利用 sed 完成 shell 中的字符串替换

shell 中经常会要用到字符串替换操作，这里的是用 sed 命令实现

# 仅替换一次
echo "hello, how are you?" | sed 's/?/!/';     # hello, how are you!

# 替换多次
echo "hello? how are you?" | sed 's/?/!/g';    # hello! how are you!

# 仅替换结尾的字符串，类似于 rtrim
echo "hello? how are you?" | sed 's/?$/!/';    # hello? how are you!

# 仅替换开头的字符串，类似于 ltrim
echo "?hello how are you?" | sed 's/^?/!/';    # !hello how are you?