added bash getopts notes

1 files changed, 36 insertions, 1 deletions

diff --git a/src/tools/bash.md b/src/tools/bash.md
index a4df7da..3f53045 100644
--- a/src/tools/bash.md
+++ b/src/tools/bash.md
@@ -98,6 +98,41 @@ command 2>&1 >file
 1. duplicate `fd 1` to `fd 2`, effectively redirecting `stderr` to `stdout`
 2. redirect `stdout` to `file`
 
+## Argument parsing with `getopts`
+
+The `getopts` builtin uses following global variables:
+- `OPTARG`, value of last option argument
+- `OPTIND`, index of the next argument to process (user must reset)
+- `OPTERR`, display errors if set to `1` 
+
+```bash
+getopts <optstring> <param> [<args>]
+```
+- `<optstring>` specifies the names of supported options, eg `f:c`
+  - `f:` means `-f` option with an argument
+  - `c` means `-c` option without an argument
+- `<param>` specifies a variable name which `getopts` fills with the last parsed option argument
+- `<args>` optionally specify argument string to parse, by default `getopts` parses `$@`
+
+### Example
+```bash
+#!/bin/bash
+function parse_args() {
+	while getopts "f:c" PARAM; do
+		case $PARAM in
+			f) echo "GOT -f $OPTARG";;
+			c) echo "GOT -c";;
+			*) echo "ERR: print usage"; exit 1;;
+		esac
+	done
+	# users responsibility to reset OPTIND
+	OPTIND=0
+}
+
+parse_args -f xxx -c
+parse_args -f yyy
+```
+
 ## Completion
 
 The `complete` builtin is used to interact with the completion system.
@@ -124,7 +159,7 @@ COMPREPLY       # array with possible completions
 The `compgen` builtin is used to generate possible matches by comparing `word`
 against words generated by `option`.
 ```bash
-compgen [option] [word]
+compgen <option> <word>
 
 # usefule options:
 # -W <list>    specify list of possible completions