In Chapters 2, 3, and 4, we covered a multitude of approaches to file I/O. In this chapter, we’ll revisit files, this time focusing not on reading from or writing to them, but rather on manipulating and managing them and their metadata.
As discussed in Chapter 1, each file is referenced by an inode, which is addressed by a filesystem-unique numerical value known as an inode number. An inode is both a physical object located on the disk of a Unix-style filesystem and a conceptual entity represented by a data structure in the Linux kernel. The inode stores the metadata associated with a file, such as the file’s access permissions, last access timestamp, owner, group, and size, as well as the location of the file’s data.
You can obtain the inode number for a file using the -i flag to the ls command:
$ ls -i 1689459 Kconfig 1689461 main.c 1680144 process.c 1689464 swsusp.c 1680137 Makefile 1680141 pm.c 1680145 smp.c 1680149 user.c 1680138 console.c 1689462 power.h 1689463 snapshot.c 1689460 disk.c 1680143 poweroff.c 1680147 swap.c
This output shows that, for example, disk.c has an inode number of 1689460. On this particular filesystem, no other file will have this inode number. On a different filesystem, however, we can make no such guarantees.
Unix provides a family of functions for obtaining the metadata of a file: