Bash – Command substitution vs I/O Redirection

You were really close:

tr "o" "a" < <(echo "Foo")

The substitution <() makes a file descriptor and just pastes the path to the shell. For comprehension just execute:

<(echo blubb)

You will see the error:

-bash: /dev/fd/63: Permission denied

That's why it just pastes /dev/fd/63 into the shell and /dev/fd/63 is not excutable, because it's a simple pipe. In the tr-example above, it's echo "Foo" that writes to the pipe and via input redirection < it's the tr command that reads from the file descriptor.

When referencing a variable, it is generally advisable to enclose its name in double quotes. This prevents reinterpretation of all special characters within the quoted string -- except $, ` (backquote), and \ (escape). Keeping $ as a special character within double quotes permits referencing a quoted variable ("$variable"), that is, replacing the variable with its value.

Use double quotes to prevent word splitting. An argument enclosed in double quotes presents itself as a single word, even if it contains whitespace separators.

e.g.

variable1="a variable containing five words"
COMMAND This is $variable1    # Executes COMMAND with 7 arguments:
# "This" "is" "a" "variable" "containing" "five" "words"

COMMAND "This is $variable1"  # Executes COMMAND with 1 argument:
# "This is a variable containing five words"

Enclosing the arguments to an echo statement in double quotes is necessary only when word splitting or preservation of whitespace is an issue.

For more info and examples go here