A Feel For YSH Syntax

A short way to describe the YSH language:

A Unix shell that's familiar to people who know Python, JavaScript, or Ruby.

This document gives you a feel for that, with brief examples. It's not a comprehensive or precise guide. Roughly speaking, YSH code has more punctuation than those 3 languages, but less than shell and Perl.

If you're totally unfamiliar with the language, read The Simplest Explanation of Oil first. (Oil was renamed YSH in 2023.)

Table of Contents
Very Common
Less Common
Opening and Closing Delimiters
Braces: Command Blocks and Dict Literal Expressions
Parens: Expression
Parens with Sigil: Command Interpolation
Brackets: Sequence, Subscript
Brackets with a Sigil: Expression
Spaces Around = ?
Naming Conventions for Identifiers
Other Punctuation Usage
Related Docs
Appendix: Table of Sigil Pairs


Different parts of YSH are parsed in either command or expression mode. Command mode is like shell:

echo $x 

Expression mode looks like Python or JavaScript, and appears on right-hand side of =:

var x = 42 + array[i]

The examples below aren't organized along those lines, but they use var and echo to remind you of the context. Some constructs are valid in both modes.


Sigils are punctuation characters that precede a name, e.g. the $ in $mystr.

Unlike Perl and PHP, Oil doesn't use sigils on the LHS of assignments, or in expression mode. The syntactic concepts doc explains this difference.

Very Common

The $ and @ sigils mean roughly what they do in shell, Perl, and PowerShell.

$ means string / scalar. These shell constructs are idiomatic in Oil:

$mvar   ${myvar}

And these Oil language extensions also use $:

echo $[42 + a[i]]            # string interpolation of expression
grep $/ digit+ /             # inline eggex (not implemented yet)

@ means array / splice an array:

echo "$@"                    # Legacy syntax; prefer @ARGV


echo @strs                   # splice array

echo @[split(x)] @[glob(x)]  # splice expressions that returns arrays

for i in @(seq 3) {          # split command sub
  echo $i

proc p(first, @rest) {       # named varargs in proc signatures
  write -- $first            # (procs are shell-like functions)
  write -- @rest

Less Common

The colon means "unquoted word" in these two lines:

var mysymbol = :key               # string, not implemented yet
var myarray = :| one two three |  # array

It's also used to pass the name of a variable to a builtin:

echo hi | read :myvar

A caret means "unevaluated":

var cmd = ^(cd /tmp; ls *.txt)
var expr = ^[42 + a[i]]  # unimplemented
var template = ^"var = $var"  # unimplemented

Opening and Closing Delimiters

The {} [] and () characters have several different meanings, but we try our best to make them consistent. They're subject to legacy constraints from Bourne shell, Korn shell, and bash.

Braces: Command Blocks and Dict Literal Expressions

In expression mode, {} are used for dict literals (aka hash tables, associative arrays), which makes Oil look like JavaScript:

var d = {name: 'Bob', age: 10}

while (x > 0) {
  setvar x -= 1

In command mode, they're used for blocks of code:

cd /tmp {
  echo $PWD

Blocks are also used for "declarative" configuration:

server www.example.com {
  port = 80
  root = '/home/www'
  section bar {

Parens: Expression

Parens are used in expressions:

var x = (42 + a[i]) * myfunc(42, 'foo')

if (x > 0) {         # compare with if test -d /tmp
  echo 'positive'

And signatures:

proc p(x, y) {
  echo $x $y

In Eggex, they mean grouping and not capture, which is consistent with other YSH expressions:

var p = / digit+ ('seconds' | 'minutes' | 'hours' ) /

Parens with Sigil: Command Interpolation

The "sigil pairs" with parens enclose commands:

echo $(ls | wc -l)             # command sub
echo @(seq 3)                  # split command usb

var myblock = ^(echo $PWD)     # block literal in expression mode

diff <(sort left.txt) <(sort right.txt)  # bash syntax

Unlike brackets and braces, the () characters can't appear in shell commands, which makes them useful as delimiters.

Brackets: Sequence, Subscript

In expression mode, [] means sequence:

var mylist = ['one', 'two', 'three']

or subscript:

var item = mylist[1]
var item = mydict['foo']

Brackets with a Sigil: Expression

In command mode, it means "expression":

echo $[1 + 2]

Spaces Around = ?

In Oil, your own variables look like this:

const x = 42
var s = 'foo'
setvar s = 'bar'

In contrast, special shell variables are written with a single NAME=value argument:

shvar PATH=/tmp {

Which is similar to the syntax of the env command:

env PYTHONPATH=/tmp ./myscript.py

Naming Conventions for Identifiers

kebab-case is for procs and filenames:

gc-test   opt-stats   gen-mypy-asdl


snake_case is for local variables:

proc foo {
  var deploy_dest = 'bar@example.com'
  echo $deploy_dest

CAPS are used for global variables built into the shell:


External programs also accept environment variables in CAPS:


Global variables that are silently mutated by the interpreter start with _:

_argv   _status   _pipeline_status   _line

As do functions to access such mutable vars:

_match()  _start()   _end()  _field()

Other Punctuation Usage

Here are other usages of the punctuation discussed:

echo *.[ch]                    # glob char and char classes
echo {alice,bob}@example.com   # brace expansion


/ [a-f A-F 0-9] /         # char classes use []

/ digit+ ('ms' | 'us') /  # non-capturing group
< digit+ >                # capturing group
< digit+ :hour >          # named capture

dot{3,4} a{+ N}           # repetition

The ~ character is used in operators that mean "pattern" or "approximate":

if (s ~ /d+/) {
  echo 'number'

if (s ~~ '*.py') {
  echo 'Python'

if (mystr ~== myint) {
  echo 'string equals number'

Extended globs are discouraged in Oil because they're a weird way of writing regular expressions. But they also use "sigil pairs" with parens:

,(*.py|*.sh)   # preferred synonym for @(*.py|*.sh)
+(...)         # bash/ksh-compatible

Shell arithmetic is also discouraged in favor of Oil arithmetic:

echo $((1 + 2))  # shell: confusing coercions, dynamically parsed
echo $[1 + 2]    # Oil: types, statically parsed

Related Docs

Appendix: Table of Sigil Pairs

This table is mainly for Oil language designers. Many constructs aren't implemented, but we reserve space for them. The Oils Reference is more complete.

Example      Description        What's Inside  Where Valid  Notes

$(hostname)  Command Sub        Command        cmd,expr
@(seq 3)     Split Command Sub  Command        cmd,expr     should decode J8

{ echo hi }  Block Literal      Command        cmd          shell requires ;
^(echo hi)   Unevaluated Block  Command        expr         rare

>(sort -n)   Process Sub        Command        cmd          rare
<(echo hi)   Process Sub        Command        cmd          rare

:|foo $bar|  Array Literal      Words          expr

$[42 + a[i]] Stringify Expr     Expression     cmd,expr
@[glob(x)]   Array-ify Expr     Expression     cmd,expr     not implemented
^[42 + a[i]] Unevaluated Expr   Expression     expr         not implemented

^"$1 $2"     Unevaluated Str    DQ String      expr         not implemented

${x %2d}     Var Sub            Formatting     cmd,expr     not implemented
${x|html}    Var Sub            Formatting     cmd,expr     not implemented

json (x)     Typed Arg List     Argument       cmd

$/d+/        Inline Eggex       Eggex Expr     cmd          not implemented

r''          Raw String         String         expr         cmd when shopt
             Literal                                        parse_raw_string

j""          JSON8 String       String         cmd,expr     not implemented

#'a'         Char Literal       UTF-8 char     expr

Discouraged / Deprecated

${x%%pre}    Shell Var Sub      Shell          cmd,expr     mostly deprecated
$((1+2))     Shell Arith Sub    Shell Arith    cmd          deprecated

@(*.py|*.sh) Extended Glob      Glob Words     cmd          deprecated

,(*.py|*.sh) Extended Glob      Glob Words     cmd          break conflict
                                                            with split command

Key to "where valid" column:

Some unused sigil pairs:

~()   -()   =()   /()   _()   .()

Generated on Wed, 13 Sep 2023 01:45:04 -0400