Fish Shell: Conditional execution based on $status?

Question

In the fish shell the syntax for conditionals is hard to find. Does anyone have a link to an explanation of how to write an if, with ands and ors?

In particular I would like to write

if not $status
  do a command
end

To do a command when the previous command returned a non-success. How do I do that?

Answer 1

See http://fishshell.com/docs/current/commands.html#if and http://fishshell.com/docs/current/tutorial.html#tut_conditionals.

Fish's if structure looks like this:

if COMMAND
   # do something if it succeeded
else
   # do something if it failed ($status != 0)
end

Then there are also the not, and and or commands, that you can use like

if not COMMAND1; or COMMAND2
    # and so on

If you really want to test a variable (e.g. $status), you need to use test as the command, like

if test $status -eq 0

Do keep in mind that $status changes after every command, so if you need to use the status of an earlier command (common in prompts) you need to do what Joachim Pileborg said, save it to another variable.

Also, test has some issues with quotes (because it's one of the few parts of fish to adhere to POSIX) - if in test $foo -eq 0 $foo is undefined, test will error out, and if it is undefined in test -n $foo, the test will be true (because POSIX demands that test with one argument be true).

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As a sidenote, in fish versions before 2.3.0, you need to add begin and end around a condition with and or or because it was interpreted weirdly.

So you'd have to do

if begin COMMAND; or COMMAND2; end # do something for status = 0