Whitespace¶

In this document, an inline whitespace is any of the following:

•

A space (U+0020);

•

A tab (U+0009);

•

A comment: starting with # and ending before (but not including) the next carriage return, newline or end of file;

•

A line continuation: a ^ followed by a newline ("\n"), or a carriage return and newline ("\r\n").

A whitespace is any of the following:

•

An inline whitespace;

•

A carriage return (U+000D);

•

A newline (U+000A).

Metacharacters¶

The following metacharacters serve to introduce or delimit syntax constructs:

The following characters are parsed as metacharacters under certain conditions:

•

~ is a metacharacter if it appears at the beginning of a compound expression, in which case it is subject to tilde expansion;

•

= is a metacharacter when used for terminating map keys or option keys, or denoting legacy assignment form or temporary assignments.

Single-quoted string¶

A single-quoted string consists of zero or more characters enclosed in single quotes ('). All enclosed characters represent themselves, except the single quote.

Two consecutive single quotes are handled as a special case: they represent one single quote, instead of terminating a single-quoted string and starting another.

Examples: '*\' evaluates to *\, and 'it''s' evaluates to it's.

Double-quoted string¶

A double-quoted string consists of zero or more characters enclosed in double quotes ("). All enclosed characters represent themselves, except backslashes (\), which introduces escape sequences. Double quotes are not allowed inside double-quoted strings, except after backslashes.

The following escape sequences are supported:

•

\cX, where X is a character with codepoint between 0x40 and 0x5F, represents the codepoint that is 0x40 lower than X. For example, \cI is the tab character: 0x49 (I) - 0x40 = 0x09 (tab). There is one special case: A question-mark is converted to del; i.e., \c? or \^? is equivalent to \x7F.

•

\^X is the same as \cX.

•

\[0..7][0..7][0..7] is a byte written as an octal value. There must be three octal digits following the backslash. For example, \000 is the nul character, and \101 is the same as A, but \0 is an invalid escape sequence (too few digits).

•

\x.. is a Unicode code point represented by two hexadecimal digits.

•

\u.... is a Unicode code point represented by four hexadecimal digits.

•

\U...... is a Unicode code point represented by eight hexadecimal digits.

•

The following single character escape sequences:

An unsupported escape sequence results in a parse error.

Note: Unlike most other shells, double-quoted strings in Elvish do not support interpolation. For instance, "$name" simply evaluates to a string containing $name. To get a similar effect, simply concatenate strings: instead of "my name is $name", write "my name is "$name. Under the hood this is a compounding operation.

Bareword¶

A string can be written without quoting – a bareword, if it only includes the characters from the following set:

•

ASCII letters (a-z and A-Z) and numbers (0-9);

•

The symbols !%+,-./:@\_;

•

Non-ASCII codepoints that are printable, as defined by unicode.IsPrint (https://godoc.org/unicode#IsPrint) in Go’s standard library.

Examples: a.txt, long-bareword, elf@elv.sh, /usr/local/bin, 你好世界.

Moreover, ~ and = are allowed to appear without quoting when they are not parsed as metacharacters.

Note: since the backslash (\) is a valid bareword character in Elvish, it cannot be used to escape metacharacter. Use quotes instead: for example, to echo a star, write echo "*" or echo '*', not echo \*. The last command just writes out \*.

String¶

A string is a (possibly empty) sequence of bytes.

Single-quoted string literals, double-quoted string literalsand barewords all evaluate to string values. Unless otherwise noted, different syntaxes of string literals are equivalent in the code. For instance, xyz, 'xyz' and "xyz" are different syntaxes for the same string with content xyz.

Strings that contain UTF-8 encoded text can be indexed with a byte index where a codepoint starts, which results in the codepoint that starts there. The index can be given as either a typed number, or a string that parses to a number. Examples:

•

In the string elv, every codepoint is encoded with only one byte, so 0, 1, 2 are all valid indices:

•

In the string 世界, each codepoint is encoded with three bytes. The first codepoint occupies byte 0 through 2, and the second occupies byte 3 through 5. Hence valid indices are 0 and 3:

Such strings may also be indexed with a slice (see documentation of list for slice syntax). The range determined by the slice is also interpreted as byte indices, and the range must begin and end at codepoint boundaries.

The behavior of indexing a string that does not contain valid UTF-8-encoded Unicode text is unspecified.

Note: String indexing will likely change.

Number¶

Elvish has a double-precision floating point number type.

There is no literal syntax for the number type; it can be constructed with the float64 builtin. The builtin takes a single argument, which should be either another float64 value, or a string in the following formats (examples below all express the same value):

•

Decimal notation, e.g. 10.

•

Hexadecimal notation, e.g. 0xA.

•

Octal notation, e.g. 0o12.

•

Binary notation, e.g. 0b1010.

•

Floating point notation, e.g. 10.0.

•

Scientific notation, e.g. 1.0e1.

The following special floating point values are also supported: +Inf, -Inf and NaN.

The float64 builtin is case-insensitive.

Numbers can contain underscores between digits to improve readability. For example, 1000000 and 1_000_000 are equivalent. As is 1.234_56e3 and 1.23456e3. You can not use an underscore as a prefix or suffix in a number.

A float64 data type can be converted to a string using (to-string $number). The resulting string is guaranteed to result in the same value when converted back to a float64. Most of the time you won’t need to perform this explicit conversion. Elvish will implicitly make the conversion when running external commands and many of the builtins (where the distinction is not important).

You usually do not need to use float64 values explicitly; see the discussion of Commands That Operate On Numbers.

List¶

A list is a value containing a sequence of values. Values in a list are called its elements. Each element has an index, starting from zero.

List literals are surrounded by square brackets [ ], with elements separated by whitespace. Examples:

~> put [lorem ipsum]
▶ [lorem ipsum]
~> put [lorem


        ipsum


        foo


        bar]
▶ [lorem ipsum foo bar]
    

Note: In Elvish, commas have no special meanings and are valid bareword characters, so don’t use them to separate elements:

~> li = [a, b]
~> put $li
▶ [a, b]
~> put $li[0]
▶ a,
    

A list can be indexed with the index of an element to obtain the element, which can take one of two forms:

•

A non-negative integer, an offset counting from the beginning of the list. For example, $li[0] is the first element of $li.

•

A negative integer, an offset counting from the back of the list. For instance, $li[-1] is the last element $li.

In both cases, the index can be given either as a typed number or a number-like string.

A list can also be indexed with a slice to obtain a sublist, which can take one of two forms:

•

A slice $a..$b, where both $a and $b are integers. The result is sublist of $li[$a] up to, but not including, $li[$b]. For instance, $li[4..7] equals [$li[4] $li[5] $li[6]], while $li[1..-1] contains all elements from $li except the first and last one.

•

A slice $a..=$b, which is similar to $a..$b, but includes $li[$b].

Examples:

~> li = [lorem ipsum foo bar]
~> put $li[0]
▶ lorem
~> put $li[-1]
▶ bar
~> put $li[0..2]
▶ [lorem ipsum]
    

Map¶

A map is a value containing unordered key-value pairs.

Map literals are surrounded by square brackets; a key/value pair is written &key=value (reminiscent to HTTP query parameters), and pairs are separated by whitespaces. Whitespaces are allowed after =, but not before =. Examples:

~> put [&foo=bar &lorem=ipsum]
▶ [&foo=bar &lorem=ipsum]
~> put [&a=   10


        &b=   23


        &sum= (+ 10 23)]
▶ [&a=10 &b=23 &sum=33]
    

The literal of an empty map is [&].

Specifying a key without = or a value following it is equivalent to specifying $true as the value. Specifying a key with = but no value following it is equivalent to specifying the empty string as the value. Example:

~> echo [&a &b=]
[&a=$true &b='']
    

A map can be indexed by any of its keys. Unlike strings and lists, there is no support for slices, and .. and ..= have no special meanings. Examples:

~> map = [&a=lorem &b=ipsum &a..b=haha]
~> echo $map[a]
lorem
~> echo $map[a..b]
haha
    

Pseudo-map¶

The concept of pseudo-map is not a concrete data type, but refer to certain data types that behave like maps in some aspects.

A pseudo-map typically has a fixed set of keys, called fields. The values of a field can be accessed by indexing. However, unlike maps, it is usually not possible to add new keys, remove existing keys, or even assigning a new value to an existing key.

The pseudo-map mechanism is often used for introspection. For example, exceptions and user-defined functions are both structmaps.

Exception¶

An exception carries information about errors during the execution of code.

There is no literal syntax for exceptions. See the discussion of exception and flow commands for more information about this data type.

An exception is a pseudo-map with a reason field, which is in turn a pseudo-map. The reason pseudo-map has has a type field identifying how the exception was raised, and further fields depending on the type:

•

If the type field is fail, the exception was raised by the fail command.

•

If the type field is flow, the exception was raised by one of the flow commands.

•

If the type field is pipeline, the exception was a result of multiple commands in the same pipeline raising exceptions.

•

If the type field starts with external-cmd/, the exception was caused by one of several conditions of an external command. In this case, the following fields are available:

•

If the type field is external-cmd/exited, the external command exited with a non-zero status code. In this case, the exit-status field contains the exit status.

•

If the type field is external-cmd/signaled, the external command was killed by a signal. In this case, the following extra fields are available:

•

If the type field is external-cmd/stopped, the external command was stopped. In this case, the following extra fields are available:

Examples:

~> put ?(fail foo)[reason]
▶ [&content=foo &type=fail]
~> put ?(return)[reason]
▶ [&name=return &type=flow]
~> put ?(false)[reason]
▶ [&cmd-name=false &exit-status=1 &pid=953421 &type=external-cmd/exited]
    

Function¶

A function encapsulates a piece of code that can be executed in an ordinary command, and takes its arguments and options. Functions are first-class values; they can be kept in variables, used as arguments, output on the value channel and embedded in other data structures. Elvish comes with a set of builtin functions, and Elvish code can also create user-defined functions.

Note: Unlike most programming languages, functions in Elvish do not have return values. Instead, they can output values, which can be captured later.

A function literal, or alternatively a lambda, evaluates to a user-defined function. The literal syntax consists of an optional signature list, followed by a code chunk that defines the body of the function.

Here is an example without a signature:

~> f = { echo "Inside a lambda" }
~> put $f
▶ <closure 0x18a1a340>
    

One or more whitespace characters after { is required: Elvish relies on the presence of whitespace to disambiguate function literals and braced lists.

Note: It is good style to put some whitespace before the closing } for symmetry, but this is not required by the syntax.

Functions defined without a signature list do not accept any arguments or options. To do so, write a signature list. Here is an example:

~> f = [a b]{ put $b $a }
~> $f lorem ipsum
▶ ipsum
▶ lorem
    

There must be no space between ] and {; otherwise Elvish will parse the signature as a list, followed by a lambda without signature:

~> put [a]{ nop }
▶ <closure 0xc420153d80>
~> put [a] { nop }
▶ [a]
▶ <closure 0xc42004a480>
    

Like in the left hand of assignments, if you prefix one of the arguments with @, it becomes a rest argument, and its value is a list containing all the remaining arguments:

~> f = [a @rest]{ put $a $rest }
~> $f lorem
▶ lorem
▶ []
~> $f lorem ipsum dolar sit
▶ lorem
▶ [ipsum dolar sit]
~> f = [a @rest b]{ put $a $rest $b }
~> $f lorem ipsum dolar sit
▶ lorem
▶ [ipsum dolar]
▶ sit
    

You can also declare options in the signature. The syntax is &name=default (like a map pair), where default is the default value for the option; the value of the option will be kept in a variable called name:

~> f = [&opt=default]{ echo "Value of $opt is "$opt }
~> $f
Value of $opt is default
~> $f &opt=foobar
Value of $opt is foobar
    

Options must have default values: Options should be optional.

If you call a function with too few arguments, too many arguments or unknown options, an exception is thrown:

~> [a]{ echo $a } foo bar
Exception: need 1 arguments, got 2
[tty], line 1: [a]{ echo $a } foo bar
~> [a b]{ echo $a $b } foo
Exception: need 2 arguments, got 1
[tty], line 1: [a b]{ echo $a $b } foo
~> [a b @rest]{ echo $a $b $rest } foo
Exception: need 2 or more arguments, got 1
[tty], line 1: [a b @rest]{ echo $a $b $rest } foo
~> [&k=v]{ echo $k } &k2=v2
Exception: unknown option k2
[tty], line 1: [&k=v]{ echo $k } &k2=v2
    

A user-defined function is a pseudo-map. If $f is a user-defined function, it has the following fields:

•

$f[arg-names] is a list containing the names of the arguments.

•

$f[rest-arg] is the index of the rest argument. If there is no rest argument, it is -1.

•

$f[opt-names] is a list containing the names of the options.

•

$f[opt-defaults] is a list containing the default values of the options, in the same order as $f[opt-names].

•

$f[def] is a string containing the definition of the function, including the signature and the body.

•

$f[body] is a string containing the body of the function, without the enclosing brackets.

•

$f[src] is a map-like data structure containing information about the source code that the function is defined in. It contains the same value that the src function would output if called from the function.

Variable suffix¶

There are two characters that have special meanings and extra type constraints when used as the suffix of a variable name:

•

If a variable name ends with ~, it can only take callable values, which are functions and external commands. Such variables are consulted when resolving ordinary commands.

•

If a variable name ends with :, it can only take namespaces as values. They are used for accessing namespaced variables.

Scoping rule¶

Elvish has lexical scoping. A file or an interactive prompt starts with a top-level scope, and a function literal introduce new lexical scopes.

When you use a variable, Elvish looks for it in the current lexical scope, then its parent lexical scope and so forth, until the outermost scope:

~> x = 12
~> { echo $x } # $x is in the global scope
12
~> { y = bar; { echo $y } } # $y is in the outer scope
bar
    

If a variable is not in any of the lexical scopes, Elvish tries to resolve it in the builtin: namespace, and if that also fails, fails with an error:

~> echo $pid # builtin
36613
~> echo $nonexistent
Compilation error: variable $nonexistent not found


  [interactive], line 1:


    echo $nonexistent
    

Note that Elvish resolves all variables in a code chunk before starting to execute any of it; that is why the error message above says compilation error. This can be more clearly observed in the following example:

~> echo pre-error; echo $nonexistent
Compilation error: variable $nonexistent not found
[tty], line 1: echo pre-error; echo $nonexistent
    

When you assign a variable, Elvish does a similar searching. If the variable cannot be found, instead of causing an error, it will be created in the current scope:

~> x = 12
~> { x = 13 } # assigns to x in the global scope
~> echo $x
13
~> { z = foo } # creates z in the inner scope
~> echo $z
Compilation error: variable $z not found
[tty], line 1: echo $z
    

One implication of this behavior is that Elvish will not shadow your variable in outer scopes.

There is a local: namespace that always refers to the current scope, and by using it it is possible to force Elvish to shadow variables:

~> x = 12
~> { local:x = 13; echo $x } # force shadowing
13
~> echo $x
12
    

After force shadowing, you can still access the variable in the outer scope using the up: namespace, which always skips the innermost scope:

~> x = 12
~> { local:x = 14; echo $x $up:x }
14 12
    

The local: and up: namespaces can also be used on unshadowed variables, although they are not useful in those cases:

~> foo = a
~> { echo $up:foo } # $up:foo is the same as $foo
a
~> { bar = b; echo $local:bar } # $local:bar is the same as $bar
b
    

It is not possible to refer to a specific outer scope.

You cannot create new variables in the builtin: namespace, although existing variables in it can be assigned new values.

Closure semantics¶

When a function literal refers to a variable in an outer scope, the function will keep that variable alive, even if that variable is the local variable of an outer function that that function has returned. This is called closure semantics (https://en.wikipedia.org/wiki/Closure_(computer_programming)), because the function literal “closes” over the environment it is defined in.

In the following example, the make-adder function outputs two functions, both referring to a local variable $n. Closure semantics means that:

1.

Both functions can continue to refer to the $n variable after make-adder has returned.

2.

Multiple calls to the make-adder function generates distinct instances of the $n variables.

~> fn make-adder {


     n = 0


     put { put $n } { n = (+ $n 1) }


   }
~> getter adder = (make-adder)
~> $getter # $getter outputs $n
▶ 0
~> $adder # $adder increments $n
~> $getter # $getter and $setter refer to the same $n
▶ 1
~> getter2 adder2 = (make-adder)
~> $getter2 # $getter2 and $getter refer to different $n
▶ 0
~> $getter
▶ 1
    

Variables that get “captured” in closures are called upvalues; this is why the pseudo-namespace for variables in outer scopes is called up:. When capturing upvalues, Elvish only captures the variables that are used. In the following example, $m is not an upvalue of $g because it is not used:

~> fn f { m = 2; n = 3; put { put $n } }
~> g = (f)
    

This effect is not currently observable, but will become so when namespaces become introspectable (https://github.com/elves/elvish/issues/492).

Literal¶

Literals of strings, lists, maps and functions all evaluate to one value of their corresponding types. They are described in their respective sections.

Variable use¶

A variable use expression is formed by a $ followed by the name of the variable. Examples:

~> foo = bar
~> x y = 3 4
~> put $foo
▶ bar
~> put $x
▶ 3
    

If the variable name only contains the following characters (a subset of bareword characters), the name can appear unquoted after $ and the variable use expression extends to the longest sequence of such characters:

•

ASCII letters (a-z and A-Z) and numbers (0-9);

•

The symbols -_:~. The colon : is special; it is normally used for separating namespaces or denoting namespace variables;

•

Non-ASCII codepoints that are printable, as defined by unicode.IsPrint (https://godoc.org/unicode#IsPrint) in Go’s standard library.

Alternatively, $ may be followed immediately by a single-quoted string (https://elv.sh/ref/language.html#single-quoted-string) or a double-quoted string (https://elv.sh/ref/language.html#double-quoted-string), in which cases the value of the string specifies the name of the variable. Examples:

~> "\n" = foo
~> put $"\n"
▶ foo
~> '!!!' = bar
~> put $'!!!'
▶ bar
    

Unlike other shells and other dynamic languages, local namespaces in Elvish are statically checked. This means that referencing a nonexistent variable results in a compilation error, which is triggered before any code is actually evaluated:

~> echo $x
Compilation error: variable $x not found
[tty], line 1: echo $x
~> f = { echo $x }
compilation error: variable $x not found
[tty 1], line 1: f = { echo $x }
    

If a variable contains a list value, you can add @ before the variable name; this evaluates to all the elements within the list. This is called exploding the variable:

~> li = [lorem ipsum foo bar]
~> put $li
▶ [lorem ipsum foo bar]
~> put $@li
▶ lorem
▶ ipsum
▶ foo
▶ bar
    

Note: Since variable uses have higher precedence than indexing, this does not work for exploding a list that is an element of another list. For doing that, and exploding the result of other expressions (such as an output capture), use the builtin all command.)

Output capture¶

An output capture expression is formed by putting parentheses () around a code chunk. It redirects the output of the chunk into an internal pipe, and evaluates to all the values that have been output.

~> + 1 10 100
▶ 111
~> x = (+ 1 10 100)
~> put $x
▶ 111
~> put lorem ipsum
▶ lorem
▶ ipsum
~> x y = (put lorem ipsum)
~> put $x
▶ lorem
~> put $y
▶ ipsum
    

If the chunk outputs bytes, Elvish strips the last newline (if any), and split them by newlines, and consider each line to be one string value:

~> put (echo "a\nb")
▶ a
▶ b
    

Trailing carriage returns are also stripped from each line, which effectively makes \r\n also valid line separators:

~> put (echo "a\r\nb")
▶ a
▶ b
    

Note 1. Only the last newline is ever removed, so empty lines are preserved; (echo "a\n") evaluates to two values, "a" and "".

Note 2. One consequence of this mechanism is that you can not distinguish outputs that lack a trailing newline from outputs that have one; (echo what) evaluates to the same value as (print what). If such a distinction is needed, use slurp to preserve the original bytes output.

If the chunk outputs both values and bytes, the values of output capture will contain both value outputs and lines. However, the ordering between value output and byte output might not agree with the order in which they happened:

~> put (put a; echo b) # value order need not be the same as output order
▶ b
▶ a
    

Exception capture¶

An exception capture expression is formed by putting ?() around a code chunk. It runs the chunk and evaluates to the exception it throws.

~> fail bad
Exception: bad
Traceback:


  [interactive], line 1:


    fail bad
~> put ?(fail bad)
▶ ?(fail bad)
    

If there was no error, it evaluates to the special value $ok:

~> nop
~> put ?(nop)
▶ $ok
    

Exceptions are booleanly false and $ok is booleanly true. This is useful in if (introduced later):

if ?(test -d ./a) {


  # ./a is a directory
}
    

Note: Exception captures do not affect the output of the code chunk. You can combine output capture and exception capture:

output = (error = ?(commands-that-may-fail))
    

Braced list¶

A braced list consists of multiple expressions separated by whitespaces and surrounded by braces ({}). There must be no space after the opening brace. A braced list evaluates to whatever the expressions inside it evaluate to. Its most typical use is grouping multiple values in a compound expression. Example:

~> put {a b}-{1 2}
▶ a-1
▶ a-2
▶ b-1
▶ b-2
    

It can also be used to affect the order of evaluation. Examples:

~> put *
▶ foo
▶ bar
~> put *o
▶ foo
~> put {*}o
▶ fooo
▶ baro
    

Note: When used to affect the order of evaluation, braced lists are very similar to parentheses in C-like languages.

Note: A braced list is an expression. It is a syntactical construct and not a separate data structure.

Elvish currently also supports using commas to separate items in a braced list. This will likely be removed in future, but it also means that literal commas must be quoted right now.

Indexing¶

An indexing expression is formed by appending one or more indices inside a pair of brackets ([]) after another expression (the indexee). Examples:

~> li = [foo bar]
~> put $li[0]
▶ foo
~> li = [[foo bar] quux]
~> put $li[0][0]
▶ foo
~> put [[foo bar]][0][0]
▶ foo
    

If the expression being indexed evaluates to multiple values, the indexing operation is applied on each value. Example:

~> put (put [foo bar] [lorem ipsum])[0]
▶ foo
▶ lorem
~> put {[foo bar] [lorem ipsum]}[0]
▶ foo
▶ lorem
    

If there are multiple index expressions, or the index expression evaluates to multiple values, the indexee is indexed once for each of the index value. Examples:

~> put elv[0 2 0..2]
▶ e
▶ v
▶ el
~> put [lorem ipsum foo bar][0 2 0..2]
▶ lorem
▶ foo
▶ [lorem ipsum]
~> put [&a=lorem &b=ipsum &a..b=haha][a a..b]
▶ lorem
▶ haha
    

If both the indexee and index evaluate to multiple values, the results generated from the first indexee appear first. Example:

~> put {[foo bar] [lorem ipsum]}[0 1]
▶ foo
▶ bar
▶ lorem
▶ ipsum
    

Compounding¶

A compound expression is formed by writing several expressions together with no space in between. A compound expression evaluates to a string concatenation of all the constituent expressions. Examples:

~> put 'a'b"c" # compounding three string literals
▶ abc
~> v = value
~> put '$v is '$v # compounding one string literal with one string variable
▶ '$v is value'
    

When one or more of the constituent expressions evaluate to multiple values, the result is all possible combinations:

~> li = [foo bar]
~> put {a b}-$li[0 1]
▶ a-foo
▶ a-bar
▶ b-foo
▶ b-bar
    

The order of the combinations is determined by first taking the first value in the leftmost expression that generates multiple values, and then taking the second value, and so on.

Tilde expansion¶

An unquoted tilde at the beginning of a compound expression triggers tilde expansion. The remainder of this expression must be a string. The part from the beginning of the string up to the first / (or the end of the word if the string does not contain /), is taken as a user name; and they together evaluate to the home directory of that user. If the user name is empty, the current user is assumed.

In the following example, the home directory of the current user is /home/xiaq, while that of the root user is /root:

~> put ~
▶ /home/xiaq
~> put ~root
▶ /root
~> put ~/xxx
▶ /home/xiaq/xxx
~> put ~root/xxx
▶ /root/xxx
    

Note that tildes are not special when they appear elsewhere in a word:

~> put a~root
▶ a~root
    

If you need them to be, use a braced list:

~> put a{~root}
▶ a/root
    

Wildcard expansion¶

Wildcard patterns are expressions that contain wildcards. Wildcard patterns evaluate to all filenames they match.

In examples in this section, we will assume that the current directory has the following structure:

.x.conf
a.cc
ax.conf
foo.cc
d/
|__ .x.conf
|__ ax.conf
|__ y.cc
.d2/
|__ .x.conf
|__ ax.conf
    

Elvish supports the following wildcards:

•

? matches one arbitrary character except /. For example, ?.cc matches a.cc;

•

* matches any number of arbitrary characters except /. For example, *.cc matches a.cc and foo.cc;

•

** matches any number of arbitrary characters including /. For example, **.cc matches a.cc, foo.cc and b/y.cc.

The following behaviors are default, although they can be altered by modifiers:

•

When the entire wildcard pattern has no match, an error is thrown.

•

None of the wildcards matches . at the beginning of filenames. For example:

Wildcards can be modified using the same syntax as indexing. For instance, in *[match-hidden] the * wildcard is modified with the match-hidden modifier. Multiple matchers can be chained like *[set:abc][range:0-9]. In which case they are OR’ed together.

There are two kinds of modifiers:

Global modifiers apply to the whole pattern and can be placed after any wildcard:

•

nomatch-ok tells Elvish not to throw an error when there is no match for the pattern. For instance, in the example directory put bad* will be an error, but put bad*[nomatch-ok] does exactly nothing.

•

but:xxx (where xxx is any filename) excludes the filename from the final result.

•

type:xxx (where xxx is a recognized file type from the list below). Only one type modifier is allowed. For example, to find the directories at any level below the current working directory: **[type:dir].

Although global modifiers affect the entire wildcard pattern, you can add it after any wildcard, and the effect is the same. For example, put */*[nomatch-ok].cpp and put *[nomatch-ok]/*.cpp do the same thing. On the other hand, you must add it after a wildcard, instead of after the entire pattern: put */*.cpp[nomatch-ok] unfortunately does not do the correct thing. (This will probably be fixed.)

Local modifiers only apply to the wildcard it immediately follows:

•

match-hidden tells the wildcard to match . at the beginning of filenames, e.g. *[match-hidden].conf matches .x.conf and ax.conf.

•

Character matchers restrict the characters to match:

Note the following caveats:

•

Local matchers chained together in separate modifiers are OR’ed. For instance, ?[set:aeoiu][digit] matches all files with the chars aeoiu or containing a digit.

•

Local matchers combined in the same modifier, such as ?[set:aeoiu digit], behave in a hard to explain manner. Do not use this form as the behavior is likely to change in the future.

•

Dots at the beginning of filenames always require an explicit match-hidden, even if the matcher includes .. For example, ?[set:.a]x.conf does not match .x.conf; you have to ?[set:.a match-hidden]x.conf.

•

Likewise, you always need to use ** to match slashes, even if the matcher includes /. For example *[set:abc/] is the same as *[set:abc].

Order of evaluation¶

An expression can use a combination of indexing, tilde expansion, wildcard and compounding. The order of evaluation is as follows:

1.

Literals, variable uses, output captures and exception captures and braced lists have the highest precedence and are evaluated first.

2.

Indexing has the next highest precedence and is then evaluated first.

3.

Expression compounding then happens. Tildes and wildcards are kept unevaluated.

4.

If the expression starts with a tilde, tilde expansion happens. If the tilde is followed by a wildcard, an exception is raised.

5.

If the expression contains any wildcard, wildcard expansion happens.

Here an example: in ~/$li[0 1]/* (where $li is a list [foo bar]), the expression is evaluated as follows:

1.

The variable use $li evaluates to the list [foo bar].

2.

The indexing expression $li[0] evaluates to two strings foo and bar.

3.

Compounding the expression, the result is ~/foo/* and ~/bar/*.

4.

Tilde expansion happens; assuming that the user’s home directory is /home/elf, the values are now /home/elf/foo/* and /home/elf/bar/*.

5.

Wildcard expansion happens, evaluating the expression to all the filenames within /home/elf/foo and /home/elf/bar. If any directory is empty or nonexistent, an exception is thrown.

To force a particular order of evaluation, group expressions using a braced list.

Ordinary command¶

An ordinary command form consists of a command head, and any number of arguments and options.

The first expression in an ordinary command is the command head. If the head is a single string literal it is subject to static resolution:

•

If a variable with name head~ (where head is the value of the head) exists, the head will evaluate as if it is $head~; i.e., a function invocation.

•

Otherwise, the head will evaluate to an external command with the name head.

If the head is not a single string literal, it is evaluated as a normal expression. The expression must evaluate to one value, and the value must be one of the following:

•

A callable value: a function or external command.

•

A string containing at least one slash, in which case it is treated like an external command with the string value as its path.

Examples of commands using static resolution:

~> put x # resolves to builtin function $put~
▶ x
~> f~ = { put 'this is f' }
~> f # resolves to user-defined function $f~
▶ 'this is f'
~> whoami # resolves to external command whoami
elf
    

Examples of commands using a dynamic callable head:

~> $put~ x
▶ x
~> (external whoami)
elf
~> { put 'this is a lambda' }
▶ 'this is a lambda'
    

Note: The last command resembles a code block in C-like languages in syntax, but is quite different under the hood: it works by defining a function on the fly and calling it immediately.

Examples of commands using a dynamic string head:

~> x = /bin/whoami
~> $x
elf
~> x = whoami
~> $x # dynamic strings can only used when containing slash
Exception: bad value: command must be callable or string containing slash, but is string
[tty 10], line 1: $x
    

The definition of barewords is relaxed when parsing the head, and includes <, >, and *. These are all names of numeric builtins:

~> < 3 5 # less-than
▶ $true
~> > 3 5 # greater-than
▶ $false
~> * 3 5 # multiplication
▶ 15
    

Arguments and options can be supplied to commands. Arguments are arbitrary words, while options have exactly the same syntax as key-value pairs in map literals. They are separated by inline whitespaces and may be intermixed:

~> echo &sep=, a b c # &seq=, is an option; a b c are arguments
a,b,c
~> echo a b &sep=, c # same, with the option mixed within arguments
a,b,c
    

Note: Since options have the same syntax as key-value pairs in maps, &key is equivalent to &key=$true:

~> fn f [&opt=$false]{ put $opt }
~> f &opt
▶ $true
    

Special command¶

A special command form has the same syntax with an ordinary command, but how it is executed depends on the command head. See special commands.

Legacy assignment form¶

If any argument in a command form is an unquoted equal sign (=), the command form is treated as an assignment form: the arguments to the left of =, including the head, are treated as lvalues, and the arguments to the right of = are treated as values to assign to those lvalues.

If any lvalue refers to a variable that doesn’t yet exist, it is created first.

This is a legacy syntax that will be deprecated in future. Use the var special command to declare variables, and the set special command set the values of variables.

Temporary assignment¶

You can prepend any command form with temporary assignments, which gives variables temporarily values during the execution of that command.

In the following example, $x and $y are temporarily assigned 100 and 200:

~> x y = 1 2
~> x=100 y=200 + $x $y
▶ 300
~> echo $x $y
1 2
    

In contrary to normal assignments, there should be no whitespaces around the equal sign =. To have multiple variables in the left-hand side, use braces:

~> x y = 1 2
~> fn f { put 100 200 }
~> {x,y}=(f) + $x $y
▶ 300
    

If you use a previously undefined variable in a temporary assignment, its value will become the empty string after the command finishes. This behavior will likely change; don’t rely on it.

Since ordinary assignments are also command forms, they can also be prepended with temporary assignments:

~> x=1
~> x=100 y = (+ 133 $x)
~> put $x $y
▶ 1
▶ 233
    

Temporary assignments must all appear at the beginning of the command form. As soon as something that is not a temporary assignments is parsed, Elvish no longer parses temporary assignments. For instance, in x=1 echo x=1, the second x=1 is not a temporary assignment, but a bareword.

Note: Elvish’s behavior differs from bash (or zsh) in one important place. In bash, temporary assignments to variables do not affect their direct appearance in the command:

bash-4.4$ x=1
bash-4.4$ x=100 echo $x
1
    

Note: Elvish currently supports using the syntax of temporary assignments for ordinary assignments, when they are not followed by a command form; for example, a=x behaves like an ordinary assignment a = x. This will likely go away; don’t rely on it.

Redirection¶

You can add redirections to any command form, to modify the file descriptors these command form operate with.

The most common form of redirections opens a file and associates it with an FD. The form consists of an optional destination FD (like 2), a redirection operator (like >) and a filename (like error.log):

•

The destination fd determines which FD to modify. It can be given either as a number, or one of stdin, stdout and stderr. There must be no space between the FD and the redirection operator; otherwise Elvish will parse it as an argument.

•

The redirection operator determines the mode to open the file, and the destination FD if it is not explicitly specified.

•

The filename names the file to open.

Possible redirection operators and their default FDs are:

•

< for reading. The default FD is 0 (stdin).

•

> for writing. The default FD is 1 (stdout).

•

>> for appending. The default FD is 1 (stdout).

•

<> for reading and writing. The default FD is 1 (stdout).

Examples:

~> echo haha > log
~> cat log
haha
~> cat < log
haha
~> ls --bad-arg 2> error
Exception: ls exited with 2
Traceback:


  [interactive], line 1:


    ls --bad-arg 2> error
~> cat error
/bin/ls: unrecognized option '--bad-arg'
Try '/bin/ls --help' for more information.
    

Redirections can also be used for closing or duplicating FDs. Instead of writing a filename, use &fd (where fd is a number, or any of stdin, stdout and stderr) for duplicating, or &- for closing. In this case, the redirection operator only determines the default destination FD (and is totally irrevelant if a destination FD is specified). Examples:

~> ls >&- # close stdout
/bin/ls: write error: Bad file descriptor
Exception: ls exited with 2
Traceback:


  [interactive], line 1:


    ls >&-
    

If you have multiple related redirections, they are applied in the order they appear. For instance:

~> fn f { echo out; echo err >&2 } # echoes "out" on stdout, "err" on stderr
~> f >log 2>&1 # use file "log" for stdout, then use (changed) stdout for stderr
~> cat log
out
err
    

Redirections may appear anywhere in the command, except at the beginning, although this may be restricted in future. It’s usually good style to write redirections at the end of command forms.

Note: Elvish only supports reading and writing bytes from/to the target of a redirection. Attemping to read values from a file or a a pipe via redirection will produce no values, and all values written to a file or a pipe will be discarded. Examples:

•

Running put foo > data will not write anything to the file data, other than truncating it.

•

Assuming the file data contains a single line hello, only-values < data will not do anything.

Declaring variables: var {#var}¶

The var special command declares local variables. It takes any number of unqualified variable names (without the leading $). The variables will start out having value $nil. Examples:

~> var a
~> put $a
▶ $nil
~> var foo bar
~> put $foo $bar
▶ $nil
▶ $nil
    

To set alternative initial values, add an unquoted = and the initial values. Examples:

~> var a b = foo bar
~> put $a $b
▶ foo
▶ bar
    

Similar to set, at most one of variables may be prefixed with @ to function as a rest variable.

When declaring a variable that already exists, the existing variable is shadowed. The shadowed variable may still be accessed indirectly if it is referenced by a function. Example:

~> var x = old
~> fn f { put $x }
~> var x = new
~> put $x
▶ new
~> f
▶ old
    

Setting the value of variables or elements: set {#set}¶

The set special command sets the value of variables or elements.

It takes any number of lvalues (which refer to either variables or elements), followed by an equal sign (=) and any number of expressions. The equal sign must appear unquoted, as a single argument.

An lvalue is one of the following:

•

A variable name (without $).

•

A variable name prefixed with @, for packing a variable number of values into a list and assigning to the variable.

•

A variable name followed by one or more indices in brackets ([]), for assigning to an element.

The number of values the expressions evaluate to and lvalues must be compatible. To be more exact:

•

If there is no rest variable, the number of values and lvalues must match exactly.

•

If there is a rest variable, the number of values should be at least the number of lvalues minus one.

All the variables to set must already exist; use the var special command to declare new variables.

Examples:

~> var x y z
~> set x = foo
~> put $x
▶ foo
~> x y = lorem ipsum
~> put $x $y
▶ lorem
▶ ipsum
~> set x @y z = a b
~> put $x $y $z
▶ a
▶ []
▶ b
~> set x @y z = a b c d
~> put $x $y $z
▶ a
▶ [b c]
▶ d
~> set y[0] = foo
~> put $y
▶ [foo c]
    

If the variable name contains any character that may not appear unquoted in variable use expressions, it must be quoted even if it is otherwise a valid bareword:

~> var 'a/b'
~> set a/b = foo
compilation error: lvalue must be valid literal variable names
[tty 23], line 1: a/b = foo
~> set 'a/b' = foo
~> put $'a/b'
▶ foo
    

Lists and maps in Elvish are immutable. As a result, when assigning to the element of a variable that contains a list or map, Elvish does not mutate the underlying list or map. Instead, Elvish creates a new list or map with the mutation applied, and assigns it to the variable. Example:

~> var li = [foo bar]
~> var li2 = $li
~> set li[0] = lorem
~> put $li $li2
▶ [lorem bar]
▶ [foo bar]
    

Deleting variables or elements: del {#del}¶

The del special command can be used to delete variables or map elements. Operands should be specified without a leading dollar sign, like the left-hand side of assignments.

Example of deleting variable:

~> x = 2
~> echo $x
2
~> del x
~> echo $x
Compilation error: variable $x not found
[tty], line 1: echo $x
    

If the variable name contains any character that cannot appear unquoted after $, it must be quoted, even if it is otherwise a valid bareword:

~> 'a/b' = foo
~> del 'a/b'
    

Deleting a variable does not affect closures that have already captured it; it only removes the name. Example:

~> x = value
~> fn f { put $x }
~> del x
~> f
▶ value
    

Example of deleting map element:

~> m = [&k=v &k2=v2]
~> del m[k2]
~> put $m
▶ [&k=v]
~> l = [[&k=v &k2=v2]]
~> del l[0][k2]
~> put $l
▶ [[&k=v]]
    

Logics: and and or {#and-or}¶

The and special command evaluates its arguments from left to right; as soon as a booleanly false value is obtained, it outputs the value and stops. When given no arguments, it outputs $true.

The or special command is the same except that it stops when a booleanly true value is obtained. When given no arguments, it outpus $false.

Condition: if {#if}¶

TODO: Document the syntax notation, and add more examples.

Syntax:

if <condition> {


    <body>
} elif <condition> {


    <body>
} else {


    <else-body>
}
    

The if special command goes through the conditions one by one: as soon as one evaluates to a booleanly true value, its corresponding body is executed. If none of conditions are booleanly true and an else body is supplied, it is executed.

The condition part is an expression, not a command like in other shells. Example:

fn tell-language [fname]{


    if (has-suffix $fname .go) {


        echo $fname" is a Go file!"


    } elif (has-suffix $fname .c) {


        echo $fname" is a C file!"


    } else {


        echo $fname" is a mysterious file!"


    }
}
    

The condition part must be syntactically a single expression, but it can evaluate to multiple values, in which case they are and’ed:

if (put $true $false) {


    echo "will not be executed"
}
    

If the expression evaluates to 0 values, it is considered true, consistent with how and works.

Tip: a combination of if and ?() gives you a semantics close to other shells:

if ?(test -d .git) {


    # do something
}
    

However, for Elvish’s builtin predicates that output values instead of throw exceptions, the output capture construct () should be used.

Conditional loop: while {#while}¶

Syntax:

while <condition> {


    <body>
} else {


    <else-body>
}
    

Execute the body as long as the condition evaluates to a booleanly true value.

The else body, if present, is executed if the body has never been executed (i.e. the condition evaluates to a booleanly false value in the very beginning).

Iterative loop: for {#for}¶

Syntax:

for <var> <container> {


    <body>
} else {


    <body>
}
    

Iterate the container (e.g. a list). In each iteration, assign the variable to an element of the container and execute the body.

The else body, if present, is executed if the body has never been executed (i.e. the iteration value has no elements).

Exception control: try {#try}¶

(If you just want to capture the exception, you can use the more concise exception capture construct ?() instead.)

Syntax:

try {


    <try-block>
} except except-varname {


    <except-block>
} else {


    <else-block>
} finally {


    <finally-block>
}
    

Only try and try-block are required. This control structure behaves as follows:

1.

The try-block is always executed first.

2.

If except is present and an exception occurs in try-block, it is caught and stored in except-varname, and except-block is then executed. Example:

3.

If no exception occurs and else is present, else-block is executed. Example:

4.

If finally-block is present, it is executed. Examples:

5.

If the exception was not caught (i.e. except is not present), it is rethrown.

Exceptions thrown in blocks other than try-block are not caught. If an exception was thrown and either except-block or finally-block throws another exception, the original exception is lost. Examples:

~> try { fail bad } except e { fail worse }
Exception: worse
Traceback:


  [tty], line 1:


    try { fail bad } except e { fail worse }
~> try { fail bad } except e { fail worse } finally { fail worst }
Exception: worst
Traceback:


  [tty], line 1:


    try { fail bad } except e { fail worse } finally { fail worst }
    

Function definition: fn {#fn}¶

Syntax:

fn <name> <lambda>
    

Define a function with a given name. The function behaves in the same way to the lambda used to define it, except that it “captures” return. In other words, return will fall through lambdas not defined with fn, and continues until it exits a function defined with fn:

~> fn f {


     { echo a; return }


     echo b # will not execute


   }
~> f
a
~> {


     f


     echo c # executed, because f "captures" the return


   }
a
c
    

TODO: Find a better way to describe this. Hopefully the example is illustrative enough, though.

The lambda may refer to the function being defined. This makes it easy to define recursive functions:

~> fn f [n]{ if (== $n 0) { put 1 } else { * $n (f (- $n 1)) } }
~> f 3
▶ (float64 6)
    

Under the hood, fn defines a variable with the given name plus ~ (see variable suffix).

IO semantics¶

For each pair of adjacent commands a | b, the output of a is connected to the input of b. Both the byte pipe and the value channel are connected, even if one of them is not used.

Command redirections are applied before the connection happens. For instance, the following writes foo to a.txt instead of the output:

~> echo foo > a.txt | cat
~> cat a.txt
foo
    

Execution flow¶

All of the commands in a pipeline are executed in parallel, and the execution of the pipeline finishes when all of its commands finish execution.

If one or more command in a pipeline throws an exception, the other commands will continue to execute as normal. After all commands finish execution, an exception is thrown, the value of which depends on the number of commands that have thrown an exception:

•

If only one command has thrown an exception, that exception is rethrown.

•

If more than one commands have thrown exceptions, a “composite exception”, containing information all exceptions involved, is thrown.

Background pipeline¶

Adding an ampersand & to the end of a pipeline will cause it to be executed in the background. In this case, the rest of the code chunk will continue to execute without waiting for the pipeline to finish. Exceptions thrown from the background pipeline do not affect the code chunk that contains it.

When a background pipeline finishes, a message is printed to the terminal if the shell is interactive.

Syntax¶

Prepend namespace: to command names and variable names to specify the namespace. The following code

e:echo $E:PATH
    

uses the echo command from the e: namespace and the PATH variable from the E: namespace. The colon is considered part of the namespace name.

Namespaces may be nested; for example, calling edit:location:start first finds the edit: namespace, and then the location: namespace inside it, and then call the start function within the nested namespace.

Special namespaces¶

The following namespaces have special meanings to the language:

•

local: and up: refer to lexical scopes, and have been documented above.

•

e: refers to externals. For instance, e:ls refers to the external command ls.

•

E: refers to environment variables. For instance, $E:USER is the environment variable USER.

•

builtin: refers to builtin functions and variables.

Modules¶

Apart from the special namespaces, the most common usage of namespaces is to reference modules, reusable pieces of code that are either shipped with Elvish itself or defined by the user.

Importing modules with use¶

Modules are imported using the use special command, which accepts a module spec and an optional alias:

use $spec $alias?
    

Both the module spec and the alias must appear as a single string literal.

The module spec specifies which module to import, and the alias, if given, specifies the namespace to import the module under. By default, the namespace is derived from the module spec, by taking the part after the last slash.

Examples:

use str # imports the "str" module as "str:"
use a/b/c # imports the "a/b/c" module as "c:"
use a/b/c foo # imports the "a/b/c" module as "foo:"
    

Pre-defined modules¶

Elvish’s standard library provides the following pre-defined modules that can be imported by the use command:

•

edit is only available in interactive mode. As a special case it does not need importing via use, but this may change in the future.

•

epm

•

math

•

path

•

platform

•

re

•

readline-binding

•

store

•

str

•

unix is only available on UNIX-like platforms (see $platform:is-unix)

User-defined modules¶

You can define your own modules in Elvish by putting them under ~/.elvish/lib and giving them a .elv extension. For instance, to define a module named a, store it in ~/.elvish/lib/a.elv:

~> cat ~/.elvish/lib/a.elv
echo "mod a loading"
fn f {


  echo "f from mod a"
}
    

This module can now be imported by use a:

~> use a
mod a loading
~> a:f
f from mod a
    

Similarly, a module defined in ~/.elvish/lib/x/y/z.elv can be imported by use x/y/z:

~> cat .elvish/lib/x/y/z.elv
fn f {


  echo "f from x/y/z"
}
~> use x/y/z
~> z:f
f from x/y/z
    

In general, a module defined in namespace will be the same as the file name (without the .elv extension).

Circular dependencies¶

Circular dependencies are allowed but has an important restriction. If a module a contains use b and module b contains use a, the top-level statements in module b will only be able to access variables that are defined before the use b in module a; other variables will be $nil.

On the other hand, functions in module b will have access to bindings in module a after it is fully evaluated.

Examples:

~> cat a.elv
var before = before
use ./b
var after = after
~> cat b.elv
use ./a
put $a:before $a:after
fn f { put $a:before $a:after }
~> use ./a
▶ before
▶ $nil
~> use ./b
~> b:f
▶ before
▶ after
    

Note that this behavior can be different depending on whether the REPL imports a or b first. In the previous example, if the REPL imports b first, it will have access to all the variables in a:

~> use ./b
▶ before
▶ after
    

Note: Elvish caches imported modules. If you are trying this locally, run a fresh Elvish instance with exec first.

When you do need to have circular dependencies, it is best to avoid using variables from the modules in top-level statements, and only use them in functions.

Relative imports¶

The module spec may being with ./ or ../, which introduce relative imports. When use is invoked from a file, this will import the file relative to the location of the file. When use is invoked at the interactive prompt, this will import the file relative to the current working directory.

Scoping of imports¶

Namespace imports are lexically scoped. For instance, if you use a module within an inner scope, it is not available outside that scope:

{


    use some-mod


    some-mod:some-func
}
some-mod:some-func # not valid
    

The imported modules themselves are also evaluated in a separate scope. That means that functions and variables defined in the module does not pollute the default namespace, and vice versa. For instance, if you define ls as a wrapper function in rc.elv:

fn ls [@a]{


    e:ls --color=auto $@a
}
    

That definition is not visible in module files: ls will still refer to the external command ls, unless you shadow it in the very same module.

Re-importing¶

Modules are cached after one import. Subsequent imports do not re-execute the module; they only serve the bring it into the current scope. Moreover, the cache is keyed by the path of the module, not the name under which it is imported. For instance, if you have the following in ~/.elvish/lib/a/b.elv:

echo importing
    

The following code only prints one importing:

{ use a/b }
use a/b # only brings mod into the lexical scope
    

As does the following:

use a/b
use a/b alias