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Oil Language vs. Shell

This doc aims to help shell users understand Oil.

If you don't know shell, see A Tour of the Oil Language.

Strictness At Parse Time and Runtime

• Parse time: statically parsed.
• Runtime: Many strict_* shell options to reduce edge cases. Oil generally fails faster than shell.

There Are 3 Core Sublanguages Instead of 4

• Shell: Command, Word, Arith, Bool
• Oil: Command, Word, Expression (Python-Like Expressions)

Expressions Replace Arith and Bool

So Many Shell Constructs Are Deprecated/Discouraged.

All of this is discouraged in favor of Oil expressions:

• [[ $x =~ $pat ]]
• x=$(( x + 1 )) and (( x = x + 1 )) and let, etc.
• declare -A assoc=(['k1']=v1 ['k2']=v2)

For details on the expression language, see Oil's Expression Language (under construction).

See Oil Languages Idioms for more rewrites.

Command Language Differences

• Curly Braces instead of then fi and do done
• Procs and Blocks for Modularity
  • Shell functions are "upgraded" into procs, e.g. with named parameters
  • Ruby-like Blocks, and metaprogramming
• Keywords for Variables like var, const, setvar instead of builtins like local, readonly, myvar=foo, etc.
  • Array literals like var a = :| ale bean | instead of local a=(ale bean).
• Multiline strings replace here docs
• fork and forkwait builtins instead of & and (). Parentheses are generally used for Python-like expressions, e.g. if (x > 0) { echo 'positive' }

See A Tour of Oil and Command Language for more details.

Word Language Differences

• Simple Word Evaluation
  • Splicing with arrays
• Expression substitution like $[42 + a[i] + f(x)]
• Inline function calls like echo $[join(['pea', nut'])]
• You can write raw strings like echo r'C:\Program Files\'

See A Tour of Oil and Word Language for more details.

Runtime

Builtins

• Oil adds long flags like read --line
• Oil has builtin Functions

Shell Options, shvar, Registers

• shopts: shopt --set parse_brace
• shvars: IFS, _DIALECT
• Registers: _pipeline_status, _match(), etc.

Big Feature Categories

• Builtin sub with ${.myproc arg1 $x} (TODO)
• String Safety (TODO)
• Modules: for organizing code into files
• Metaprogramming with Blocks (TODO)

Some Shell Features That Oil Retains

Here's an incomplete list of bash features that are preserved:

• C-style strings like $'line\n'
• Brace expansion like {alice,bob}@example.com
• Process Substitution like diff <(sort left.txt) <(sort right.txt)

Related Links

• Oil Languages Idioms shows example of Oil and shell side by side.
• What Breaks When You Upgrade to Oil. These are breaking changes.
• Oil Expressions vs. Python