Tutorials on Filter

Commonly, we process JSON data by writing a program to load, deserialize and manipulate this data. Depending on the programming language, this program may require an additional compilation step before being executed within a terminal. For simple operations, such as filtering and mapping, we don't need to write an additional program to perform these operations on our JSON data. Rather, we can directly manipulate our JSON data within a terminal via the jq command-line utility, which allows the editing of streamed JSON data without an interactive text editor interface (" sed for JSON"). If you're looking for a tool to retrieve JSON data from an API endpoint, process this data and save the result to a CSV, TSV or JSON file, then jq easily accomplishes this task in a single-line command. Below, I'm going to show you how to process JSON data with jq . Install the jq command-line utility by visiting the homepage of the jq website, downloading a prebuilt binary (compatible with your operating system) and executing this binary once the download is complete. Alternatively... To verify the installation was successful, restart the terminal, and inside of this terminal, enter the command jq . This should print an overview of the jq command: For extensive documentation, enter the command man jq , which summons man ual pages for the jq command: To get started, let's pretty-print a JSON dataset (with formatting and syntax-highlighting). The jq command must be passed a filter as its first argument. A filter is a program that tells jq what output should be returned given the input JSON data. The most basic filter is the pre-defined identity filter . , which tells jq to do nothing to the input JSON data and return it as is. To run jq on a JSON dataset, pipe the stringified JSON to jq (e.g., the file content of a .json file via the cat command or the JSON response from an API endpoint via the cURL command). If we pipe the JSON response of a cURL command to jq . , then jq pretty-prints this response in the terminal. Suppose we only wanted a single element from the JSON data. To access a single element from a JSON array, pass the array index filter to jq , which follows the syntax .[x] with x representing an index value (positive and negative integer). To access the first element: To access the last element: To access the penultimate (second to last) element: To access the element at index 3 : If the index value is outside of the JSON array's bounds, then no element is returned by the array index filter: Here, the dataset only contains 41 rows. Therefore, any index beyond 40 causes the filter to return no element. If an index value is omitted, then all of the elements are returned by the array index filter: Additionally, the .[] filter can be used on JSON objects to return all top-level values within the object. In case you are unsure whether the input data is not valid JSON, then append a ? to the empty square brackets to suppress errors. For example, if the input data is a stringified integer value... Without the ? , the error jq: error (at <stdin>:1): Cannot iterate over number (1) will be thrown. With the ? , this error is suppressed as if no error occurred. Suppose we only wanted a subset of the JSON data. To extract a sub-array from a JSON array, pass the array/string slice filter to jq , which follows the syntax .[x:y] with x and y representing starting (inclusive) and ending (exclusive) index values respectively (positive and negative integers). It behaves similar to JavaScript's .slice() method. To extract the first element only: To extract the last element only: To extract all elements but the first element (omit the first element): To extract all elements but the last element (omit the last element): To extract the elements at indices 3 - 5 : To retrieve the length of a JSON array, pipe the output of an identity filter to the built-in length function: This returns the total number of elements within the JSON array. For our example dataset, the total number of records returned by the NYC Open Data API is 41 . For a JSON object, the length function returns the total number of top-level keys within this object. To retrieve the length of each item of a JSON array, pipe the output of a .[] slice filter to the length function: This returns a list of each element's length. For our example dataset, each record contains four pieces of information: the year, the population of NYC for that year, the total number of gallons (in millions) of water consumed by NYC residents per day and the average number of gallons of water consumed by a NYC resident per day. If an element is a string, then length returns the string's length. If an element is a null value, then length returns zero. To retrieve the top-level keys from JSON, use the built-in keys function. These keys are returned as an array of strings. Unlike the length function, the keys function requires no filter piping. By default, these keys are sorted alphabetically. Alternatively, the keys_unsorted function does not sort keys alphabetically and returns the keys in their original order. For JSON arrays, this function returns a list of indices. Experiment with these techniques on other JSON data sources/files.

Disclaimer - If you are unfamiliar with FFmpeg, then please read this blog post before proceeding. When you upload a video to a platform such as Youtube , you can select and add a custom thumbnail image to display within its result item. Amongst the many recommended videos, a professionally-made thumbnail captures the attention of undecided users and improves the chances of your video being played. At a low-level, a thumbnail consists of an image, a title and a duration (placed within a faded black box and fixed to the lower-right corner): To generate a thumbnail from a video with ffmpeg : Let's test the drawtext filter by extracting the thumbnail image from the beginning of the video and writing "Test Text" to the center of this image. This thumbnail image will be a JPEG file. Notice that the drawtext filter accepts the parameters text , fontcolor , fontsize , x and y for configuring it: The parameters are delimited by a colon. To see a full list of drawtext parameters, click here . Now that we've covered the basics, let's add a duration to this thumbnail: Unfortunately, there's no convenient variable like w or tw for accessing the input's duration. Therefore, we must extract the duration from the input's information, which is outputted by the -i option. 2>&1 redirects standard error ( 2 for stderr ) to standard output ( 1 for stdout ). We pipe the information outputted by the -i option directly to grep to search for the line containing the text "Duration" and pipe it to cut to extract the duration (i.e., 00:00:10 for ten seconds) from this line. This duration is stored within a variable DURATION so that it can be injected into the text passed to drawtext . Here, we use two drawtext filters to modify the input media: one for writing the title text "Test Text" and one for writing the duration "00:00:10". The filters are comma delimited. To place the duration within a box, provide the box parameter and set it to 1 to enable it. To set the background color of this box, provide the boxcolor parameter. Note : Alternatively, you could get the video's duration via the ffprobe command. Let's tidy up this thumbnail by substituting the placeholder title with the actual title, uppercasing this title, changing the font to "Open Sans" and moving the duration box to the bottom-right corner. Like the duration, the title must also be extracted from the input media's information. To uppercase every letter in the title, place the ^^ symbol of Bash 4 at the end of the title's variable via parameter expansion ( ${TITLE^^} ). Since Bash is required for the uppercasing, let's place these commands inside of a .sh file beginning with a Bash shebang , which determines how the script will be executed. To find the location of the Bash interpreter for the shebang, run the following command: ( thumbnail.sh ) To specify a font weight for a custom font, reference that font weight's file as the fontfile . Don't forget to replace <username> with your own username! Additionally, several changes were made to the thumbnail box. The box color has a subtle opacity of 0.625. This number (any number between 0 and 1) proceeds the @ in the boxcolor . A border width of 8px provides a bit of spacing between the edges of the box and the text itself. Note : If you run into a bash: Bad Substitution error, update Bash to version 4+ and verify the Bash shebang correctly points to the Bash executable. When you hover over a recommended video's thumbnail, a brief clip appears and plays to give you an idea of what the video's content is. With the ffmpeg command, generating a clip from a video is relatively easy. Just provide a starting timestamp via the -ss option (from the original video, -ss seeks until it reaches this timestamp, which will serve as the point the clip begins at) and an ending timestamp via the -to option (from the original video at which the clip should end). Because video previews on Youtube are three seconds long, let's extract a three second segment starting from the four second mark and ending at the seven second mark. Since the clip lasts for a few seconds, we must re-encode the video (exclude -c copy ) to accurately capture instances when no keyframes exist. To clip a video without re-encoding, ffmpeg must capture a sufficient number of keyframes from the video. Since MP4s are encoded with the H.264 video codec ( h264 (High) is stated under the video's metadata printed by ffmpeg -i <input> ), if we assume that there are 250 frames between any two keyframes ("a GOP size of 250"), then for the ten second Big Buck Bunny video with a frame rate of 30 fps, there is one keyframe each eight to nine seconds. Clipping a video less than nine seconds with -c copy results in no keyframes being captured, and thus, the outputted clip contains no video ( 0 kB of video). Eight Second Clip (with -c copy ): Nine Second Clip (with -c copy ): Note : Alternatively, the -t option can be used in place of the -to option. With the -t option, you must specify the duration rather than the ending timestamp. So instead of 00:00:07 with -to , it would be 00:00:03 with -t for a three second clip. Suppose you want to add your brand's logo, custom-made title graphics or watermark to the thumbnail. To overlay such an image on top of a thumbnail, pass this image as an input file via the i option and apply the overlay filter. Position the image on top of the thumbnail accordingly with the x and y parameters. ( thumbnail.sh ) Passing multiple inputs (in this case, a video and watermark image) requires the -filter_complex option in place of the -vf option. The main_h and overlay_h variables represent the main input's height (from the input video) and the overlay's height (from the input watermark image) respectively. Here, we place the watermark image in the lower-left corner of the thumbnail. The watermark image looks a bit large compared to the other elements on the thumbnail. Let's scale down the watermark image to half its original size by first scaling it down before any of the existing chained filters are executed. ( thumbnail.sh ) To scale the watermark image to half its size, we must explicitly tell the scale filter to only scale this image and not the video. This is done by prepending [1:v] to the scale filter to have the scale filter target our second input -i ./watermark-ex.png . The iw and ih variables will represent the watermark image's width and height respectively. Once the scaling is done, the scaled watermark image is outputted to ovrl , which can be referenced by other filters for consumption as a filter input. Because the overlay filter takes two inputs, an input video and an input image overlay, we prepend the overlay filter with these inputs: [0:v] for the first input -i ./Big_Buck_Bunny_360_10s_30MB.mp4 and ovrl for our scaled watermark image. Imagine having a large repository of videos that needs to be processed and uploaded during continuous integration. Write a Bash script to automate this process.