| echo | |
| Author: | Douglas McIlroy (AT&T Bell Laboratories) |
| Developer: | Various open-source and commercial developers |
| Operating System: | Multics, Unix, Unix-like, V, Plan 9, Inferno, FLEX, TRIPOS, AmigaDOS, Z80-RIO, OS-9, DOS, MSX-DOS, Panos, FlexOS, SISNE plus, OS/2, Windows, ReactOS, MPE/iX, KolibriOS, SymbOS |
| Platform: | Cross-platform |
| Genre: | Command |
In computing, echo 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.
The command is available in the following 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 echo as a builtin command.
The command is also available in the EFI shell.
echo 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. echo -n in Version 7 replaced prompt, (which behaved like echo but without terminating its output with a line delimiter).
On PWB/UNIX and later Unix System III, echo started expanding C escape sequences such as \n with the notable difference that octal escape sequences were expressed as \0ooo instead of \ooo in C.
Eighth Edition Unix echo only did the escape expansion when passed a -e option, and that behaviour was copied by a few other implementations such as the builtin echo command of Bash or zsh and GNU echo.
On MS-DOS, the command is available in versions 2 and later.
Nowadays, several incompatible implementations of echo 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 echo leaves the behaviour unspecified if the first argument is -n 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 echo implementations are not compliant in the default environment.
Because of these variations in behaviour, echo is considered a non-portable command on Unix-like systems and the printf command (where available, introduced by Ninth Edition Unix) is preferred instead.
Using ANSI escape code SGR sequences, compatible terminals can print out colored text.
Using a UNIX System III-style implementation: