使用tee同时输出多个命令到同一文件是否安全？

如果您在append（-a）模式下使用tee ，则没有数据丢失的风险，但没有附加操作的文件系统（例如NFS）除外。在append模式下，tee打开带有O_APPEND标志的文件。

$ echo 1 | strace tee -a /tmp/file 2>&1 | grep open | grep /tmp/file
openat(AT_FDCWD, "/tmp/file", O_WRONLY|O_CREAT|O_APPEND, 0666) = 3

来自man 2 openat：

O_APPEND
     The file is opened in append mode.  Before each write(2), the
     file offset is positioned at the end of the file, as if with
     lseek(2).  The modification of the file offset and the write
     operation are performed as a single atomic step.

其中的关键句是文件偏移量的修改和写操作是作为单个原子步骤执行的。保证write()从哪个实例进行的每次调用tee都将原子偏移量自动定位到文件末尾，然后写入数据。

该openat手册页呢注意，使用的这种模式是不安全的NFS文件系统中（正如我前面提到的）：

     O_APPEND may lead to corrupted files on NFS filesystems if more
     than one process appends data to a file at once.  This is be‐
     cause  NFS does not support appending to a file, so the client
     kernel has to simulate it, which can't be done without a race
     condition.