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Oil Language FAQ

Here are some common questions about the Oil language. Many of the answers boil down to the fact that Oil is a smooth upgrade from bash.

Old and new constructs exist side-by-side. New constructs have fewer "gotchas".

What's the difference myvar, $myvar, and "$myvar" ?

Oil is more like Python/JavaScript rather than PHP/Perl, so it doesn't use the $ sigil as much.

Never use $ on the left-hand side:

var mystr = "foo"   # not var $mystr

Use $ to substitute vars into commands:

echo $mystr
echo $mystr/subdir  # no quotes in commands

or quoted strings:

echo "$mystr/subdir"
var x = "$mystr/subdir"

Rarely use $ on the right-hand side:

var x = mystr       # preferred
var x = $mystr      # ILLEGAL -- use remove $
var x = ${mystr:-}  # occasionally useful

var x = $?          # allowed

See Command vs. Expression Mode for more details.

What's the difference between $(dirname $x) and $len(x) ?

Superficially, both of these syntaxes take an argument x and return a string. But they are different:

• $(dirname $x) is a shell command substitution that returns a string, and starts another process.
• $len(x) is a function call, and doesn't need to start a process.
  • Note that len(x) is an expression that evaluates to an integer, and $len(x) converts it to a string.

(Note: builtin subs like ${.myproc $x} are meant to eliminate process overhead, but they're not yet implemented.)

How can I return rich values from shell functions / Oil procs?

There are two primary ways:

• Print the "returned" data to stdout. Retrieve it with a command sub like $(myproc) or a pipeline like myproc | read --line.
• Use an "out param" with setref.

(Oil may grow true functions with the func keyword, but it will be built on top of proc and the builtin sub mechanism.)

Send us feedback if this doesn't make sense, or if you want a longer explanation.

Why doesn't a raw string work here: ${array[r'\']} ?

This boils down to the difference between OSH and Oil, and not being able to mix the two. Though they look similar, ${array[i]} syntax (with braces) is fundamentally different than $[array[i]] syntax (with brackets).

• OSH supports ${array[i]}.
  • The index is legacy/deprecated shell arithmetic like ${array[i++]} or ${assoc["$key"]}.
  • The index cannot be a raw string like r'\'.
• Oil supports both, but expression substitution syntax $[array[i]] is preferred.
  • It accepts Oil expressions like $[array[i + 1] or $[mydict[key]].
  • A raw string like r'\' is a valid key, e.g. $[mydict[r'\']].

Of course, Oil style is preferred when compatibility isn't an issue.

No:

echo ${array[r'\']}

Yes:

echo $[array[r'\']]

A similar issue exists with arithmetic.

Old:

echo $((1 + 2))   # shell arithmetic

New:

echo $[1 + 2]     # Oil expression

Related

• Oil Language FAQ on the wiki has more answers. They may be migrated here at some point.