理解shell的eval命令

Posted 2021-01-28 一见

看以下两条命令：
[[email protected] /sys]$ a="ls";b="$a";c="$b";"$c"
Hey! No command ‘$a‘ found, did you mean ‘ia‘?

[[email protected] /sys]$ a="ls";b="$a";c="$b";eval "$c"
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eval命令的作用是再次扫描解释，可多次叠加，如：
[[email protected] /sys]$ a="ls";b="$a";c="$b";eval "$c"
Hey! No command ‘$a‘ found, did you mean ‘ia‘?
[[email protected] /sys]$ a="ls";b="$a";c="$b";eval eval "$c"
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附: 关闭文件描述符函数，并导出该函数给子进程使用
close_all_fd()
{
    # 0, 1, 2, 255
    # compgen -G "/proc/$BASHPID/fd/*
    for fd in $(ls /proc/$$/fd); do
        if test $fd -gt 2; then
            # 关闭文件描述符fd
            eval "exec $fd>&-"
        fi
    done
}
# 导出close_all_fd
export -f close_all_fd
实际应用，可参见：https://github.com/eyjian/libmooon/blob/master/shell/process_monitor.sh。