Echo (command)

In computing,   is a command that outputs the strings that are passed to it as arguments. It is a command available in various operating system shells and typically used in shell scripts and batch files to output status text to the screen or a computer file, or as a source part of a pipeline.

Implementations
The command is available in the following operating systems:
 * Multics
 * TSC FLEX
 * MetaComCo TRIPOS
 * Zilog Z80-RIO
 * Microware OS-9
 * DOS
 * Acorn Computers Panos
 * Digital Research FlexOS
 * IBM OS/2
 * Microsoft Windows
 * ReactOS
 * HP MPE/iX
 * KolibriOS
 * SymbOS
 * Unix and Unix-like operating systems

Many shells, including all Bourne-like (such as Bash or zsh ) and Csh-like shells as well as COMMAND.COM and cmd.exe implement  as a builtin command.

The command is also available in the EFI shell.

History
began within Multics. After it was programmed in C by Doug McIlroy as a "finger exercise" and proved to be useful, it became part of Version 2 Unix. in Version 7 replaced, (which behaved like   but without terminating its output with a line delimiter).

On PWB/UNIX and later Unix System III,  started expanding C escape sequences such as   with the notable difference that octal escape sequences were expressed as   instead of   in C.

Eighth Edition Unix  only did the escape expansion when passed a   option, and that behaviour was copied by a few other implementations such as the builtin   command of Bash or zsh and GNU.

On MS-DOS, the command is available in versions 2 and later.

Nowadays, several incompatible implementations of  exist on different operating systems (often several on the same system), some of them expanding escape sequences by default, some of them not, some of them accepting options (the list of which varying with implementations), some of them not.

The POSIX specification of leaves the behaviour unspecified if the first argument is   or any argument contain backslash characters while the Unix specification (XSI option in POSIX) mandates the expansion of (some) sequences and does not allow any option processing. In practice, many  implementations are not compliant in the default environment.

Because of these variations in behaviour,  is considered a non-portable command on Unix-like systems and the   command (where available, introduced by Ninth Edition Unix) is preferred instead.

Usage examples
Using ANSI escape code SGR sequences, compatible terminals can print out colored text.

Using a UNIX System III-style implementation:

Or a Unix Version 8-style implementation (such as Bash when not in Unix-conformance mode):

and after:

Portably with :