理解shell的eval命令

看以下两条命令： [zhangsan@XEN /sys]$ a="ls";b="\$a";c="$b";"$c" Hey! No command '$a' found, did you mean 'ia'? [zhangsan@XEN /sys]$ a="ls";b="\$a";c="$b";eval "$c" block  bus  class  dev  devices  firmware  fs  kernel  module  power eval命令的作用是再次扫描解释，可多次叠加，如： [zhangsan@XEN /sys]$ a="ls";b="\$a";c="\$b";eval "$c"      Hey! No command '$a' found, did you mean 'ia'? [zhangsan@XEN /sys]$ a="ls";b="\$a";c="\$b";eval eval "$c" block  bus  class  dev  devices  firmware  fs  kernel  module  power 附: 关闭文件描述符函数，并导出该函数给子进程使用 close_all_fd() {     # 0, 1, 2, 255     # compgen -G "/proc/$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。

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