NAME

echo — write arguments to standard output

SYNOPSIS

echo [string...]

DESCRIPTION

The echo utility writes its arguments to standard output, followed by a <newline>. If there are no arguments, only the <newline> is written.

OPTIONS

The echo utility shall not recognize the "--" argument in the manner specified by Guideline 10 of XBD 12.2 Utility Syntax Guidelines; "--" shall be recognized as a string operand.

Implementations shall not support any options.

OPERANDS

The following operands shall be supported:

string

A string to be written to standard output. If the first operand consists of a '-' followed by one or more characters from the set {'e', 'E', 'n'}, or if any of the operands contain a <backslash> character, the results are implementation-defined.

^[XSI] On XSI-conformant systems, if the first operand consists of a '-' followed by one or more characters from the set {'e', 'E', 'n'}, it shall be treated as a string to be written. The following character sequences shall be recognized on XSI-conformant systems within any of the arguments:

\a

Write an <alert>.

\b

Write a <backspace>.

\c

Suppress the <newline> that otherwise follows the final argument in the output. All characters following the '\c' in the arguments shall be ignored.

\f

Write a <form-feed>.

\n

Write a <newline>.

\r

Write a <carriage-return>.

\t

Write a <tab>.

\v

Write a <vertical-tab>.

\\

Write a <backslash> character.

\0num

Write an 8-bit value that is the zero, one, two, or three-digit octal number num.

STDIN

Not used.

INPUT FILES

None.

ENVIRONMENT VARIABLES

The following environment variables shall affect the execution of echo:

LANG

Provide a default value for the internationalization variables that are unset or null. (See XBD 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

^[XSI] 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

^[XSI] Determine the location of messages objects and message catalogs.

ASYNCHRONOUS EVENTS

Default.

STDOUT

The echo utility arguments shall be separated by single <space> characters and a <newline> character shall follow the last argument. ^[XSI]  Output transformations shall occur based on the escape sequences in the input. See the OPERANDS section.

STDERR

The standard error shall be used only for diagnostic messages.

OUTPUT FILES

None.

EXTENDED DESCRIPTION

None.

EXIT STATUS

The following exit values shall be returned:

 0

Successful completion.

>0

An error occurred.

CONSEQUENCES OF ERRORS

Default.

APPLICATION USAGE

It is not possible to use echo portably across all POSIX systems unless escape sequences are omitted, and the first argument does not consist of a '-' followed by one or more characters from the set {'e', 'E', 'n'}.

The printf utility can be used portably to emulate any of the traditional behaviors of the echo utility as follows (assuming that IFS has its standard value or is unset):

• The historic System V echo and the requirements on XSI implementations in this volume of POSIX.1-2024 are equivalent to:

  printf "%b\n" "$*"
  

• The BSD echo is equivalent to:

  if [ "X$1" = "X-n" ]
  then
      shift
      printf "%s" "$*"
  else
      printf "%s\n" "$*"
  fi
  

New applications are encouraged to use printf instead of echo.

EXAMPLES

None.

RATIONALE

The echo utility has not been made obsolescent because of its extremely widespread use in historical applications. Conforming applications that wish to do prompting without <newline> characters or that could possibly be expecting to echo a string consisting of a '-' followed by one or more characters from the set {'e', 'E', 'n'} should use the printf utility.

At the time that the IEEE Std 1003.2-1992 standard was being developed, the two different historical versions of echo that were considered for standardization varied in incompatible ways.

The BSD echo checked the first argument for the string -n which caused it to suppress the <newline> that would otherwise follow the final argument in the output.

The System V echo treated all arguments as strings to be written, but allowed escape sequences within them, as described for XSI implementations in the OPERANDS section, including \c to suppress a trailing <newline>.

Thus the IEEE Std 1003.2-1992 standard said that the behavior was implementation-defined if the first operand is -n or if any of the operands contain a <backslash> character. It also specified that the echo utility does not support Utility Syntax Guideline 10 because historical applications depended on echo to echo all of its arguments, except for the -n first argument in the BSD version.

The Single UNIX Specification, Version 1 required the System V behavior, and this became the XSI requirement when Version 2 and POSIX.2 were merged with POSIX.1 to form the joint IEEE Std 1003.1-2001 / Single UNIX Specification, Version 3 standard.

This standard now treats a first operand of -e or -E the same as -n in recognition that support for them has become more widespread in non-XSI implementations. Where supported, -e enables processing of escape sequences in the remaining operands (in situations where it is disabled by default), and -E disables it (in situations where it is enabled by default). A first operand containing a combination of these three letters, in the same manner as option grouping, also results in implementation-defined behavior.

FUTURE DIRECTIONS

None.

SEE ALSO

printf

XBD 8. Environment Variables, 12.2 Utility Syntax Guidelines

CHANGE HISTORY

First released in Issue 2.

Issue 5

In the OPTIONS section, the last sentence is changed to indicate that implementations "do not" support any options; in the previous issue this said "need not".

Issue 6

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

• A set of character sequences is defined as string operands.

• LC_CTYPE is added to the list of environment variables affecting echo.

• In the OPTIONS section, implementations shall not support any options.

IEEE Std 1003.1-2001/Cor 1-2002, item XCU/TC1/D6/21 is applied, so that the echo utility can accommodate historical BSD behavior.

Issue 7

SD5-XCU-ERN-97 is applied, updating the SYNOPSIS.

Issue 8

Austin Group Defect 1122 is applied, changing the description of NLSPATH .

Austin Group Defect 1222 is applied, making the results implementation-defined, on systems that are not XSI-conformant, if the first operand consists of a '-' followed by one or more characters from the set {'e', 'E', 'n'}.