echo — print strings
The echo command can be used to print strings. The standard
usage is echo firstString secondString .... Also, it provides
additional parameters for formatting of the output.
Abuse of echo
If you find yourself about to use echo in an ebuild, stop and
reconsider. It is almost always unnecessary.
First of all, for printing messages in standard Portage scripts, you
can use the einfo, and eerror functions along with their
corresponding functions, einfon, eerrorn, etc, which are
the same as the former, but they won't print the trailing newline
(\n).
All usage of the form echo ${somevar} | grep substring just to
check if the content of the ${somevar} variable contains
substring, or more often, echo ${somevar} | command,
is deprecated and should be (and in most cases, can be) avoided:
doing so involves for no reason an additional shell session and a pipe.
The "here strings" section describes the preferred way
of dealing with such cases.
Here strings
Using "here strings", you can pass contents of an
environment variable to the standard input of an application, using
<<<word redirection: what actually happens is
that bash expands word and passes the result to the standard
input.
A common example would be verifying if a variable ${foo}
contains the bar substring with the following construct: grep bar
<<< ${foo}. This replaces the deprecated and more
wasteful behaviour of using echo ${foo} | grep bar.
Standard usage of echo
In standard calls, the echo command with no additional options,
outputs the arguments passed to the standard output, separated by
whitespace and with a trailing newline character (\n).
If one wants to exclude the trailing newline character, the -n
option can be passed, as in: echo -n "no trailing newline".
Special characters, like tabs (\t), newline characters
(\n), carriage return characters (\r) (i.e. for
translating newlines from DOS format to Unix format), and any other
ASCII character given by its code value) can be passed to the echo
command with the -e option. For example, to output three
strings, each on different lines, in a single echo command,
use: echo -e "first line\nsecond line\nthird line".
Other escape sequences and additional parameters for the echo
command are available in the bash(1) man page.