How can I write if/else with Boolean in Bash? [duplicate]

Question

How can I write an 'if then' statement to switch between these to variables as in the following?

if(switch){
  server_var_shortname=$server_shared_shortname
  server_var=$server_shared
  server_var_bare=$server_shared_bare
} else {
  server_var_shortname=$server_vps_shortname
  server_var=$server_vps
  server_var_bare=$server_vps_bare
}

I'm not familiar with Bash syntax and basically just need an 'if/else' statement on a Boolean. Also, can I use true / false values as such? Also how do I do the 'else' statement?

$switch=true;
if $switch
then
  server_var_shortname=$server_shared_shortname
  server_var=$server_shared
  server_var_bare=$server_shared_bare
fi

Answer 1

First, shells (including Bash) don't have Booleans; they don't even have integers (although they can sort of fake it). Mostly, they have strings.

Bash also has arrays... of strings. There are a number of ways of faking Booleans; my favorite is to use the strings "true" and "false". These also happen to be the names of commands that always succeed and fail respectively, which comes in handy, because the if statement actually takes a command, and runs the then clause if it succeeds and the else clause if it fails. Thus, you can "run" the Boolean, and it'll succeed if set to "true" and fail if set to "false". Like this:

switch=true  # This doesn't have quotes around it, but it's a string anyway.
# ...
if $switch; then
    server_var_shortname=$server_shared_shortname
    server_var=$server_shared
    server_var_bare=$server_shared_bare
else
    server_var_shortname=$server_vps_shortname
    server_var=$server_vps
    server_var_bare=$server_vps_bare
fi

Note that the more usual format you'll see for if has square-brackets, like if [ something ]; then. In this case, [ is actually a command (not some funny sort of grouping operator) that evaluates its argument as an expression; thus [ "some string" = "some other string" ] is a command that will fail because the strings aren't equal. You could use if [ "$switch" = true ]; then, but I prefer to cheat and use the fake Boolean directly.

Caveat: if you do use the cheat I'm suggesting, make sure your "Boolean" variable is set to either "true" or "false" -- not unset, not set to something else. If it's set to anything else, I take no responsibility for the results.

Some other syntax notes:

• Use $ on variables when fetching their values, not when assigning to them. You have $switch=true; up there, which will get you an error.
• Also, you have a semicolon at the end of that line. This is unnecessary; semicolons are used to separate multiple commands on the same line (and a few other places), but they aren't needed to end the last (/only) command on a line.

• The [ command (which is also known as test) has a kind of weird syntax. Mostly because it's a command, so it goes through the usual command parsing, so e.g. [ 5 > 19 ] is parsed as [ 5 ] with output sent to a file named "19" (and is then true, because "5" is nonblank). [ 5 ">" 19 ] is better, but still evaluates to true because > does string (alphabetical) comparisons, and "5" is alphabetically after "19". [ 5 -gt 19 ] does the expected thing.

  There's also [[ ]] (similar, but cleaner syntax and not available in all shells) and (( )) (for math, not strings; also not in all shells). See Bash FAQ #31.

• Putting commands in variables is generally a bad idea. See Bash FAQ #50.

• shellcheck.net is your friend.