Added introduction.

manuscript/chapter4.txt

@@ -1,50 +1,188 @@
-# File Paths
+# File handling
 
-## Get the directory name of a file path
+**CAVEAT:** `bash` doesn't handle binary data properly in versions `< 4.4`.
 
-Alternative to the `dirname` command.
+## Read a file to a string
+
+Alternative to the `cat` command.
+
+```shell
+file_data="$(<"file")"
+```
+
+## Read a file to an array (*by line*)
+
+Alternative to the `cat` command.
+
+```shell
+# Bash <4
+IFS=$'\n' read -d "" -ra file_data < "file"
+
+# Bash 4+
+mapfile -t file_data < "file"
+```
+
+## Get the first N lines of a file
+
+Alternative to the `head` command.
+
+**CAVEAT:** Requires `bash` 4+
 
 **Example Function:**
 
 ```sh
-dirname() {
-    # Usage: dirname "path"
-    printf '%s\n' "${1%/*}/"
+head() {
+    # Usage: head "n" "file"
+    mapfile -tn "$1" line < "$2"
+    printf '%s\n' "${line[@]}"
 }
 ```
 
 **Example Usage:**
 
 ```shell
-$ dirname ~/Pictures/Wallpapers/1.jpg
-/home/black/Pictures/Wallpapers/
+$ head 2 ~/.bashrc
+# Prompt
+PS1='➜ '
 
-$ dirname ~/Pictures/Downloads/
-/home/black/Pictures/
+$ head 1 ~/.bashrc
+# Prompt
 ```
 
-## Get the base-name of a file path
+## Get the last N lines of a file
 
-Alternative to the `basename` command.
+Alternative to the `tail` command.
+
+**CAVEAT:** Requires `bash` 4+
 
 **Example Function:**
 
 ```sh
-basename() {
-    # Usage: basename "path"
-    : "${1%/}"
-    printf '%s\n' "${_##*/}"
+tail() {
+    # Usage: tail "n" "file"
+    mapfile -tn 0 line < "$2"
+    printf '%s\n' "${line[@]: -$1}"
 }
 ```
 
 **Example Usage:**
 
 ```shell
-$ basename ~/Pictures/Wallpapers/1.jpg
-1.jpg
+$ tail 2 ~/.bashrc
+# Enable tmux.
+# [[ -z "$TMUX"  ]] && exec tmux
 
-$ basename ~/Pictures/Downloads/
-Downloads
+$ tail 1 ~/.bashrc
+# [[ -z "$TMUX"  ]] && exec tmux
+```
+
+## Get the number of lines in a file
+
+Alternative to `wc -l`.
+
+**Example Function (bash 4):**
+
+```sh
+lines() {
+    # Usage: lines "file"
+    mapfile -tn 0 lines < "$1"
+    printf '%s\n' "${#lines[@]}"
+}
+```
+
+**Example Function (bash 3):**
+
+This method uses less memory than the `mapfile` method and it's more
+compatible but it's slower for bigger files.
+
+```sh
+lines_loop() {
+    # Usage: lines_loop "file"
+    count=0
+    while IFS= read -r _; do
+        ((count++))
+    done < "$1"
+    printf '%s\n' "$count"
+}
+```
+
+**Example Usage:**
+
+```shell
+$ lines ~/.bashrc
+48
+
+$ lines_loop ~/.bashrc
+48
+```
+
+## Count files or directories in directory
+
+This works by passing the output of the glob as function arguments. We
+then count the arguments and print the number.
+
+**Example Function:**
+
+```sh
+count() {
+    # Usage: count /path/to/dir/*
+    #        count /path/to/dir/*/
+    printf '%s\n' "$#"
+}
+```
+
+**Example Usage:**
+
+```shell
+# Count all files in dir.
+$ count ~/Downloads/*
+232
+
+# Count all dirs in dir.
+$ count ~/Downloads/*/
+45
+
+# Count all jpg files in dir.
+$ count ~/Pictures/*.jpg
+64
+```
+
+## Create an empty file
+
+Alternative to `touch`.
+
+```shell
+# Shortest.
+:> file
+
+# Longer alternatives:
+echo -n > file
+printf '' > file
+```
+
+## Extract lines between two markers
+
+**Example Function:**
+
+```sh
+extract() {
+    # Usage: extract file "opening marker" "closing marker"
+    while IFS=$'\n' read -r line; do
+        [[ "$extract" && "$line" != "$3" ]] && \
+            printf '%s\n' "$line"
+
+        [[ "$line" == "$2" ]] && extract=1
+        [[ "$line" == "$3" ]] && extract=
+    done < "$1"
+}
+```
+
+**Example Usage:**
+
+```shell
+# Extract code blocks from MarkDown file.
+$ extract ~/projects/pure-bash/README.md '```sh' '```'
+# Output here...
 ```
 
 <!-- CHAPTER END -->