Add Find All Objects In An Array Where Key Is Set as a jq TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1289 TILs and counting..._
+_1291 TILs and counting..._
 
 ---
 
@@ -488,6 +488,7 @@ _1289 TILs and counting..._
 - [Count Each Collection In A JSON Object](jq/count-each-collection-in-a-json-object.md)
 - [Count The Number Of Things In A JSON File](jq/count-the-number-of-things-in-a-json-file.md)
 - [Extract A List Of Values](jq/extract-a-list-of-values.md)
+- [Find All Objects In An Array Where Key Is Set](jq/find-all-objects-in-an-array-where-key-is-set.md)
 - [Find All Objects With A Matching Key Value Pair](jq/find-all-objects-with-a-matching-key-value-pair.md)
 - [Reduce Object To Just Entries Of A Specific Type](jq/reduce-object-to-just-entries-of-a-specific-type.md)
 
@@ -502,6 +503,7 @@ _1289 TILs and counting..._
 - [Configure Your Server Timezone](linux/configure-your-server-timezone.md)
 - [List The Statuses Of All Upstart Jobs](linux/list-the-statuses-of-all-upstart-jobs.md)
 - [Show Current System Time And Settings](linux/show-current-system-time-and-settings.md)
+- [Show Used And Available System Memory](linux/show-used-and-available-system-memory.md)
 - [Upgrading Ubuntu](linux/upgrading-ubuntu.md)
 
 ### Mac

jq/find-all-objects-in-an-array-where-key-is-set.md

@@ -0,0 +1,50 @@
+# Find All Objects In An Array Where Key Is Set
+
+Let's say we have a large array of objects. And the data in those objects,
+while generally having the same shape, does not always have certain values set.
+
+For instance, here is some data where the `token` is sometimes set, sometimes
+`null`, and sometimes missing altogether.
+
+```json
+[
+  {
+    "id": 1,
+    "token": "abc"
+  },
+  {
+    "id": 3,
+    "token": null
+  },
+  {
+    "id": 5
+  },
+  ...
+]
+```
+
+We can find out how many objects in this collection have the `token` key set to
+an actual value with [a `jq` query](https://stedolan.github.io/jq/manual) like
+the following.
+
+```bash
+jq '. | map(select(has("token") and .token != null)) | length' data.json
+```
+
+This maps over each object selecting those where it has `token` and `token` is
+not `null`.
+
+We can instead produce the inverse count—those objects where `token` is not set
+to a value—with the `not` operator.
+
+```bash
+jq '. | map(select(has("token") and .token != null | not)) | length' data.json
+```
+
+If you want to inspect the array of objects that either of these queries filters down to, you can drop the `| length` part.
+
+```bash
+jq '. | map(select(has("token") and .token != null))' data.json
+```
+
+Here is [a live example](https://jqterm.com/?query=.%20%7C%20map%28select%28has%28%22token%22%29%20and%20.token%20!%3D%20null%20%7C%20not%29%29).

linux/show-used-and-available-system-memory.md

@@ -0,0 +1,24 @@
+# Show Used And Available System Memory
+
+The `free` command will display the total RAM on your system as well as how
+much of it is used and how much is still freely available.
+
+```bash
+$ free
+              total        used        free      shared  buff/cache   available
+Mem:        1000260      166004      552808       14440      281448      664724
+Swap:             0           0           0
+```
+
+The default output format of `free` is not very readable though. To make it
+more human readable, you can add the `-h` (_human_) and `--si` (_International
+System of Units_) flags.
+
+```bash
+$ free -h --si
+              total        used        free      shared  buff/cache   available
+Mem:           976M        162M        547M         14M        266M        649M
+Swap:            0B          0B          0B
+```
+
+See `man free` for more details.