关于FFMPEG工程中configure脚本文件判断shell类型的疑问

cusheller

关于FFMPEG工程中configure脚本文件判断shell类型的疑问
谁能帮忙解释一下是如何判断shell是否支持的？(: ${foo%%bar})  这句该如何理解？ 谢谢了先。

unset foo
(: ${foo%%bar}) 2> /dev/null
E1="$?"

(: ${foo?}) 2> /dev/null
E2="$?"

if test "$E1" != 0 || test "$E2" = 0; then
    echo "Broken shell detected.  Trying alternatives."
    export FF_CONF_EXEC
    if test "0$FF_CONF_EXEC" -lt 1; then
        FF_CONF_EXEC=1
        try_exec bash "$0" "$@"
    fi
    if test "0$FF_CONF_EXEC" -lt 2; then
        FF_CONF_EXEC=2
        try_exec ksh "$0" "$@"
    fi
    if test "0$FF_CONF_EXEC" -lt 3; then
        FF_CONF_EXEC=3
        try_exec /usr/xpg4/bin/sh "$0" "$@"
    fi
    echo "No compatible shell script interpreter found."
    echo "This configure script requires a POSIX-compatible shell"
    echo "such as bash or ksh."
    echo "THIS IS NOT A BUG IN FFMPEG, DO NOT REPORT IT AS SUCH."
    echo "Instead, install a working POSIX-compatible shell."
    echo "Disabling this configure test will create a broken FFmpeg."
    if test "$BASH_VERSION" = '2.04.0(1)-release'; then
        echo "This bash version ($BASH_VERSION) is broken on your platform."
        echo "Upgrade to a later version if available."
    fi
    exit 1
fi　

waker

()是在子shell中运行命令
:是一个命令
${foo%bar}是变量替换

如果当前shell不支持${foo%bar}，说明它不是一个bsh风格的shell，脚本尝试用其它三种bsh风格的shell解析脚本

cusheller

解决，谢谢LS
在不支持 %% 用法的 shell 中, 应该会导致语法错误, 从而 $? 为非 0