echo, redirection and variable expansion

[Dominique_71]

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$ echo fvwm user dir is "${FVWM_USERDIR}"       
fvwm user dir is /home/dom/.fvwm-crystal
$ echo 'fvwm user dir is "${FVWM_USERDIR}"'                 
fvwm user dir is "${FVWM_USERDIR}"
$ echo 'fvwm user dir is "${FVWM_USERDIR}"' | sed 's:user::'
fvwm  dir is "${FVWM_USERDIR}"
$ echo fvwm user dir is "${FVWM_USERDIR}" | sed 's:dom::'
fvwm user dir is /home//.fvwm-crystal
$ echo 'fvwm user dir is "${FVWM_USERDIR}"' | echo

 $ echo fvwm user dir is "${FVWM_USERDIR}" | echo

The last 2 commands only returns a newline.
As man echo, and man builtin, say nothing about redirection with echo, I was waiting that echo will understand it and return some content in addition to the new line. I am a little bit puzzled here.

[Zucca]

echo doesn't do anything with stdin. I think you want to use cat instead, but that "useless use of cat" then. So just leave the trailing echo and the pipe out?

[flexibeast]

If you need to pass the output of a command to `echo` (or indeed, `printf`), you can put that command in a subshell, e.g.:

Code: Select all

echo $(echo 'something')

Whether a pipeline involves creating a subshell for each component of the pipeline is shell-specific. According to the bash(1) man page, Bash creates a subshell:

Each command in a multi-command pipeline, where pipes are created, is executed in a subshell, which is a separate process.

Dash doesn't, or at least that's how i read the following in the dash(1) man page:

Note that unlike some other shells, each process in the pipeline is a child of the invoking shell (unless it is a shell builtin, in which case it executes in the current shell – but any effect it has on the environment is wiped).

The zshall(1) man page implies that Zsh does:

If a pipeline is preceded by `coproc', it is executed as a coprocess; a two-way pipe is established between it and the parent shell.

When the option is not set, job information from the parent shell is saved for output within a subshell (for example, within a pipeline).

[Dominique_71]

I will not use that into a script, I was just testing things around and surprised by the result.