JSON One-Liners

2026-04-13

JSON is the universal data exchange format. Raku can parse and manipulate JSON from the command line using the built-in JSON::Fast module (included with Rakudo), making it a handy alternative to jq for quick data extraction.

Reading JSON

Parse a JSON file and pretty-print it:

raku -MJSON::Fast -e 'say to-json(from-json(slurp("data.json")), :pretty)'

Parse JSON from STDIN:

echo '{"name":"Alice","age":30}' | raku -MJSON::Fast -e 'say from-json($*IN.slurp).raku'

Extracting Fields

Get a single field from a JSON object:

echo '{"name":"Alice","age":30}' | raku -MJSON::Fast -e 'say from-json($*IN.slurp)<name>'

Get a nested field:

echo '{"user":{"name":"Alice","address":{"city":"Toronto"}}}' | \
  raku -MJSON::Fast -e 'say from-json($*IN.slurp)<user><address><city>'

Working with Arrays

Print each element of a JSON array:

echo '[1,2,3,4,5]' | raku -MJSON::Fast -e '.say for from-json($*IN.slurp).list'

Get a specific index:

echo '["alpha","beta","gamma"]' | raku -MJSON::Fast -e 'say from-json($*IN.slurp)[1]'

Count elements:

echo '[1,2,3,4,5]' | raku -MJSON::Fast -e 'say from-json($*IN.slurp).elems'

Filtering Array Objects

Given an array of objects, filter by a field:

cat <;<'EOF' | raku -MJSON::Fast -e '
    my @data = from-json($*IN.slurp).list;
    .say for @data.grep(*<active>;).map(*<name>;);
'
[{"name":"Alice","active":true},{"name":"Bob","active":false},{"name":"Carol","active":true}]
EOF

Output:

Alice
Carol

Extracting a Field from Each Object

Get all names from an array of objects:

raku -MJSON::Fast -e '
    my @data = from-json(slurp("users.json")).list;
    .say for @data.map(*<name>;);
'

Sorting JSON Arrays

Sort objects by a field:

raku -MJSON::Fast -e '
    my @data = from-json(slurp("users.json")).list;
    say to-json(@data.sort(*<age>;), :pretty);
'

Sort descending:

raku -MJSON::Fast -e '
    my @data = from-json(slurp("users.json")).list;
    say to-json(@data.sort(-*<age>;), :pretty);
'

Aggregating Data

Sum a numeric field:

raku -MJSON::Fast -e '
    my @items = from-json(slurp("orders.json")).list;
    say "Total: " ~ @items.map(*<amount>;).sum;
'

Average:

raku -MJSON::Fast -e '
    my @items = from-json(slurp("scores.json")).list;
    my @vals = @items.map(*<score>;);
    say "Average: {(@vals.sum / @vals.elems).round(0.01)}";
'

Group and count:

raku -MJSON::Fast -e '
    my @data = from-json(slurp("events.json")).list;
    my %groups;
    %groups{$_<;type>;}++ for @data;
    say "$_.key(): $_.value()" for %groups.sort(-*.value);
'

Transforming JSON

Add a field to each object:

raku -MJSON::Fast -e '
    my @data = from-json(slurp("users.json")).list;
    my @enhanced = @data.map({ %( |$_, status => "processed") });
    say to-json(@enhanced, :pretty);
'

Remove a field:

raku -MJSON::Fast -e '
    my @data = from-json(slurp("users.json")).list;
    my @clean = @data.map({ my %h = $_.Hash; %h<;password>;:delete; %h });
    say to-json(@clean, :pretty);
'

Rename a field:

raku -MJSON::Fast -e '
    my @data = from-json(slurp("data.json")).list;
    my @renamed = @data.map({
        my %h = $_.Hash;
        %h<;full_name>; = %h<;name>;:delete;
        %h
    });
    say to-json(@renamed, :pretty);
'

Converting JSON to CSV

raku -MJSON::Fast -e '
    my @data = from-json(slurp("users.json")).list;
    my @keys = @data[0].keys.sort;
    say @keys.join(",");
    for @data -> %row {
        say @keys.map({ %row{$_} // "" }).join(",");
    }
'

Converting CSV to JSON

raku -MJSON::Fast -e '
    my @lines = "data.csv".IO.lines;
    my @headers = @lines.shift.split(",");
    my @data = @lines.map(-> $line {
        my @vals = $line.split(",");
        my %row;
        for @headers.kv -> $i, $h { %row{$h} = @vals[$i] // "" }
        %row
    });
    say to-json(@data, :pretty);
'

Merging JSON Files

Combine two JSON arrays:

raku -MJSON::Fast -e '
    my @a = from-json(slurp("file1.json")).list;
    my @b = from-json(slurp("file2.json")).list;
    say to-json([|@a, |@b], :pretty);
'

Merge two JSON objects (shallow):

raku -MJSON::Fast -e '
    my %a = from-json(slurp("defaults.json")).Hash;
    my %b = from-json(slurp("overrides.json")).Hash;
    say to-json(%( |%a, |%b ), :pretty);
'

Validating JSON

Check if a string is valid JSON:

echo '{"valid": true}' | raku -MJSON::Fast -e '
    try {
        from-json($*IN.slurp);
        say "Valid JSON";
        CATCH { default { say "Invalid JSON: {.message}" } }
    }
'

Piping with Other Tools

Combine with curl to process API responses:

curl -s https://httpbin.org/get | raku -MJSON::Fast -e '
    my %data = from-json($*IN.slurp).Hash;
    say "Origin: {%data<origin>}";
    say "Headers:";
    for %data<;headers>;.sort -> $h {
        say "  {$h.key}: {$h.value}";
    }
'

Pretty vs Compact Output

# Pretty (indented)
raku -MJSON::Fast -e 'say to-json(from-json(slurp("data.json")), :pretty)'

# Compact (single line)
raku -MJSON::Fast -e 'say to-json(from-json(slurp("data.json")))'

# Sorted keys
raku -MJSON::Fast -e 'say to-json(from-json(slurp("data.json")), :pretty, :sorted-keys)'

These one-liners make Raku a viable command-line JSON processor. For quick extraction and transformation tasks, they are often easier to write than the equivalent jq expressions, especially when you need to combine JSON processing with other logic.