yq/usage/tips-and-tricks.md

Tips, Tricks, Troubleshooting

Validating yaml files

Yaml files can be surprisingly lenient in what can be parsed as a yaml file. A reasonable way of validation a yaml file is to ensure the top level is a map or array (although it is valid yaml to have scalars at the top level, but often this is not what you want). This can be done by:

yq --exit-status 'tag == "!!map" or tag== "!!seq"' file.txt > /dev/null

Split expressions over multiple lines to improve readability

Feel free to use multiple lines in your expression to improve readability.

Use with if you need to make several updates to the same path.

Use # comments to explain things

yq --inplace '
  with(.a.deeply.nested;
    . = "newValue" | . style="single" # line comment about styles
  ) |
  #
  # Block comment that explains what is happening.
  #
  with(.b.another.nested; 
    . = "cool" | . style="folded"
  )
' my_file.yaml

Create bash array

Given a yaml file like

coolActions:
  - create
  - edit
  - delete

You can create a bash array named actions by:

> readarray actions < <(yq '.coolActions[]' sample.yaml)
> echo "${actions[1]}"
edit

yq in a bash loop

For a given yaml file like:

identities:
- arn: "arn:aws:iam::ARN1"
  group: "system:masters"
  user: "user1"
- arn: "arn:aws:iam::ARN2"
  group: "system:masters"
  user: "user2"

You can loop over the results in a bash loop like:

# load array into a bash array
# output each entry as a single line json
readarray identityMappings < <(yq -o=j -I=0 '.identities[]' test.yml )

for identityMapping in "${identityMappings[@]}"; do
    # identity mapping is a single json snippet representing a single entry
    roleArn=$(echo "$identityMapping" | yq '.arn' -)
    echo "roleArn: $roleArn"
done

Set contents from another file

Use the load operator to load contents from another file.

Special characters in strings

The strenv operator is a great way to handle special characters in strings:

VAL='.a |!@  == "string2"' yq '.a = strenv(VAL)' example.yaml

Update multiple files

yq doesn't have a way of updating multiple files in a single command (yet?) - but you can use your shell's built in tools like find:

find *.yaml -exec yq '. += "cow"' -i {} \;

This will run the '. += "cow"' expression against every matching file, and update it in place (-i).

String blocks and newline issues

There are a couple of tricks to getting the right string representation, take a look at string operators for more details:

Quotes in Windows Powershell

Powershell has its own way of handling quotes:

PS > yq -n '.test = ""something""'
test: something
PS >

See https://github.com/mikefarah/yq/issues/747 for more trickery.

Merge / combine all documents into one

To merge all given yaml files into one, use the reduce operator with the * (multiply) operator. Note the use of ea or eval-all to load all files into memory so that they can be merged.

yq ea '. as $item ireduce ({}; . * $item )' file1.yml file2.yml ...

Merge - showing the source file and line

To see the original source file and line number of your merged result, you can pre-process the files and add that information in as line comments, then perform the merge.

yq ea '(..  lineComment |= filename + ":" + line) | select(fi==0) * select(fi==1)' data1.yaml data2.yaml

Merge an array of objects by key

See here for a working example.

Deeply prune a tree

See here for a working example.

Creating a new file / working with blank documents

To create a new yaml file simply:

yq -n '.someNew="content"' > newfile.yml

Comparing yaml files

The best way to run a diff is to use yq to normalise the yaml files and then just use diff. Here is a simple example of using pretty print -P to normalise the styling and running diff:

diff <(yq -P 'sort_keys(..)' -o=props file1.yaml) <(yq -P 'sort_keys(..)' -o=props file2.yaml)

This way you can use the full power of diff and normalise the yaml files as you like.

You may also want to remove all comments using ... comments=""

Reading multiple streams (STDINs)

Like diff and other bash commands, you can use <(exp) to pipe in multiple streams of data into yq. instance:

yq '.apple' <(curl -s https://somewhere/data1.yaml) <(cat file.yml)

Updating deeply selected paths

or why is yq only returning the updated yaml

The most important thing to remember to do is to have brackets around the LHS expression - otherwise what yq will do is first filter by the selection, and then, separately, update the filtered result and return that subset.

yq '(.foo.bar[] | select(name == "fred) | .apple) = "cool"'

Combining multiple files into one

In order to combine multiple yaml files into a single file (with --- separators) you can just:

yq '.' somewhere/*.yaml

Multiple updates to the same path

You can use the with operator to set a nested context:

yq 'with(.a.deeply ; .nested = "newValue" | .other= "newThing")' sample.yml

The first argument expression sets the root context, and the second expression runs against that root context.

Logic without if/elif/else

yq has not yet added if expressions - however you should be able to use with and select to achieve the same outcome. Lets use an example:

- animal: cat
- animal: dog
- animal: frog

Now, if you were using good ol' jq - you may have a script with ifs like so:

jq ' .[] |=
if (.animal == "cat") then
  .noise = "meow" |
  .whiskers = true
elif (.animal == "dog") then
  .noise = "woof" |
  .happy = true
else 
  .noise = "??"
end
' < file.yaml

Using yq - you can get the same result by:

yq '.[] |= (
  with(select(.animal == "cat"); 
    .noise = "meow" | 
    .whiskers = true
  ) |
  with(select(.animal == "dog"); 
    .noise = "woof" | 
    .happy = true
  ) |
  with(select(.noise == null); 
    .noise = "???"
  )
)' < file.yml

Note that the logic isn't quite the same, as there is no concept of 'else'. So you may need to put additional logic in the expressions, as this has for the 'else' logic.

yq adds a !!merge tag automatically

The merge functionality from yaml v1.1 (e.g. <<:has actually been removed in the 1.2 spec. Thankfully, yq underlying yaml parser still supports that tag - and it's extra nice in that it explicitly puts the !!merge tag on key of the map entry. This tag tells other yaml parsers that this entry is a merge entry, as opposed to a regular string key that happens to have a value of <<:. This is backwards compatible with the 1.1 spec of yaml, it's simply an explicit way of specifying the type (for instance, you can use a !!str tag to enforce a particular value to be a string.

Although this does affect the readability of the yaml to humans, it still works and processes fine with various yaml processors.