Bash命令行参数解析方法

技术背景

在Bash脚本编程中，解析命令行参数是一项常见的需求。通过解析命令行参数，脚本可以根据用户的输入动态调整其行为。不同的参数格式和解析方法适用于不同的场景，掌握这些方法可以提高脚本的灵活性和可维护性。

实现步骤

1. 空格分隔参数解析

#!/bin/bash

POSITIONAL_ARGS=()

while [[ $# -gt 0 ]]; do
  case $1 in
    -e|--extension)
      EXTENSION="$2"
      shift # past argument
      shift # past value
      ;;
    -s|--searchpath)
      SEARCHPATH="$2"
      shift # past argument
      shift # past value
      ;;
    --default)
      DEFAULT=YES
      shift # past argument
      ;;
    -*|--*)
      echo "Unknown option $1"
      exit 1
      ;;
    *)
      POSITIONAL_ARGS+=("$1") # save positional arg
      shift # past argument
      ;;
  esac
done

set -- "${POSITIONAL_ARGS[@]}" # restore positional parameters

echo "FILE EXTENSION  = ${EXTENSION}"
echo "SEARCH PATH     = ${SEARCHPATH}"
echo "DEFAULT         = ${DEFAULT}"
echo "Number files in SEARCH PATH with EXTENSION:" $(ls -1 "${SEARCHPATH}"/*."${EXTENSION}" | wc -l)

if [[ -n $1 ]]; then
    echo "Last line of file specified as non-opt/last argument:"
    tail -1 "$1"
fi

2. 等号分隔参数解析

#!/bin/bash

for i in "$@"; do
  case $i in
    -e=*|--extension=*)
      EXTENSION="${i#*=}"
      shift # past argument=value
      ;;
    -s=*|--searchpath=*)
      SEARCHPATH="${i#*=}"
      shift # past argument=value
      ;;
    --default)
      DEFAULT=YES
      shift # past argument with no value
      ;;
    -*|--*)
      echo "Unknown option $i"
      exit 1
      ;;
    *)
      ;;
  esac
done

echo "FILE EXTENSION  = ${EXTENSION}"
echo "SEARCH PATH     = ${SEARCHPATH}"
echo "DEFAULT         = ${DEFAULT}"
echo "Number files in SEARCH PATH with EXTENSION:" $(ls -1 "${SEARCHPATH}"/*."${EXTENSION}" | wc -l)

if [[ -n $1 ]]; then
    echo "Last line of file specified as non-opt/last argument:"
    tail -1 $1
fi

3. 使用getopt[s]解析

#!/bin/bash
# More safety, by turning some bugs into errors.
set -o errexit -o pipefail -o noclobber -o nounset

# ignore errexit with `&& true`
getopt --test > /dev/null && true
if [[ $? -ne 4 ]]; then
    echo 'I’m sorry, `getopt --test` failed in this environment.'
    exit 1
fi

# option --output/-o requires 1 argument
LONGOPTS=debug,force,output:,verbose
OPTIONS=dfo:v

# -temporarily store output to be able to check for errors
# -activate quoting/enhanced mode (e.g. by writing out “--options”)
# -pass arguments only via   -- "$@"   to separate them correctly
# -if getopt fails, it complains itself to stderr
PARSED=$(getopt --options=$OPTIONS --longoptions=$LONGOPTS --name "$0" -- "$@") || exit 2
# read getopt’s output this way to handle the quoting right:
eval set -- "$PARSED"

d=n f=n v=n outFile=-
# now enjoy the options in order and nicely split until we see --
while true; do
    case "$1" in
        -d|--debug)
            d=y
            shift
            ;;
        -f|--force)
            f=y
            shift
            ;;
        -v|--verbose)
            v=y
            shift
            ;;
        -o|--output)
            outFile="$2"
            shift 2
            ;;
        --)
            shift
            break
            ;;
        *)
            echo "Programming error"
            exit 3
            ;;
    esac
done

# handle non-option arguments
if [[ $# -ne 1 ]]; then
    echo "$0: A single input file is required."
    exit 4
fi

echo "verbose: $v, force: $f, debug: $d, in: $1, out: $outFile"

4. 使用POSIX getopts解析

#!/bin/sh

# A POSIX variable
OPTIND=1         # Reset in case getopts has been used previously in the shell.

# Initialize our own variables:
output_file=""
verbose=0

while getopts "h?vf:" opt; do
  case "$opt" in
    h|\?)
      show_help
      exit 0
      ;;
    v)  verbose=1
      ;;
    f)  output_file=$OPTARG
      ;;
  esac
done

shift $((OPTIND-1))

[ "${1:-}" = "--" ] && shift

echo "verbose=$verbose, output_file='$output_file', Leftovers: $@"

核心代码

以下是几种常见解析方法的核心代码片段：

空格分隔参数解析核心代码

while [[ $# -gt 0 ]]; do
  case $1 in
    # 处理不同参数
    ...
  esac
  shift
done

等号分隔参数解析核心代码

for i in "$@"; do
  case $i in
    # 处理不同参数
    ...
  esac
done

getopt[s]解析核心代码

PARSED=$(getopt --options=$OPTIONS --longoptions=$LONGOPTS --name "$0" -- "$@") || exit 2
eval set -- "$PARSED"

while true; do
  case "$1" in
    # 处理不同参数
    ...
  esac
done

getopts解析核心代码

while getopts "h?vf:" opt; do
  case "$opt" in
    # 处理不同参数
    ...
  esac
done
shift $((OPTIND-1))

最佳实践

选择合适的解析方法：根据参数的格式和需求选择合适的解析方法。如果需要支持长选项和更复杂的参数格式，getopt或getopts是不错的选择；如果只是简单的参数解析，手动处理也是可行的。
错误处理：在解析过程中，要对未知参数和缺少参数的情况进行处理，避免脚本因参数错误而崩溃。
代码可读性：使用case语句和清晰的注释可以提高代码的可读性和可维护性。