Template functions

The text template format used in | line_format and | label_format support the usage of functions.

All labels are added as variables in the template engine. They can be referenced using they label name prefixed by a .(e.g .label_name). For example the following template will output the value of the path label:

{{ .path }}

Additionally you can also access the log line using the __line__ function and the timestamp using the __timestamp__ function.

You can take advantage of pipeline to join together multiple functions. In a chained pipeline, the result of each command is passed as the last argument of the following command.

Example:

{{ .path | replace " " "_" | trunc 5 | upper }}

line

This function returns the current log line.

Signature:

line() string

Examples:

"{{ __line__ | lower }}"
`{{ __line__ }}`

timestamp

This function returns the current log lines timestamp.

Signature:

timestamp() time.Time

"{{ __timestamp__ }}"
`{{ __timestamp__ | date "2006-01-02T15:04:05.00Z-07:00" }}`
`{{ __timestamp__ | unixEpoch }}`

See the blog: [Parsing and formatting date/time in Go](https://www.pauladamsmith.com/blog/2011/05/go_time.html) for more information.

## ToLower and ToUpper

This function converts the entire string to lowercase or uppercase.

Signatures:

- `ToLower(string) string`
- `ToUpper(string) string`

Examples:

```template
"{{.request_method | ToLower}}"
"{{.request_method | ToUpper}}"
`{{ToUpper "This is a string" | ToLower}}`

Replace string

Use this function to perform a simple string replacement.

Signature:

Replace(s, old, new string, n int) string

It takes four arguments:

s source string
old string to replace
new string to replace with
n the maximun amount of replacement (-1 for all)

Example:

`{{ Replace "This is a string" " " "-" -1 }}`

The results in This-is-a-string.

Trim, TrimLeft, TrimRight, and TrimSpace

Trim returns a slice of the string s with all leading and trailing Unicode code points contained in cutset removed.

Signature: Trim(value, cutset string) string

TrimLeft and TrimRight are the same as Trim except that it trims only leading and trailing characters respectively.

`{{ Trim .query ",. " }}`
`{{ TrimLeft .uri ":" }}`
`{{ TrimRight .path "/" }}`

TrimSpace TrimSpace returns string s with all leading and trailing white space removed, as defined by Unicode.

Signature: TrimSpace(value string) string

{{ TrimSpace .latency }}

TrimPrefix and TrimSuffix will trim respectively the prefix or suffix supplied.

Signature:

TrimPrefix(value string, prefix string) string
TrimSuffix(value string, suffix string) string

{{ TrimPrefix .path "/" }}

regexReplaceAll and regexReplaceAllLiteral

regexReplaceAll 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 sub-match. See the golang Regexp.replaceAll documentation for more examples.

`{{ regexReplaceAll "(a*)bc" .some_label "${1}a" }}`

regexReplaceAllLiteral function returns a copy of the input string and replaces matches of the Regexp with the replacement string replacement. The replacement string is substituted directly, without using Expand.

`{{ regexReplaceAllLiteral "(ts=)" .timestamp "timestamp=" }}`

lower

Use this function to convert to lower case.

Signature:

lower(string) string

Examples:

"{{ .request_method | lower }}"
`{{ lower  "HELLO"}}`

The last example will return hello.

upper

Use this function to convert to upper case.

Signature:

upper(string) string

Examples:

"{{ .request_method | upper }}"
`{{ upper  "hello"}}`

This results in HELLO.

title

Convert to title case.

Signature:

title(string) string

Examples:

"{{.request_method | title}}"
`{{ title "hello world"}}`

The last example will return Hello World.

trunc

Truncate a string and add no suffix.

Signature:

trunc(count int,value string) string

Examples:

"{{ .path | trunc 2 }}"
`{{ trunc 5 "hello world"}}`   // output: hello
`{{ trunc -5 "hello world"}}`  // output: world

substr

Get a substring from a string.

Signature:

substr(start int,end int,value string) string

If start is < 0, this calls value[:end]. If start is >= 0 and end < 0 or end bigger than s length, this calls value[start:] Otherwise, this calls value[start, end].

Examples:

"{{ .path | substr 2 5 }}"
`{{ substr 0 5 "hello world"}}`  // output: hello
`{{ substr 6 11 "hello world"}}` // output: world

replace

This function performs simple string replacement.

Signature: replace(old string, new string, src string) string

It takes three arguments:

old string to replace
new string to replace with
src source string

Examples:

{{ .cluster | replace "-cluster" "" }}
{{ replace "hello" "world" "hello world" }}

The last example will return world world.

trim

The trim function removes space from either side of a string.

Signature: trim(string) string

Examples:

{{ .ip | trim }}
{{ trim "   hello    " }} // output: hello

trimAll

Use this function to remove given characters from the front or back of a string.

Signature: trimAll(chars string,src string) string

Examples:

{{ .path | trimAll "/" }}
{{ trimAll "$" "$5.00" }} // output: 5.00

trimSuffix

Use this function to trim just the suffix from a string.

Signature: trimSuffix(suffix string, src string) string

Examples:

{{  .path | trimSuffix "/" }}
{{ trimSuffix "-" "hello-" }} // output: hello

trimPrefix

Use this function to trim just the prefix from a string.

Signature: trimPrefix(prefix string, src string) string

Examples:

{{  .path | trimPrefix "/" }}
{{ trimPrefix "-" "-hello" }} // output: hello

indent

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

Signature: indent(spaces int,src string) string

{{ indent 4 .query }}

This indents each line contained in the .query by four (4) spaces.

nindent

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

Signature: nindent(spaces int,src string) string

{{ nindent 4 .query }}

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

repeat

Use this function to repeat a string multiple times.

Signature: repeat(c int,value string) string

{{ repeat 3 "hello" }} // output: hellohellohello

contains

Use this function to test to see if one string is contained inside of another.

Signature: contains(s string, src string) bool

Examples:

{{ if .err contains "ErrTimeout" }} timeout {{end}}
{{ if contains "he" "hello" }} yes {{end}}

hasPrefix and hasSuffix

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

Signatures:

hasPrefix(prefix string, src string) bool
hasSuffix(suffix string, src string) bool

Examples:

{{ if .err hasSuffix "Timeout" }} timeout {{end}}
{{ if hasPrefix "he" "hello" }} yes {{end}}

add

Sum numbers. Supports multiple numbers

Signature: func(i ...interface{}) int64

{{ add 3 2 5 }} // output: 10

sub

Subtract numbers.

Signature: func(a, b interface{}) int64

{{ sub 5 2 }} // output: 3

mul

Mulitply numbers. Supports multiple numbers.

Signature: func(a interface{}, v ...interface{}) int64

{{ mul 5 2 3}} // output: 30

div

Integer divide numbers.

Signature: func(a, b interface{}) int64

{{ div 10 2}} // output: 5

addf

Sum numbers. Supports multiple numbers.

Signature: func(i ...interface{}) float64

{{ addf 3.5 2 5 }} // output: 10.5

subf

Subtract numbers. Supports multiple numbers.

Signature: func(a interface{}, v ...interface{}) float64

{{ subf  5.5 2 1.5 }} // output: 2

mulf

Mulitply numbers. Supports multiple numbers

Signature: func(a interface{}, v ...interface{}) float64

{{ mulf 5.5 2 2.5 }} // output: 27.5

divf

Divide numbers. Supports multiple numbers.

Signature: func(a interface{}, v ...interface{}) float64

{{ divf 10 2 4}} // output: 1.25

mod

Modulo wit mod.

Signature: func(a, b interface{}) int64

{{ mod 10 3}} // output: 1

max

Return the largest of a series of integers:

Signature: max(a interface{}, i ...interface{}) int64

{{ max 1 2 3 }} //output 3

min

Return the smallest of a series of integers.

Signature: min(a interface{}, i ...interface{}) int64

{{ max 1 2 3 }} //output 1

maxf

Return the largest of a series of floats:

Signature: maxf(a interface{}, i ...interface{}) float64

{{ maxf 1 2.5 3 }} //output 3

minf

Return the smallest of a series of floats.

Signature: minf(a interface{}, i ...interface{}) float64

{{ minf 1 2.5 3 }} //output 1.5

ceil

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

Signature: ceil(a interface{}) float64

{{ ceil 123.001 }} //output 124.0

floor

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

Signature: floor(a interface{}) float64

{{ floor 123.9999 }} //output 123.0

round

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

Signature: round(a interface{}, p int, rOpt ...float64) float64

{{ round 123.555555 3 }} //output 123.556

We can also provide a roundOn number as third parameter

{{ round 123.88571428571 5 .2 }} //output 123.88572

With default roundOn of .5 the above value would be 123.88571

int

Convert value to an int.

Signature: toInt(v interface{}) int

{{ "3" | int }} //output 3

float64

Convert to a float64.

Signature: toFloat64(v interface{}) float64

{{ "3.5" | float64 }} //output 3.5

fromJson

fromJson decodes a JSON document into a structure. If the input cannot be decoded as JSON the function will return an empty string.

fromJson "{\"foo\": 55}"

Example of a query to print a newline per queries stored as a json array in the log line:

{job="loki/querier"} |= "finish in prometheus" | logfmt | line_format "{{ range $q := fromJson .queries }} {{ $q.query }} {{ end }}"

now

now returns the current local time.

{{ now }}

toDate

toDate parses a formatted string and returns the time value it represents.

{{ toDate "2006-01-02" "2021-11-02" }}

date

date returns a textual representation of the time value formatted according to the provided golang datetime layout.

{{ date "2006-01-02" now }}

unixEpoch

unixEpoch returns the number of seconds elapsed since January 1, 1970 UTC.

{{ unixEpoch now }}

Example of a query to filter Loki querier jobs which create time is 1 day before:

{job="loki/querier"} | label_format nowEpoch=`{{(unixEpoch now)}}`,createDateEpoch=`{{unixEpoch (toDate "2006-01-02" .createDate)}}` | label_format dateTimeDiff="{{sub .nowEpoch .createDateEpoch}}" | dateTimeDiff > 86400

default

default checks whether the string(src) is set, and returns default(d) if not set.

Signature: default(d string, src string) string

Examples:

{{ default "-" "" }} // output: -
{{ default "-" "foo" }} // output: foo

Example of a query to print a - if the http_request_headers_x_forwarded_for label is empty:

{job="access_log"} | json | line_format `{{.http_request_headers_x_forwarded_for | default "-"}}`

Template functions

line

timestamp

Replace string

Trim, TrimLeft, TrimRight, and TrimSpace

regexReplaceAll and regexReplaceAllLiteral

lower

upper

title

trunc

substr

replace

trim

trimAll

trimSuffix

trimPrefix

indent

nindent

repeat

contains

hasPrefix and hasSuffix

add

sub

mul

div

addf

subf

mulf

divf

mod

max

min

maxf

minf

ceil

floor

round

int

float64

fromJson

now

toDate

date

unixEpoch

default

Related Loki resources

Getting started with logging and Grafana Loki (Part 3 of 4)

Logging with Loki: Essential configuration settings

Observability with logs & Grafana