Expression Syntax¶

JMESPath expression syntax reference.

Identifiers¶

Access object properties by name:

echo '{"name": "Alice", "age": 30}' | jpx 'name'
# "Alice"

Index Expressions¶

Access array elements by index (0-based):

echo '{"items": ["a", "b", "c"]}' | jpx 'items[0]'
# "a"

echo '{"items": ["a", "b", "c"]}' | jpx 'items[-1]'
# "c"

Slicing¶

Extract array slices with [start:stop:step]:

echo '[0, 1, 2, 3, 4, 5]' | jpx '[0:3]'
# [0, 1, 2]

echo '[0, 1, 2, 3, 4, 5]' | jpx '[::2]'
# [0, 2, 4]

echo '[0, 1, 2, 3, 4, 5]' | jpx '[::-1]'
# [5, 4, 3, 2, 1, 0]

Wildcard¶

Project all elements with *:

echo '{"users": [{"name": "Alice"}, {"name": "Bob"}]}' | jpx 'users[*].name'
# ["Alice", "Bob"]

Flatten¶

Flatten nested arrays with []:

echo '[[1, 2], [3, 4]]' | jpx '[]'
# [1, 2, 3, 4]

Filter Expressions¶

Filter arrays with [?expression]:

echo '[{"age": 25}, {"age": 17}, {"age": 30}]' | jpx '[?age > `18`]'
# [{"age": 25}, {"age": 30}]

echo '[{"name": "Alice", "active": true}, {"name": "Bob", "active": false}]' | jpx '[?active].name'
# ["Alice"]

Comparison Operators¶

Operator	Description
`==`	Equal
`!=`	Not equal
`<`	Less than
`<=`	Less than or equal
`>`	Greater than
`>=`	Greater than or equal

Logical Operators¶

Operator	Description
`&&`	Logical AND
`\\|\\|`	Logical OR
`!`	Logical NOT

Pipe Expressions¶

Chain expressions with |:

echo '{"items": [3, 1, 2]}' | jpx 'items | sort(@) | reverse(@)'
# [3, 2, 1]

Multi-Select List¶

Create arrays from multiple expressions:

echo '{"a": 1, "b": 2, "c": 3}' | jpx '[a, b]'
# [1, 2]

Multi-Select Hash¶

Create objects from multiple expressions:

echo '{"first": "Alice", "last": "Smith", "age": 30}' | jpx '{name: first, surname: last}'
# {"name": "Alice", "surname": "Smith"}

Literal Values¶

Use backticks for literal values:

# Literal number
echo '{"items": [1, 2, 3]}' | jpx '[?@ > `1`]'

# Literal string
echo '{"items": ["a", "b"]}' | jpx 'contains(items, `"a"`)'

# Literal array
echo '{}' | jpx '`[1, 2, 3]`'

# Literal object
echo '{}' | jpx '`{"key": "value"}`'

Current Node¶

Reference the current node with @:

echo '[1, 2, 3, 4, 5]' | jpx '[?@ > `2`]'
# [3, 4, 5]

echo '"hello"' | jpx 'upper(@)'
# "HELLO"

Function Calls¶

Call functions with function_name(arg1, arg2, ...):

echo '{"items": [1, 2, 3]}' | jpx 'length(items)'
# 3

echo '{"name": "hello"}' | jpx 'upper(name)'
# "HELLO"

echo '{"a": [1, 2], "b": [3, 4]}' | jpx 'merge(a, b)'
# [1, 2, 3, 4]

Functions can take literal JSON values as arguments using backticks:

# Apply default values to an object
echo '{"name": "Alice"}' | jpx 'defaults(@, `{"role": "user", "active": true}`)'
# {"name": "Alice", "role": "user", "active": true}

# Build an object from key-value pairs
echo 'null' | jpx 'from_items(`[["a", 1], ["b", 2]]`)'
# {"a": 1, "b": 2}

Expression References¶

Use & for expression references (used with higher-order functions):

echo '[{"age": 30}, {"age": 20}]' | jpx 'sort_by(@, &age)'
# [{"age": 20}, {"age": 30}]

echo '[1, 2, 3]' | jpx 'map(&@ * `2`, @)'
# [2, 4, 6]

Let Expressions (JEP-18)¶

Bind intermediate results to variables with let:

echo '{"people": [{"name": "Alice", "age": 30}, {"name": "Bob", "age": 25}]}' \
  | jpx 'let $adults = people[?age > `28`] in $adults[*].name'
# ["Alice"]

echo '{"data": [3, 1, 4]}' \
  | jpx 'let $s = sort(data), $n = length(data) in {sorted: $s, count: $n}'
# {"sorted": [1, 3, 4], "count": 3}

Variables are prefixed with $ and scoped to the in body. Multiple bindings are separated by commas. See the Let Expressions guide for full details.

Note

Let expressions are disabled in strict mode since they are a JEP-18 extension.