---

1. Files and Directories

API:

open(), read(), write(), dup(), lseek(), fsync(), close()

stat(), link(), symlink(), unlink(), rename(), chmod()

mkdir(), rmdir(), opendir(), readdir(), closedir()

---

2. strace

strace - trace system calls and signals

$ echo hi > j
$ strace -e trace=open,openat,read,close cat j
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
close(3)                                = 0
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\260\34\2\0\0\0\0\0"..., 832) = 832
close(3)                                = 0
openat(AT_FDCWD, "/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
close(3)                                = 0
openat(AT_FDCWD, "j", O_RDONLY)         = 3
read(3, "hi\n", 131072)                 = 3
hi
read(3, "", 131072)                     = 0
close(3)                                = 0
close(1)                                = 0
close(2)                                = 0
+++ exited with 0 +++
$

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3. Shared Files

---

4. open, read, offset examples

Assume file contains 300 bytes:

open more than once:

open and reposition:

---

5. Inode Metadata

inode = index node (information node)

inode number is index into file system inode table

stat() system call, stat command

$ stat j
  File: j
  Size: 3               Blocks: 8          IO Block: 4096   regular file
Device: 902h/2306d      Inode: 9981236     Links: 1
Access: (0644/-rw-r--r--)  Uid: ( 1000/   perry)   Gid: ( 1000/   perry)
Access: 2020-06-17 11:06:52.799937997 -0400
Modify: 2020-06-17 11:06:42.512146065 -0400
Change: 2020-06-17 11:06:42.512146065 -0400
 Birth: -
$

---

6. Reading Directories

The only standard fields in a directory entry structure are d_ino and d_name:

struct dirent
{
  ino_t  d_ino;       // Inode number
  char   d_name[256]; // Null-terminated filename
  //...
};

Example program from chapter 39 page 16:

#include <stdio.h>
#include <dirent.h>
#include <assert.h>

int main(int argc, char *argv[]) {
  DIR *dp = opendir(".");
  assert(dp != NULL);
  struct dirent *d;
  while ((d = readdir(dp)) != NULL) {
    printf("%lu %s\n", (unsigned long) d->d_ino, d->d_name);
  }
  closedir(dp);
  return 0;
}

---

7. Permission Bits

chmod() system call, chmod command

$ ls -ld . notes.html /bin/ls
drwxr-xr-x 2 perry perry   4096 Jun 17 14:20 .
-rwxr-xr-x 1 root  root  133792 Jan 18  2018 /bin/ls
-rw-r--r-- 1 perry perry   1118 Jun 17 14:18 notes.html
$

user, group, other; read, write, execute:

  -rwxr-xr-x
   uuugggooo
   111101101 = 0755

  -rw-r--r--
   uuugggooo
   110100100 = 0644

---

8. Mounted File Systems

$ df | egrep '/dev/md|File'
Filesystem     1K-blocks      Used Available Use% Mounted on
/dev/md0        49687632  13045168  34088764  28% /
/dev/md3       960249880 514337568 397064492  57% /a
/dev/md2       402626372  13579452 368524324   4% /home
$ mount | grep /dev/md
/dev/md0 on / type ext4 (rw,relatime,errors=remount-ro,data=ordered)
/dev/md3 on /a type ext4 (rw,relatime,data=ordered)
/dev/md2 on /home type ext4 (rw,relatime,data=ordered)
$ grep md2 /proc/mdstat
md2 : active raid1 sda3[0] sdb3[1]
$ sudo fdisk -l | grep '^Disk /dev/sd'
Disk /dev/sdb: 447.1 GiB, 480103981056 bytes, 937703088 sectors
Disk /dev/sda: 447.1 GiB, 480103981056 bytes, 937703088 sectors
Disk /dev/sdc: 931.5 GiB, 1000204886016 bytes, 1953525168 sectors
Disk /dev/sdd: 931.5 GiB, 1000204886016 bytes, 1953525168 sectors
$ df
Filesystem     1K-blocks      Used Available Use% Mounted on
udev             8060648         0   8060648   0% /dev
tmpfs            1618832      2088   1616744   1% /run
/dev/md0        49687632  13045168  34088764  28% /
tmpfs            8094148     40092   8054056   1% /dev/shm
tmpfs               5120         4      5116   1% /run/lock
tmpfs            8094148         0   8094148   0% /sys/fs/cgroup
/dev/loop0         99456     99456         0 100% /snap/core/9289
/dev/loop1         96256     96256         0 100% /snap/core/9066
/dev/loop2         18304     18304         0 100% /snap/pdftk/9
/dev/md3       960249880 514337568 397064492  57% /a
/dev/md2       402626372  13579452 368524324   4% /home
tmpfs            1618828        16   1618812   1% /run/user/125
tmpfs            1618828        96   1618732   1% /run/user/1000
tmpfs            1618828         0   1618828   0% /run/user/0
$