Shell脚本的第一行空白:解释UID变量的行为 [英] Blank first line of shell script: explain behavior of UID variable

问题描述

我有两个非常简单的脚本，不同之处仅在于第一行空白:

I have two very simple scripts, differing only by the presence of a blank first line:

$ cat test.bash
#!/bin/bash
echo ${UID}
$ cat test_blank.bash

#!/bin/bash
echo ${UID}

现在，无论是否包含nice，我现在都可以运行:

Now I run then, with and without nice:

$ ./test.bash
1060

$ ./test_blank.bash
1060

$ nice ./test.bash
1060

$ nice ./test_blank.bash

请说明为什么在最后的情况下未设置UID变量的原因.将nice替换为sudo或nohup时，行为是相同的.

Please explain why, in the final case, the UID variable is unset. The behavior is the same when replacing nice with sudo or nohup.

推荐答案

观察:

$ bash test_blank.bash
1060
$ dash test_blank.bash

bash产生输出，但dash(在类似debian的系统上是默认的sh)不会输出.这是因为bash设置了UID，而破折号没有设置. ( POSIX 不需要shell来设置UID.)因此，问题就变成了哪个shell执行脚本.

bash produces output but dash, which is the default sh on debian-like systems, does not. This is because bash sets UID but dash does not. (POSIX does not require a shell to set UID.) So, the question becomes which shell executes the script.

当bash看到./test.sh时，它(bash)运行脚本.当另一个命令(例如nice)接收脚本作为参数并且该脚本的第一行没有有效的shebang时，将运行默认的shell(可能为dash).

When bash sees ./test.sh, it (bash) runs the script. When another command, such as nice, receives the script as an argument and the script does not have a valid shebang as the first line, then the default shell, likely dash, is run.

如果要使用破折号的UID或任何其他不提供UID的外壳，请使用id命令:

If you want UID in dash, or any other shell that does not provide it, use the id command:

UID=$(id -u)

找出哪个外壳程序正在运行脚本

要查看哪个外壳程序正在运行脚本，请使用:

Finding out which shell is running a script

To see which shell is running a script, use:

$ cat test2.sh

#!/bin/bash
ps $$
echo UID=${UID}

在bash下:

$ ./test2.sh
  PID TTY      STAT   TIME COMMAND
 1652 pts/12   S+     0:00 bash -rcfile .bashrc
UID=1060

相比之下，如果使用nice调用它，则可以看到它在/bin/sh下运行，并且未分配UID变量:

If we invoke it using nice, by contrast, we can see that it is running under /bin/sh and the UID variable is not assigned:

$ nice test2.sh
  PID TTY      STAT   TIME COMMAND
 1659 pts/12   SN+    0:00 /bin/sh test2.sh
UID=

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