Concept: Looping (#713)

concepts/looping/.meta/config.json

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+{
+  "authors": [
+    "glennj"
+  ],
+  "contributors": [
+  ],
+  "blurb": "Looping constructs."
+}

concepts/looping/about.md

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+# TODO

concepts/looping/introduction.md

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+# Looping
+
+Bash has two forms of looping:
+
+* Repeat some commands _for each item in a list of items_,
+* Repeat some commands _while some condition is true (or false)_.
+
+## For Loops
+
+To iterate over a list of items, we use the `for` loop.
+
+```bash
+for varname in words; do COMMANDS; done
+```
+
+`words` is expanded using word splitting and filename expansion (that we learned about in the [Quoting concept][quoting]).
+Each word is assigned to the `varname` in turn, and the COMMANDS are executed.
+
+Here are some examples that show when the for loop can be useful
+
+* Perform some commands on a group of files.
+
+    ```bash
+    # source all the bash library files in the current directory
+    for file in ./*.bash; do
+        source "$file"
+    done
+    ```
+
+* Perform some command on each of the parameters to a script or function.
+
+    ```bash
+    for arg in "$@"; do
+        echo "argument: ${arg}"
+    done
+    ```
+
+* Perform some command for each whitespace-separated word in the output of a command.
+
+    ```bash
+    for i in $(seq 10); do
+        echo "${i} squared is $((i * i))"
+    done
+    ```
+
+    We'll see some warnings about this style later on.
+
+### Arithmetic For Loop
+
+Bash does have an arithmetic loop.
+This will look somewhat familiar to other programming languages.
+
+```bash
+for ((INITIALIZATION; CONDITION; INCREMENT)); do COMMANDS; done
+```
+
+The double-parentheses in bash represents an arithmetic context (we'll see more about bash arithmetic in a later concept).
+The above example using `seq` can be written like this
+
+```bash
+for ((i = 1; i <= 10; i++)); do
+    echo "${i} squared is $((i * i))"
+done
+```
+
+### When Not To Use For
+
+[Don't read lines with `for`][bashfaq1].
+
+This is an anti-pattern you'll often see: iterating over the output of `cat`.
+
+```bash
+for line in $(cat some.file); do 
+    do_something_with "$line"
+done
+```
+
+This is wrong on 2 counts.
+
+1. The command substitution is unquoted, so it is subject to word splitting.
+   Word splitting, by default, splits on _whitespace_ not just newlines.
+   This for loop is iterating over the **words** in the file, not the lines.
+2. The command substitution is unquoted, so it is subject to filename expansion.
+   Every word in the file will be matched as a glob pattern.
+
+As we learned in the [Quoting concept][quoting], word splitting can be controlled with the `IFS` variable, and filename expansion can be turned off.
+But this tends to be a fragile solution.
+`for` is the wrong method to iterate over the lines of a file.
+The idiomatic way to read a file is with a `while` loop.
+
+## While Loops
+
+Use `while` to repeat a sequence of commands _while_ some condition is true.
+
+```bash
+while CONDITION_COMMANDS; do COMMANDS; done
+```
+
+As with `if` (as we learned in the [Conditionals concept][conditionals], there is no special syntax for CONDITION_COMMANDS.
+The exit status of the command list will determine "true" or "false".
+
+## Controlling Loops
+
+The `break` command jumps out of the loop.
+Control resumes with the command following the loop's `done` terminator.
+
+The `continue` command jumps to the next iteration of the loop.
+
+## Reading the Lines of a File
+
+As mentioned, a while loop is the idiomatic way to read a file. 
+
+```bash
+while IFS= read -r line; do
+    do_something_with "$line"
+done < some.file
+```
+
+* The content of the file is provided as input to the loop with the `<` redirection.
+* The `read` command returns "true" if it can read a full line from its input.
+  When the input is consumed, or if the last line ends without a trailing newline, then read returns "false".
+  * The _truly_ idiomatic way to read the lines of a file, even if the last line does not end with a newline, is
+
+    ```bash
+    while IFS= read -r line || [[ -n "$line" ]]; do ...
+    ```
+
+* The `-r` option tells `read` to not substitute backslash sequences: a backslash is just a plain character.
+* `IFS=` assigns the empty string to IFS only for the duration of the read command.
+  This temporarily turns off word splitting so that any leading or trailing whitespace in the incoming line of text is preserved.
+
+## Until
+
+The `until` construct repeats a sequence of commands while some condition is **false**.
+The loop repeats _until_ the condition becomes true.
+
+```bash
+until CONDITION_COMMANDS; do COMMANDS; done
+```
+
+It is rare to use `until`.
+It is more common to use a "while not" loop.
+
+```bash
+while ! CONDITION_COMANDS; do ...
+```
+
+## Infinite Loops
+
+Sometimes you want to loop forever.
+Here are two ways to do it.
+
+1. a while loop with a condition that always has a success exit status
+
+    ```bash
+    while true; do ...
+    ```
+
+2.  an arithmetic for loop with empty expressions
+
+    ```bash
+    for ((;;)); do ...
+    ```
+
+## Do-While
+
+There is no explicit do-while construct, but you can achieve the same effect:
+
+```bash
+while true; do
+    COMMANDS
+
+    if END_CONDITION; then
+        break
+    fi
+done
+```
+
+[bashfaq1]: https://mywiki.wooledge.org/DontReadLinesWithFor
+[quoting]: https://exercism.org/tracks/bash/concepts/quoting
+[conditionals]: https://exercism.org/tracks/bash/concepts/conditionals

concepts/looping/links.json

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+[
+  {
+    "description": "\"Loops\" in the Bash Guide",
+    "url": "https://mywiki.wooledge.org/BashGuide/TestsAndConditionals#Conditional_Loops_.28while.2C_until_and_for.29"
+  },
+  {
+    "description": "\"Looping constructs\" in the bash manual",
+    "url": "https://www.gnu.org/software/bash/manual/bash.html#Looping-Constructs"
+  }
+]

config.json

@@ -1238,6 +1238,11 @@
       "uuid": "fcd13bb3-3557-4f3a-82d8-5ba588a51cf4",
       "slug": "conditionals",
       "name": "Conditionals"
+    },
+    {
+      "uuid": "ae9f3e82-bcdd-4c09-9788-bc543235fd52",
+      "slug": "looping",
+      "name": "Looping"
     }
   ],
   "key_features": [