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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 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: command subs may be optimized later, as ksh does.)

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 at some point. However, that must be done carefully, as a proc composes with processes, but a func doesn't.

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'\']} ?

Oil has two array index syntax:

• The legacy shell-like syntax ${array[i]}, which accepts shell arithmetic expressions (which consist of number-like strings).
• The Oil expression subsitution syntax $[array[i]], which accepts Oil expressions (which consist of typed data).

No:

echo ${array[r'\']}

Yes:

echo $[array[r'\']]

A similar issue exists with arithmetic.

Old:

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

New:

echo $[1 + 2]     # Oil expression