Template Function List

Helm includes many template functions you can take advantage of in templates. They are listed here and broken down by the following categories:

Logic and Flow Control Functions

Helm includes numerous logic and control flow functions including and, coalesce, default, empty, eq, fail, ge, gt, le, lt, ne, not, and or.

and

Returns a boolean and of the two arguments.

and .Arg1 .Arg2

or

Returns the boolean or of the two arguments. It returns the first non-empty argument or the last argument.

or .Arg1 .Arg2

not

Returns the boolean negation of its argument.

not .Arg

eq

Returns the boolean equality of the arguments (e.g., Arg1 == Arg2).

eq .Arg1 .Arg2

ne

Returns the boolean inequality of the arguments (e.g., Arg1 != Arg2)

ne .Arg1 .Arg2

lt

Returns a boolean true if the first argument is less than the second. False is returned otherwise (e.g., Arg1 < Arg2).

lt .Arg1 .Arg2

le

Returns a boolean true if the first argument is less than or equal to the second. False is returned otherwise (e.g., Arg1 <= Arg2).

le .Arg1 .Arg2

gt

Returns a boolean true if the first argument is greater than the second. False is returned otherwise (e.g., Arg1 > Arg2).

gt .Arg1 .Arg2

ge

Returns a boolean true if the first argument is greater than or equal to the second. False is returned otherwise (e.g., Arg1 >= Arg2).

ge .Arg1 .Arg2

default

To set a simple default value, use default:

default "foo" .Bar

In the above, if .Bar evaluates to a non-empty value, it will be used. But if it is empty, foo will be returned instead.

The definition of "empty" depends on type:

  • Numeric: 0
  • String: ""
  • Lists: []
  • Dicts: {}
  • Boolean: false
  • And always nil (aka null)

For structs, there is no definition of empty, so a struct will never return the default.

empty

The empty function returns true if the given value is considered empty, and false otherwise. The empty values are listed in the default section.

empty .Foo

Note that in Go template conditionals, emptiness is calculated for you. Thus, you rarely need if empty .Foo. Instead, just use if .Foo.

fail

Unconditionally returns an empty string and an error with the specified text. This is useful in scenarios where other conditionals have determined that template rendering should fail.

fail "Please accept the end user license agreement"

coalesce

The coalesce function takes a list of values and returns the first non-empty one.

coalesce 0 1 2

The above returns 1.

This function is useful for scanning through multiple variables or values:

coalesce .name .parent.name "Matt"

The above will first check to see if .name is empty. If it is not, it will return that value. If it is empty, coalesce will evaluate .parent.name for emptiness. Finally, if both .name and .parent.name are empty, it will return Matt.

ternary

The ternary function takes two values, and a test value. If the test value is true, the first value will be returned. If the test value is empty, the second value will be returned. This is similar to the c ternary operator.

true test value

ternary "foo" "bar" true

or

true | ternary "foo" "bar"

The above returns "foo".

false test value

ternary "foo" "bar" false

or

false | ternary "foo" "bar"

The above returns "bar".

String Functions

Helm includes the following string functions: abbrev, abbrevboth, camelcase, cat, contains, hasPrefix, hasSuffix, indent, initials, kebabcase, lower, nindent, nospace, plural, print, printf, println, quote, randAlpha, randAlphaNum, randAscii, randNumeric, repeat, replace, shuffle, snakecase, squote, substr, swapcase, title, trim, trimAll, trimPrefix, trimSuffix, trunc, untitle, upper, wrap, and wrapWith.

print

Returns a string from the combination of its parts.

print "Matt has " .Dogs " dogs"

Types that are not strings are converted to strings where possible.

Note, when two arguments next to each other are not strings a space is added between them.

println

Works the same way as print but adds a new line at the end.

printf

Returns a string based on a formatting string and the arguments to pass to it in order.

printf "%s has %d dogs." .Name .NumberDogs

The placeholder to use depends on the type for the argument being passed in. This includes:

General purpose:

  • %v the value in a default format
    • when printing dicts, the plus flag (%+v) adds field names
  • %% a literal percent sign; consumes no value

Boolean:

  • %t the word true or false

Integer:

  • %b base 2
  • %c the character represented by the corresponding Unicode code point
  • %d base 10
  • %o base 8
  • %O base 8 with 0o prefix
  • %q a single-quoted character literal safely escaped
  • %x base 16, with lower-case letters for a-f
  • %X base 16, with upper-case letters for A-F
  • %U Unicode format: U+1234; same as "U+%04X"

Floating-point and complex constituents:

  • %b decimalless scientific notation with exponent a power of two, e.g. -123456p-78
  • %e scientific notation, e.g. -1.234456e+78
  • %E scientific notation, e.g. -1.234456E+78
  • %f decimal point but no exponent, e.g. 123.456
  • %F synonym for %f
  • %g %e for large exponents, %f otherwise.
  • %G %E for large exponents, %F otherwise
  • %x hexadecimal notation (with decimal power of two exponent), e.g. -0x1.23abcp+20
  • %X upper-case hexadecimal notation, e.g. -0X1.23ABCP+20

String and slice of bytes (treated equivalently with these verbs):

  • %s the uninterpreted bytes of the string or slice
  • %q a double-quoted string safely escaped
  • %x base 16, lower-case, two characters per byte
  • %X base 16, upper-case, two characters per byte

Slice:

  • %p address of 0th element in base 16 notation, with leading 0x

trim

The trim function removes white space from both sides of a string:

trim "   hello    "

The above produces hello

trimAll

Removes the given characters from the front and back of a string:

trimAll "$" "$5.00"

The above returns 5.00 (as a string).

trimPrefix

Trim just the prefix from a string:

trimPrefix "-" "-hello"

The above returns hello

trimSuffix

Trim just the suffix from a string:

trimSuffix "-" "hello-"

The above returns hello

lower

Convert the entire string to lowercase:

lower "HELLO"

The above returns hello

upper

Convert the entire string to uppercase:

upper "hello"

The above returns HELLO

title

Convert to title case:

title "hello world"

The above returns Hello World

untitle

Remove title casing. untitle "Hello World" produces hello world.

repeat

Repeat a string multiple times:

repeat 3 "hello"

The above returns hellohellohello

substr

Get a substring from a string. It takes three parameters:

  • start (int)
  • end (int)
  • string (string)
substr 0 5 "hello world"

The above returns hello

nospace

Remove all whitespace from a string.

nospace "hello w o r l d"

The above returns helloworld

trunc

Truncate a string

trunc 5 "hello world"

The above produces hello.

trunc -5 "hello world"

The above produces world.

abbrev

Truncate a string with ellipses (...)

Parameters:

  • max length
  • the string
abbrev 5 "hello world"

The above returns he..., since it counts the width of the ellipses against the maximum length.

abbrevboth

Abbreviate both sides:

abbrevboth 5 10 "1234 5678 9123"

the above produces ...5678...

It takes:

  • left offset
  • max length
  • the string

initials

Given multiple words, take the first letter of each word and combine.

initials "First Try"

The above returns FT

randAlphaNum, randAlpha, randNumeric, and randAscii

These four functions generate cryptographically secure (uses crypto/rand) random strings, but with different base character sets:

  • randAlphaNum uses 0-9a-zA-Z
  • randAlpha uses a-zA-Z
  • randNumeric uses 0-9
  • randAscii uses all printable ASCII characters

Each of them takes one parameter: the integer length of the string.

randNumeric 3

The above will produce a random string with three digits.

wrap

Wrap text at a given column count:

wrap 80 $someText

The above will wrap the string in $someText at 80 columns.

wrapWith

wrapWith works as wrap, but lets you specify the string to wrap with. (wrap uses \n)

wrapWith 5 "\t" "Hello World"

The above produces hello world (where the whitespace is an ASCII tab character)

contains

Test to see if one string is contained inside of another:

contains "cat" "catch"

The above returns true because catch contains cat.

hasPrefix and hasSuffix

The hasPrefix and hasSuffix functions test whether a string has a given prefix or suffix:

hasPrefix "cat" "catch"

The above returns true because catch has the prefix cat.

quote and squote

These functions wrap a string in double quotes (quote) or single quotes (squote).

cat

The cat function concatenates multiple strings together into one, separating them with spaces:

cat "hello" "beautiful" "world"

The above produces hello beautiful world

indent

The indent function indents every line in a given string to the specified indent width. This is useful when aligning multi-line strings:

indent 4 $lots_of_text

The above will indent every line of text by 4 space characters.

nindent

The nindent function is the same as the indent function, but prepends a new line to the beginning of the string.

nindent 4 $lots_of_text

The above will indent every line of text by 4 space characters and add a new line to the beginning.

replace

Perform simple string replacement.

It takes three arguments:

  • string to replace
  • string to replace with
  • source string
"I Am Henry VIII" | replace " " "-"

The above will produce I-Am-Henry-VIII

plural

Pluralize a string.

len $fish | plural "one anchovy" "many anchovies"

In the above, if the length of the string is 1, the first argument will be printed (one anchovy). Otherwise, the second argument will be printed (many anchovies).

The arguments are:

  • singular string
  • plural string
  • length integer

NOTE: Helm does not currently support languages with more complex pluralization rules. And 0 is considered a plural because the English language treats it as such (zero anchovies).

snakecase

Convert string from camelCase to snake_case.

snakecase "FirstName"

This above will produce first_name.

camelcase

Convert string from snake_case to CamelCase

camelcase "http_server"

This above will produce HttpServer.

kebabcase

Convert string from camelCase to kebab-case.

kebabcase "FirstName"

This above will produce first-name.

swapcase

Swap the case of a string using a word based algorithm.

Conversion algorithm:

  • Upper case character converts to Lower case
  • Title case character converts to Lower case
  • Lower case character after Whitespace or at start converts to Title case
  • Other Lower case character converts to Upper case
  • Whitespace is defined by unicode.IsSpace(char)
swapcase "This Is A.Test"

This above will produce tHIS iS a.tEST.

shuffle

Shuffle a string.

shuffle "hello"

The above will randomize the letters in hello, perhaps producing oelhl.

Type Conversion Functions

The following type conversion functions are provided by Helm:

  • atoi: Convert a string to an integer.
  • float64: Convert to a float64.
  • int: Convert to an int at the system's width.
  • int64: Convert to an int64.
  • toDecimal: Convert a unix octal to a int64.
  • toString: Convert to a string.
  • toStrings: Convert a list, slice, or array to a list of strings.
  • toJson (mustToJson): Convert list, slice, array, dict, or object to JSON.
  • toPrettyJson (mustToPrettyJson): Convert list, slice, array, dict, or object to indented JSON.
  • toRawJson (mustToRawJson): Convert list, slice, array, dict, or object to JSON with HTML characters unescaped.

Only atoi requires that the input be a specific type. The others will attempt to convert from any type to the destination type. For example, int64 can convert floats to ints, and it can also convert strings to ints.

toStrings

Given a list-like collection, produce a slice of strings.

list 1 2 3 | toStrings

The above converts 1 to "1", 2 to "2", and so on, and then returns them as a list.

toDecimal

Given a unix octal permission, produce a decimal.

"0777" | toDecimal

The above converts 0777 to 511 and returns the value as an int64.

toJson, mustToJson

The toJson function encodes an item into a JSON string. If the item cannot be converted to JSON the function will return an empty string. mustToJson will return an error in case the item cannot be encoded in JSON.

toJson .Item

The above returns JSON string representation of .Item.

toPrettyJson, mustToPrettyJson

The toPrettyJson function encodes an item into a pretty (indented) JSON string.

toPrettyJson .Item

The above returns indented JSON string representation of .Item.

toRawJson, mustToRawJson

The toRawJson function encodes an item into JSON string with HTML characters unescaped.

toRawJson .Item

The above returns unescaped JSON string representation of .Item.

Regular Expressions

Helm includes the following regular expression functions: regexFind (mustRegexFind), regexFindAll (mustRegexFindAll), regexMatch (mustRegexMatch), regexReplaceAll (mustRegexReplaceAll), regexReplaceAllLiteral (mustRegexReplaceAllLiteral), regexSplit (mustRegexSplit).

regexMatch, mustRegexMatch

Returns true if the input string contains any match of the regular expression.

regexMatch "^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,}$" "test@acme.com"

The above produces true

regexMatch panics if there is a problem and mustRegexMatch returns an error to the template engine if there is a problem.

regexFindAll, mustRegexFindAll

Returns a slice of all matches of the regular expression in the input string. The last parameter n determines the number of substrings to return, where -1 means return all matches

regexFindAll "[2,4,6,8]" "123456789" -1

The above produces [2 4 6 8]

regexFindAll panics if there is a problem and mustRegexFindAll returns an error to the template engine if there is a problem.

regexFind, mustRegexFind

Return the first (left most) match of the regular expression in the input string

regexFind "[a-zA-Z][1-9]" "abcd1234"

The above produces d1

regexFind panics if there is a problem and mustRegexFind returns an error to the template engine if there is a problem.

regexReplaceAll, mustRegexReplaceAll

Returns a copy of the input string, replacing matches of the Regexp with the replacement string replacement. Inside string replacement, $ signs are interpreted as in Expand, so for instance $1 represents the text of the first submatch

regexReplaceAll "a(x*)b" "-ab-axxb-" "${1}W"

The above produces -W-xxW-

regexReplaceAll panics if there is a problem and mustRegexReplaceAll returns an error to the template engine if there is a problem.

regexReplaceAllLiteral, mustRegexReplaceAllLiteral

Returns a copy of the input string, replacing matches of the Regexp with the replacement string replacement The replacement string is substituted directly, without using Expand

regexReplaceAllLiteral "a(x*)b" "-ab-axxb-" "${1}"

The above produces -${1}-${1}-

regexReplaceAllLiteral panics if there is a problem and mustRegexReplaceAllLiteral returns an error to the template engine if there is a problem.

regexSplit, mustRegexSplit

Slices the input string into substrings separated by the expression and returns a slice of the substrings between those expression matches. The last parameter n determines the number of substrings to return, where -1 means return all matches

regexSplit "z+" "pizza" -1

The above produces [pi a]

regexSplit panics if there is a problem and mustRegexSplit returns an error to the template engine if there is a problem.

Cryptographic and Security Functions

Helm provides some advanced cryptographic functions. They include adler32sum, buildCustomCert, decryptAES, derivePassword, encryptAES, genCA, genPrivateKey, genSelfSignedCert, genSignedCert, htpasswd, sha1sum, and sha256sum.

sha1sum

The sha1sum function receives a string, and computes it's SHA1 digest.

sha1sum "Hello world!"

sha256sum

The sha256sum function receives a string, and computes it's SHA256 digest.

sha256sum "Hello world!"

The above will compute the SHA 256 sum in an "ASCII armored" format that is safe to print.

adler32sum

The adler32sum function receives a string, and computes its Adler-32 checksum.

adler32sum "Hello world!"

htpasswd

The htpasswd function takes a username and password and generates a bcrypt hash of the password. The result can be used for basic authentication on an Apache HTTP Server.

htpasswd "myUser" "myPassword"

Note that it is insecure to store the password directly in the template.

derivePassword

The derivePassword function can be used to derive a specific password based on some shared "master password" constraints. The algorithm for this is well specified.

derivePassword 1 "long" "password" "user" "example.com"

Note that it is considered insecure to store the parts directly in the template.

genPrivateKey

The genPrivateKey function generates a new private key encoded into a PEM block.

It takes one of the values for its first param:

  • ecdsa: Generate an elliptic curve DSA key (P256)
  • dsa: Generate a DSA key (L2048N256)
  • rsa: Generate an RSA 4096 key

buildCustomCert

The buildCustomCert function allows customizing the certificate.

It takes the following string parameters:

  • A base64 encoded PEM format certificate
  • A base64 encoded PEM format private key

It returns a certificate object with the following attributes:

  • Cert: A PEM-encoded certificate
  • Key: A PEM-encoded private key

Example:

$ca := buildCustomCert "base64-encoded-ca-crt" "base64-encoded-ca-key"

Note that the returned object can be passed to the genSignedCert function to sign a certificate using this CA.

genCA

The genCA function generates a new, self-signed x509 certificate authority.

It takes the following parameters:

  • Subject's common name (cn)
  • Cert validity duration in days

It returns an object with the following attributes:

  • Cert: A PEM-encoded certificate
  • Key: A PEM-encoded private key

Example:

$ca := genCA "foo-ca" 365

Note that the returned object can be passed to the genSignedCert function to sign a certificate using this CA.

genSelfSignedCert

The genSelfSignedCert function generates a new, self-signed x509 certificate.

It takes the following parameters:

  • Subject's common name (cn)
  • Optional list of IPs; may be nil
  • Optional list of alternate DNS names; may be nil
  • Cert validity duration in days

It returns an object with the following attributes:

  • Cert: A PEM-encoded certificate
  • Key: A PEM-encoded private key

Example:

$cert := genSelfSignedCert "foo.com" (list "10.0.0.1" "10.0.0.2") (list "bar.com" "bat.com") 365

genSignedCert

The genSignedCert function generates a new, x509 certificate signed by the specified CA.

It takes the following parameters:

  • Subject's common name (cn)
  • Optional list of IPs; may be nil
  • Optional list of alternate DNS names; may be nil
  • Cert validity duration in days
  • CA (see genCA)

Example:

$ca := genCA "foo-ca" 365
$cert := genSignedCert "foo.com" (list "10.0.0.1" "10.0.0.2") (list "bar.com" "bat.com") 365 $ca

encryptAES

The encryptAES function encrypts text with AES-256 CBC and returns a base64 encoded string.

encryptAES "secretkey" "plaintext"

decryptAES

The decryptAES function receives a base64 string encoded by the AES-256 CBC algorithm and returns the decoded text.

"30tEfhuJSVRhpG97XCuWgz2okj7L8vQ1s6V9zVUPeDQ=" | decryptAES "secretkey"

Date Functions

Helm includes the following date functions you can use in templates: ago, date, dateInZone, dateModify (mustDateModify), duration, durationRound, htmlDate, htmlDateInZone, now, toDate (mustToDate), and unixEpoch.

now

The current date/time. Use this in conjunction with other date functions.

ago

The ago function returns duration from time.Now in seconds resolution.

ago .CreatedAt"

returns in time.Duration String() format

2h34m7s

date

The date function formats a date.

Format the date to YEAR-MONTH-DAY:

now | date "2006-01-02"

Date formatting in Go is a little bit different.

In short, take this as the base date:

Mon Jan 2 15:04:05 MST 2006

Write it in the format you want. Above, 2006-01-02 is the same date, but in the format we want.

dateInZone

Same as date, but with a timezone.

dateInZone "2006-01-02" (now) "UTC"

duration

Formats a given amount of seconds as a time.Duration.

This returns 1m35s

duration 95

durationRound

Rounds a given duration to the most significant unit. Strings and time.Duration gets parsed as a duration, while a time.Time is calculated as the duration since.

This return 2h

durationRound "2h10m5s"

This returns 3mo

durationRound "2400h10m5s"

unixEpoch

Returns the seconds since the unix epoch for a time.Time.

now | unixEpoch

dateModify, mustDateModify

The dateModify takes a modification and a date and returns the timestamp.

Subtract an hour and thirty minutes from the current time:

now | date_modify "-1.5h"

If the modification format is wrong dateModify will return the date unmodified. mustDateModify will return an error otherwise.

htmlDate

The htmlDate function formats a date for inserting into an HTML date picker input field.

now | htmlDate

htmlDateInZone

Same as htmlDate, but with a timezone.

htmlDateInZone (now) "UTC"

toDate, mustToDate

toDate converts a string to a date. The first argument is the date layout and the second the date string. If the string can't be convert it returns the zero value. mustToDate will return an error in case the string cannot be converted.

This is useful when you want to convert a string date to another format (using pipe). The example below converts "2017-12-31" to "31/12/2017".

toDate "2006-01-02" "2017-12-31" | date "02/01/2006"

Dictionaries and Dict Functions

Helm provides a key/value storage type called a dict (short for "dictionary", as in Python). A dict is an unorder type.

The key to a dictionary must be a string. However, the value can be any type, even another dict or list.

Unlike lists, dicts are not immutable. The set and unset functions will modify the contents of a dictionary.

Helm provides the following functions to support working with dicts: deepCopy (mustDeepCopy), dict, get, hasKey, keys, merge (mustMerge), mergeOverwrite (mustMergeOverwrite), omit, pick, pluck, set, unset, and values.

dict

Creating dictionaries is done by calling the dict function and passing it a list of pairs.

The following creates a dictionary with three items:

$myDict := dict "name1" "value1" "name2" "value2" "name3" "value 3"

get

Given a map and a key, get the value from the map.

get $myDict "key1"

The above returns "value1"

Note that if the key is not found, this operation will simply return "". No error will be generated.

set

Use set to add a new key/value pair to a dictionary.

$_ := set $myDict "name4" "value4"

Note that set returns the dictionary (a requirement of Go template functions), so you may need to trap the value as done above with the $_ assignment.

unset

Given a map and a key, delete the key from the map.

$_ := unset $myDict "name4"

As with set, this returns the dictionary.

Note that if the key is not found, this operation will simply return. No error will be generated.

hasKey

The hasKey function returns true if the given dict contains the given key.

hasKey $myDict "name1"

If the key is not found, this returns false.

pluck

The pluck function makes it possible to give one key and multiple maps, and get a list of all of the matches:

pluck "name1" $myDict $myOtherDict

The above will return a list containing every found value ([value1 otherValue1]).

If the give key is not found in a map, that map will not have an item in the list (and the length of the returned list will be less than the number of dicts in the call to pluck.

If the key is found but the value is an empty value, that value will be inserted.

A common idiom in Helm templates is to uses pluck... | first to get the first matching key out of a collection of dictionaries.

merge, mustMerge

Merge two or more dictionaries into one, giving precedence to the dest dictionary:

$newdict := merge $dest $source1 $source2

This is a deep merge operation but not a deep copy operation. Nested objects that are merged are the same instance on both dicts. If you want a deep copy along with the merge than use the deepCopy function along with merging. For example,

deepCopy $source | merge $dest

mustMerge will return an error in case of unsuccessful merge.

mergeOverwrite, mustMergeOverwrite

Merge two or more dictionaries into one, giving precedence from right to left, effectively overwriting values in the dest dictionary:

Given:

dst:
  default: default
  overwrite: me
  key: true

src:
  overwrite: overwritten
  key: false

will result in:

newdict:
  default: default
  overwrite: overwritten
  key: false
$newdict := mergeOverwrite $dest $source1 $source2

This is a deep merge operation but not a deep copy operation. Nested objects that are merged are the same instance on both dicts. If you want a deep copy along with the merge than use the deepCopy function along with merging. For example,

deepCopy $source | mergeOverwrite $dest

mustMergeOverwrite will return an error in case of unsuccessful merge.

keys

The keys function will return a list of all of the keys in one or more dict types. Since a dictionary is unordered, the keys will not be in a predictable order. They can be sorted with sortAlpha.

keys $myDict | sortAlpha

When supplying multiple dictionaries, the keys will be concatenated. Use the uniq function along with sortAlpha to get a unqiue, sorted list of keys.

keys $myDict $myOtherDict | uniq | sortAlpha

pick

The pick function selects just the given keys out of a dictionary, creating a new dict.

$new := pick $myDict "name1" "name2"

The above returns {name1: value1, name2: value2}

omit

The omit function is similar to pick, except it returns a new dict with all the keys that do not match the given keys.

$new := omit $myDict "name1" "name3"

The above returns {name2: value2}

values

The values function is similar to keys, except it returns a new list with all the values of the source dict (only one dictionary is supported).

$vals := values $myDict

The above returns list["value1", "value2", "value 3"]. Note that the values function gives no guarantees about the result ordering- if you care about this, then use sortAlpha.

deepCopy, mustDeepCopy

The deepCopy and mustDeepCopy functions takes a value and makes a deep copy of the value. This includes dicts and other structures. deepCopy panics when there is a problem while mustDeepCopy returns an error to the template system when there is an error.

dict "a" 1 "b" 2 | deepCopy

A Note on Dict Internals

A dict is implemented in Go as a map[string]interface{}. Go developers can pass map[string]interface{} values into the context to make them available to templates as dicts.

Encoding Functions

Helm has the following encoding and decoding functions:

  • b64enc/b64dec: Encode or decode with Base64
  • b32enc/b32dec: Encode or decode with Base32

Lists and List Functions

Helm provides a simple list type that can contain arbitrary sequential lists of data. This is similar to arrays or slices, but lists are designed to be used as immutable data types.

Create a list of integers:

$myList := list 1 2 3 4 5

The above creates a list of [1 2 3 4 5].

Helm provides the following list functions: append (mustAppend), compact (mustCompact), concat, first (mustFirst), has (mustHas), initial (mustInitial), last (mustLast), prepend (mustPrepend), rest (mustRest), reverse (mustReverse), seq, slice (mustSlice), uniq (mustUniq), until, untilStep, and without (mustWithout).

first, mustFirst

To get the head item on a list, use first.

first $myList returns 1

first panics if there is a problem while mustFirst returns an error to the template engine if there is a problem.

rest, mustRest

To get the tail of the list (everything but the first item), use rest.

rest $myList returns [2 3 4 5]

rest panics if there is a problem while mustRest returns an error to the template engine if there is a problem.

last, mustLast

To get the last item on a list, use last:

last $myList returns 5. This is roughly analogous to reversing a list and then calling first.

initial, mustInitial

This compliments last by returning all but the last element. initial $myList returns [1 2 3 4].

initial panics if there is a problem while mustInitial returns an error to the template engine if there is a problem.

append, mustAppend

Append a new item to an existing list, creating a new list.

$new = append $myList 6

The above would set $new to [1 2 3 4 5 6]. $myList would remain unaltered.

append panics if there is a problem while mustAppend returns an error to the template engine if there is a problem.

prepend, mustPrepend

Push an element onto the front of a list, creating a new list.

prepend $myList 0

The above would produce [0 1 2 3 4 5]. $myList would remain unaltered.

prepend panics if there is a problem while mustPrepend returns an error to the template engine if there is a problem.

concat

Concatenate arbitrary number of lists into one.

concat $myList ( list 6 7 ) ( list 8 )

The above would produce [1 2 3 4 5 6 7 8]. $myList would remain unaltered.

reverse, mustReverse

Produce a new list with the reversed elements of the given list.

reverse $myList

The above would generate the list [5 4 3 2 1].

reverse panics if there is a problem while mustReverse returns an error to the template engine if there is a problem.

uniq, mustUniq

Generate a list with all of the duplicates removed.

list 1 1 1 2 | uniq

The above would produce [1 2]

uniq panics if there is a problem while mustUniq returns an error to the template engine if there is a problem.

without, mustWithout

The without function filters items out of a list.

without $myList 3

The above would produce [1 2 4 5]

Without can take more than one filter:

without $myList 1 3 5

That would produce [2 4]

without panics if there is a problem while mustWithout returns an error to the template engine if there is a problem.

has, mustHas

Test to see if a list has a particular element.

has 4 $myList

The above would return true, while has "hello" $myList would return false.

has panics if there is a problem while mustHas returns an error to the template engine if there is a problem.

compact, mustCompact

Accepts a list and removes entries with empty values.

$list := list 1 "a" "foo" ""
$copy := compact $list

compact will return a new list with the empty (i.e., "") item removed.

compact panics if there is a problem and mustCompact returns an error to the template engine if there is a problem.

slice, mustSlice

To get partial elements of a list, use slice list [n] [m]. It is equivalent of list[n:m].

  • slice $myList returns [1 2 3 4 5]. It is same as myList[:].
  • slice $myList 3 returns [4 5]. It is same as myList[3:].
  • slice $myList 1 3 returns [2 3]. It is same as myList[1:3].
  • slice $myList 0 3 returns [1 2 3]. It is same as myList[:3].

slice panics if there is a problem while mustSlice returns an error to the template engine if there is a problem.

until

The until function builds a range of integers.

until 5

The above generates the list [0, 1, 2, 3, 4].

This is useful for looping with range $i, $e := until 5.

untilStep

Like until, untilStep generates a list of counting integers. But it allows you to define a start, stop, and step:

untilStep 3 6 2

The above will produce [3 5] by starting with 3, and adding 2 until it is equal or greater than 6. This is similar to Python's range function.

seq

Works like the bash seq command.

  • 1 parameter (end) - will generate all counting integers between 1 and end inclusive.
  • 2 parameters (start, end) - will generate all counting integers between start and end inclusive incrementing or decrementing by 1.
  • 3 parameters (start, step, end) - will generate all counting integers between start and end inclusive incrementing or decrementing by step.
seq 5       => 1 2 3 4 5
seq -3      => 1 0 -1 -2 -3
seq 0 2     => 0 1 2
seq 2 -2    => 2 1 0 -1 -2
seq 0 2 10  => 0 2 4 6 8 10
seq 0 -2 -5 => 0 -2 -4

Math Functions

All math functions operate on int64 values unless specified otherwise.

The following math functions are available: add, add1, ceil, div, floor, len, max, min, mod, mul, round, and sub.

add

Sum numbers with add. Accepts two or more inputs.

add 1 2 3

add1

To increment by 1, use add1

sub

To subtract, use sub

div

Perform integer division with div

mod

Modulo with mod

mul

Multiply with mul. Accepts two or more inputs.

mul 1 2 3

max

Return the largest of a series of integers:

This will return 3:

max 1 2 3

min

Return the smallest of a series of integers.

min 1 2 3 will return 1

floor

Returns the greatest float value less than or equal to input value

floor 123.9999 will return 123.0

ceil

Returns the greatest float value greater than or equal to input value

ceil 123.001 will return 124.0

round

Returns a float value with the remainder rounded to the given number to digits after the decimal point.

round 123.555555 3 will return 123.556

len

Returns the length of the argument as an integer.

len .Arg

Network Functions

Helm has a single network function, getHostByName.

The getHostByName receives a domain name and returns the ip address.

getHostByName "www.google.com" would return the corresponding ip address of www.google.com

File Path Functions

While Helm template functions do not grant access to the filesystem, they do provide functions for working with strings that follow file path conventions. Those include base, clean, dir, ext, and isAbs.

base

Return the last element of a path.

base "foo/bar/baz"

The above prints "baz"

dir

Return the directory, stripping the last part of the path. So dir "foo/bar/baz" returns foo/bar

clean

Clean up a path.

clean "foo/bar/../baz"

The above resolves the .. and returns foo/baz

ext

Return the file extension.

ext "foo.bar"

The above returns .bar.

isAbs

To check whether a file path is absolute, use isAbs.

Reflection Functions

Helm provides rudimentary reflection tools. These help advanced template developers understand the underlying Go type information for a particular value. Helm is written in Go and is strongly typed. The type system applies within templates.

Go has several primitive kinds, like string, slice, int64, and bool.

Go has an open type system that allows developers to create their own types.

Helm provides a set of functions for each via kind functions and type functions. A deepEqual function is also provided to compare to values.

Kind Functions

There are two Kind functions: kindOf returns the kind of an object.

kindOf "hello"

The above would return string. For simple tests (like in if blocks), the isKind function will let you verify that a value is a particular kind:

kindIs "int" 123

The above will return true

Type Functions

Types are slightly harder to work with, so there are three different functions:

  • typeOf returns the underlying type of a value: typeOf $foo
  • typeIs is like kindIs, but for types: typeIs "*io.Buffer" $myVal
  • typeIsLike works as typeIs, except that it also dereferences pointers.

Note: None of these can test whether or not something implements a given interface, since doing so would require compiling the interface in ahead of time.

deepEqual

deepEqual returns true if two values are "deeply equal"

Works for non-primitive types as well (compared to the built-in eq).

deepEqual (list 1 2 3) (list 1 2 3)

The above will return true

Semantic Version Functions

Some version schemes are easily parseable and comparable. Helm provides functions for working with SemVer 2 versions. These include semver and semverCompare. Below you will also find details on using ranges for comparisons.

semver

The semver function parses a string into a Semantic Version:

$version := semver "1.2.3-alpha.1+123"

If the parser fails, it will cause template execution to halt with an error.

At this point, $version is a pointer to a Version object with the following properties:

  • $version.Major: The major number (1 above)
  • $version.Minor: The minor number (2 above)
  • $version.Patch: The patch number (3 above)
  • $version.Prerelease: The prerelease (alpha.1 above)
  • $version.Metadata: The build metadata (123 above)
  • $version.Original: The original version as a string

Additionally, you can compare a Version to another version using the Compare function:

semver "1.4.3" | (semver "1.2.3").Compare

The above will return -1.

The return values are:

  • -1 if the given semver is greater than the semver whose Compare method was called
  • 1 if the version who's Compare function was called is greater.
  • 0 if they are the same version

(Note that in SemVer, the Metadata field is not compared during version comparison operations.)

semverCompare

A more robust comparison function is provided as semverCompare. This version supports version ranges:

  • semverCompare "1.2.3" "1.2.3" checks for an exact match
  • semverCompare "^1.2.0" "1.2.3" checks that the major and minor versions match, and that the patch number of the second version is greater than or equal to the first parameter.

The SemVer functions use the Masterminds semver library, from the creators of Sprig.

Basic Comparisons

There are two elements to the comparisons. First, a comparison string is a list of space or comma separated AND comparisons. These are then separated by || (OR) comparisons. For example, ">= 1.2 < 3.0.0 || >= 4.2.3" is looking for a comparison that's greater than or equal to 1.2 and less than 3.0.0 or is greater than or equal to 4.2.3.

The basic comparisons are:

  • =: equal (aliased to no operator)
  • !=: not equal
  • >: greater than
  • <: less than
  • >=: greater than or equal to
  • <=: less than or equal to

Note, according to the Semantic Version specification pre-releases may not be API compliant with their release counterpart. It says,

Working With Prerelease Versions

Pre-releases, for those not familiar with them, are used for software releases prior to stable or generally available releases. Examples of prereleases include development, alpha, beta, and release candidate releases. A prerelease may be a version such as 1.2.3-beta.1 while the stable release would be 1.2.3. In the order of precedence, prereleases come before their associated releases. In this example 1.2.3-beta.1 < 1.2.3.

According to the Semantic Version specification prereleases may not be API compliant with their release counterpart. It says,

A pre-release version indicates that the version is unstable and might not satisfy the intended compatibility requirements as denoted by its associated normal version.

SemVer comparisons using constraints without a prerelease comparator will skip prerelease versions. For example, >=1.2.3 will skip prereleases when looking at a list of releases while >=1.2.3-0 will evaluate and find prereleases.

The reason for the 0 as a pre-release version in the example comparison is because pre-releases can only contain ASCII alphanumerics and hyphens (along with . separators), per the spec. Sorting happens in ASCII sort order, again per the spec. The lowest character is a 0 in ASCII sort order (see an ASCII Table)

Understanding ASCII sort ordering is important because A-Z comes before a-z. That means >=1.2.3-BETA will return 1.2.3-alpha. What you might expect from case sensitivity doesn't apply here. This is due to ASCII sort ordering which is what the spec specifies.

Hyphen Range Comparisons

There are multiple methods to handle ranges and the first is hyphens ranges. These look like:

  • 1.2 - 1.4.5 which is equivalent to >= 1.2 <= 1.4.5
  • 2.3.4 - 4.5 which is equivalent to >= 2.3.4 <= 4.5

Wildcards In Comparisons

The x, X, and * characters can be used as a wildcard character. This works for all comparison operators. When used on the = operator it falls back to the patch level comparison (see tilde below). For example,

  • 1.2.x is equivalent to >= 1.2.0, < 1.3.0
  • >= 1.2.x is equivalent to >= 1.2.0
  • <= 2.x is equivalent to < 3
  • * is equivalent to >= 0.0.0

Tilde Range Comparisons (Patch)

The tilde (~) comparison operator is for patch level ranges when a minor version is specified and major level changes when the minor number is missing. For example,

  • ~1.2.3 is equivalent to >= 1.2.3, < 1.3.0
  • ~1 is equivalent to >= 1, < 2
  • ~2.3 is equivalent to >= 2.3, < 2.4
  • ~1.2.x is equivalent to >= 1.2.0, < 1.3.0
  • ~1.x is equivalent to >= 1, < 2

Caret Range Comparisons (Major)

The caret (^) comparison operator is for major level changes once a stable (1.0.0) release has occurred. Prior to a 1.0.0 release the minor versions acts as the API stability level. This is useful when comparisons of API versions as a major change is API breaking. For example,

  • ^1.2.3 is equivalent to >= 1.2.3, < 2.0.0
  • ^1.2.x is equivalent to >= 1.2.0, < 2.0.0
  • ^2.3 is equivalent to >= 2.3, < 3
  • ^2.x is equivalent to >= 2.0.0, < 3
  • ^0.2.3 is equivalent to >=0.2.3 <0.3.0
  • ^0.2 is equivalent to >=0.2.0 <0.3.0
  • ^0.0.3 is equivalent to >=0.0.3 <0.0.4
  • ^0.0 is equivalent to >=0.0.0 <0.1.0
  • ^0 is equivalent to >=0.0.0 <1.0.0

URL Functions

Helm includes the urlParse, urlJoin, and urlquery functions enabling you to work with URL parts.

urlParse

Parses string for URL and produces dict with URL parts

urlParse "http://admin:secret@server.com:8080/api?list=false#anchor"

The above returns a dict, containing URL object:

scheme:   'http'
host:     'server.com:8080'
path:     '/api'
query:    'list=false'
opaque:   nil
fragment: 'anchor'
userinfo: 'admin:secret'

This is implemented used the URL packages from the Go standard library. For more info, check https://golang.org/pkg/net/url/#URL

urlJoin

Joins map (produced by urlParse) to produce URL string

urlJoin (dict "fragment" "fragment" "host" "host:80" "path" "/path" "query" "query" "scheme" "http")

The above returns the following string:

proto://host:80/path?query#fragment

urlquery

Returns the escaped version of the value passed in as an argument so that it is suitable for embedding in the query portion of a URL.

$var := urlquery "string for query"

UUID Functions

Helm can generate UUID v4 universally unique IDs.

uuidv4

The above returns a new UUID of the v4 (randomly generated) type.

Kubernetes and Chart Functions

Helm includes functions for working with Kubernetes including .Capabilities.APIVersions.Has, Files, and lookup.

lookup

lookup is used to look up resource in a running cluster. When used with the helm template command it always returns an empty response.

You can find more detail in the documentation on the lookup function.

.Capabilities.APIVersions.Has

Returns if an API version or resource is available in a cluster.

.Capabilities.APIVersions.Has "apps/v1"
.Capabilities.APIVersions.Has "apps/v1/Deployment"

More information is available on the built-in object documentation.

File Functions

There are several functions that enable you to get to non-special files within a chart. For example, to access application configuration files. These are documented in Accessing Files Inside Templates.

Note, the documentation for many of these functions come from Sprig. Sprig is a template function library available to Go applications.