Question

[ -f /tmp/myfile.txt ] && echo "File exists" || echo "No such file"

How does this work? Specifically the the evaluation of && and ||.

Answer 1

&& and || are control operators that determine whether the following command should be executed based on the exit status of the prior command. From man (1) bash:

The control operators && and || denote AND lists and OR lists, respectively. An AND list has the form

  command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero.

An OR list has the form

  command1 || command2

command2 is executed if and only if command1 returns a non-zero exit status.
The return status of AND and OR lists is the exit status of the last
command executed in the list.

[, or test, returns a zero exit status if the command passes the test. From help test:

test: test [expr]
Exits with a status of 0 (true) or 1 (false) depending on
the evaluation of EXPR.

So when you do

[ -f /tmp/myfile.txt ] && echo foo || echo bar

what you’re really saying is (read this carefully, please) If the file exists, echo foo. If the file does not exist or the echo foo command fails, echo bar.

This is the subtle, but critical difference between these control operators and an if…then command.