Concept
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 s to output status text to the screen or a , or as a source part of a pipeline. 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 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.