -r option was not
specified.
Асинхронные события (Asynchronous events)
Default.
Стандартный вывод (Stdout)
Not used.
Стандартный вывод сообщений (Stderr)
The standard error shall be used for diagnostic messages and
prompts for continued input.
Выходные файлы (Output files)
None.
Расширенное описание (Extended description)
None.
Статус выхода (Exit)
The following exit values shall be returned:
0 Successful completion.
>0 End-of-file was detected or an error occurred.
Последствия ошибок (Consequences of errors)
Default.
The following sections are informative.
Использование в приложениях (Application usage)
The -r option is included to enable read to subsume the purpose
of the line utility, which is not included in POSIX.1‐2008.
Примеры (Examples)
The following command:
while read -r xx yy
do
printf "%s %s\n$yy$xx"
done < input_file
prints a file with the first field of each line moved to the end
of the line.
Обоснование (Rationale)
The read utility historically has been a shell built-in. It was
separated off into its own utility to take advantage of the
richer description of functionality introduced by this volume of
POSIX.1‐2017.
Since read affects the current shell execution environment, it is
generally provided as a shell regular built-in. If it is called
in a subshell or separate utility execution environment, such as
one of the following:
(read foo)
nohup read ...
find . -exec read ... \;
it does not affect the shell variables in the environment of the
caller.
Although the standard input is required to be a text file, and
therefore will always end with a <newline> (unless it is an empty
file), the processing of continuation lines when the -r option is
not used can result in the input not ending with a <newline>.
This occurs if the last line of the input file ends with a
<backslash> <newline>. It is for this reason that ``if any'' is
used in ``The terminating <newline> (if any) shall be removed
from the input'' in the description. It is not a relaxation of
the requirement for standard input to be a text file.
Будущие направления (Future directions)
None.
Смотри также (See also)
Chapter 2, Shell Command Language
The Base Definitions volume of POSIX.1‐2017, Chapter 8,
Environment Variables, Section 12.2, Utility Syntax Guidelines
