Echo (command)

src: i.ytimg.com

In computing, echo is a command in DOS, OS/2, Microsoft Windows, Unix and Unix-like operating systems that outputs the strings it is being passed as arguments. It is a command typically used in shell scripts and batch files to output status text to the screen or a file, or as a source part of a pipeline.

Many shells, including all Bourne-like (such as Bash or zsh,) and Csh-like shells implement echo as a builtin command.

Video Echo (command)

History

echo began within Multics, and 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.

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.

Maps Echo (command)

Usage example

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

Using a System III-style implementation of echo):

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

and after:

Portably with printf:

src: img.wonderhowto.com

See also

• List of Unix Programs
• List of DOS commands

src: i.ytimg.com

References

src: images.slideplayer.com

External links

• echo: write arguments to standard output - Commands & Utilities Reference, The Single UNIX Specification, Issue 7 from The Open Group
• Microsoft TechNet Echo article

Source of the article : Wikipedia