bash – 用空格缩进heredocs

对于shell脚本,使用选项卡不是偏好或风格的问题;这是语言的定义.

usage () {
    # Lines between EOF are each indented with the same number of tabs
    # Spaces can follow the tabs for in-document indentation
    cat <<-EOF
        Hello,this is a cool program.
        This should get unindented.
        This code should stay indented:
            something() {
                echo It works,yo!;
            }
        That's all.
    EOF
}

另一个选择是避免使用这些文档,而不必使用更多的引号和行延续：

usage () {
    printf '%sn' 
        "Hello,this is a cool program." 
        "This should get unindented." 
        "This code should stay indented:" 
        "    something() {" 
        "        echo It works,yo!" 
        "    }" 
        "That's all."
}

如果您愿意放弃POSIX兼容性,可以使用数组来避免显式的行延续：

usage () {
    message=(
        "Hello,this is a cool program."
        "This should get unindented."
        "This code should stay indented:"
        "    something() {"
        "        echo It works,yo!"
        "    }"
        "That's all."
    )
    printf '%sn' "${message[@]}"
}

以下再次使用这里的文档,但这次用bash 4的readarray命令来填充数组.参数扩展从每个谎言的开始处理去除固定数量的空格.

usage () {
    # No tabs necessary!
    readarray message <<'    EOF'
        Hello,yo!;
            }
        That's all.
    EOF
    # Each line is indented an extra 8 spaces,so strip them
    printf '%s' "${message[@]#        }"
}

最后一个变体：您可以使用扩展模式来简化参数扩展.而不必计算用于缩进的空格数量,只需用选定的非空格字符结束缩进,然后匹配固定的前缀.我用 ：. (下面的空格
冒号是为了可读性;它可以删除与前缀模式稍微改变.)

(另外,除此之外,您使用以白色开头的文档定界符的非常好的方法的一个缺点是它阻止您在文档中执行扩展,如果要这样做,您可以要么将分隔符保留为未绑定,或者对您的无标签规则造成一个小小的例外,请使用< -EOF和制表符缩进的关闭分隔符.)

usage () {
    # No tabs necessary!
    closing="That's all"
    readarray message <<EOF
       : Hello,this is a cool program.
       : This should get unindented.
       : This code should stay indented:
       :      something() {
       :          echo It works,yo!;
       :      }
       : $closing
EOF
    shopt -s extglob
    printf '%s' "${message[@]#+( ): }"
    shopt -u extglob
}