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Take snapshots via CLI

How to grab Percy snapshots via CLI

The Percy snapshot is a powerful command that allows you to capture visual snapshots of web pages, lists, sitemaps, or static directories.

On this page, we’ll explore the different usage of Percy snapshot command through CLI and respective examples for the same. You have the option to either capture a full screenshot or take a screenshot of a single element.

Percy snapshot command

Run the Percy snapshot command via the command-line interface (CLI) as shown below. You can specify a snapshots file, a static directory, or the URL of a sitemap that you wish to capture snapshots of.

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Options

Use the below options along with percy snapshot command. For details, refer the below table:

Option Description
General -b, –base-url : Set the base URL for the pages when taking snapshots.
--include : Define one or more globs/patterns to include specific snapshots.
--exclude : Specify one or more globs/patterns to exclude specific snapshots.
Static –clean-urls: Rewrite static index and filepath URLs to be clean.
Percy -c, –config : Specify the path to the Percy configuration file.
-d, --dry-run: Print snapshot names only, useful for testing without actually taking snapshots.
-h, --allowed-hostname : Set allowed hostnames to capture during asset discovery.
-t, --network-idle-timeout : Define the asset discovery network idle timeout.
--disable-cache: Disable asset discovery caches.
--debug: Enable debug mode for asset discovery and prevent uploading snapshots.
Global -v, –verbose: Log all activity.
-q, –quiet: Log errors only.
-s, –silent: Disable all logging.
–help: Display command help.

Examples

Snapshot a file

When you have a file containing a list of snapshots, it must be in YAML, JSON, or a JavaScript format exporting a list of pages. Each snapshot should include at least a URL that can be navigated to using a browser.

  • snapshots.yml (YAML):
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Snapshot options

You can provide a name, waitForTimeout, and waitForSelector to customize snapshot behavior.

  • snapshots.json (JSON):
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Advanced options

With the snapshot command, you can interact with the page by providing an execute option. This can be any valid JavaScript you run inside of a browser.

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The execute option can also accept an object with these keys:

  • afterNavigation - called after the page navigates to the given URL
  • beforeResize - called before the page viewport is resized to one of the passed width(s)
  • afterResize - called after the page viewport is resized to one of the passed width(s)
  • beforeSnapshot - called before Percy captures a snapshot
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For advanced use cases, you can specify an execute function and additionalSnapshots for each snapshot to execute JavaScript within the page execution context before taking subsequent snapshots.

All options are also accepted by other file formats. For execute however, a string containing a function body can be provided when the file format prevents normal functions.

  • snapshots.js (JavaScript):
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The snapshot command also accepts a JavaScript file that exports an array of objects (with the keys name & url present). You can export sync or async functions from this file.

For example, a common use case is to build an array of pages dynamically and then iterate over that array to snapshot pages. You can export that array to the snapshot command. As a snapshots.js file:

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Instead of an array of snapshots, list files can also contain an object that defines additional top-level options along with a snapshots option containing the array of snapshots. This allows dynamically filtering lists with include/exclude options, and enables utilizing features such as YAML anchors and references.

  • Example snapshots.yml
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Option Description
url (required) URL of the page to visit and snapshot.
name Name of the snapshot (must be unique).
execute JavaScript you would like to execute in browser before capturing a snapshot.
waitForTimeout An amount of time to wait before capturing a snapshot.
waitForSelector A CSS selector to wait to appear on page before capturing a snapshot.

See per-snapshot configuration options for additional common per-snapshot options (like widths, percy-css, etc).

Snapshot a static directory

When providing a static directory, it will be served locally and with below use cases:

  • Pages matching the files argument (excluding the ignore argument) will be navigated to and snapshotted.
  • Pages matching the include argument (and excluding the exclude argument) will be navigated to and snapshotted.
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  • Static Options
    For snapshotting static directories, you can configure options in the Percy configuration file.

.percy.yml (YAML):

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  • Example using include/exclude options
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  • Example using rewrite
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  • Example using options
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Option Description
base-url The base URL path the static site should be served under.
clean-urls When true, rewrite index and filepath URLs to be clean.
include/exclude A predicate or an array of predicates matching snapshots to include/exclude.
A predicate can be a string glob or pattern, a regular expression, or a function that accepts a snapshot object and returns true or false if the snapshot is considered matching or not.
rewrites An object containing source-destination pairs for rewriting URLs.
Paths for resources can sometimes be expected to be in a certain format that may not be covered by the clean-urls option. For such paths, rewrites can map a short, clean, or pretty path to a specific resource. Paths are matched using path-to-regexp.
overrides An array of per-snapshot option overrides.
Just like page listing options, static snapshots may also contain per-snapshot configuration options. However, since pages are matched against the files option, so are per-snapshot configuration options via an array of overrides. If multiple overrides match a snapshot, they will be merged with previously matched overrides.

Snapshot a sitemap URL

When providing a sitemap URL, the document must be an XML document. You can use --include and --exclude flags to filter snapshots. The Percy configuration file also accepts options.

Sitemaps can contain a lot of URLs, so its best to always start with the --dry-run flag while fine tuning the include and exclude options.

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  • Sitemap options:
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JavaScript files may also export sync or async functions that return a list of pages to snapshot.

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Screenshot a single element

Sometimes capturing a full-page screenshot isn’t necessary. For example, if there are dynamic parts of the page that you don’t need to test or are only interested in a very specific region to test. For these cases, you can pass a scope snapshot option and Percy will only capture the scoped element on the given widths. This can be passed as a per-snapshot option. The scope selector accepts any valid selector you would be able to pass to document querySelector.

  • If there are multiple matching selectors on the page, Percy will select the first matching element.

Screenshot a single element using selector

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Screenshot a single element using xpath

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Screenshot a scrollable single element

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The Percy Snapshot Command is a versatile command for capturing and comparing visual snapshots, making it an indispensable part of your web development and testing workflow.

Use cases

Multiple elements with the same selector

If you would like to scope a screenshot to a specific element that has the same matching selector as other elements on the page you’ll have to get more specific with your selector. This can be done by either adding another unique selector to that element or by using standard CSS selectors to get more specific. This is the same way you would write CSS – Percy doesn’t add anything to this process.

For example, given the below DOM:

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Instead of using just .underline to select the element, you would want to either specify the element type (h1 / p) or by using CSS tree-structural pseudo-classes like :last-of-type or :nth-child.

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Likewise,

  • p.underline scope selects the ‘first’ paragraph
  • p.underline:last-of-type scope selects last paragraph

Selector for elements serialized by Percy

It may happen that your single-element screenshots are not working correctly with canvas/video elements.

During DOM serialization <canvas>, <video> elements are converted to <img>. Check canvas elements in Percy’s SDK Workflow. This can cause the selector to not match, and an incorrect screenshot.

To work around this, we can use CSS tree structural pseudo-classes as suggested in the previous section.

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  • Instead of canvas or video you’ll need to change it to img
  • If there is only a single canvas/video tag inside div we can directly use .selector as the scope.
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Likewise,

  • .selector img:nth-of-type(2) scope selects the ‘second’ canvas
  • .selector img:last-of-type scope selects the ‘last’ video

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