11.2.1 Determine the File Type: file

The command file determines the type of a file or a list of files by checking /etc/magic.

tux@mercury:~> file /usr/bin/file
/usr/bin/file: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), \
    for GNU/Linux 2.6.4, dynamically linked (uses shared libs), stripped

The parameter -f list specifies a file with a list of filenames to examine. The -z allows file to look inside compressed files:

tux@mercury:~> file /usr/share/man/man1/file.1.gz
usr/share/man/man1/file.1.gz: gzip compressed data, from Unix, max compression
tux@mercury:~> file -z /usr/share/man/man1/file.1.gz
/usr/share/man/man1/file.1.gz: ASCII troff or preprocessor input text \
    (gzip compressed data, from Unix, max compression)

11.2.2 File Systems and Their Usage: mount, df, and du

The command mount shows which file system (device and type) is mounted at which mount point:

tux@mercury:~> mount
/dev/sda3 on / type reiserfs (rw,acl,user_xattr)
proc on /proc type proc (rw)
sysfs on /sys type sysfs (rw)
udev on /dev type tmpfs (rw)
devpts on /dev/pts type devpts (rw,mode=0620,gid=5)
/dev/sda1 on /boot type ext2 (rw,acl,user_xattr)
/dev/sda4 on /local type reiserfs (rw,acl,user_xattr)
/dev/fd0 on /media/floppy type subfs (rw,nosuid,nodev,noatime,fs=floppyfss,p

Obtain information about total usage of the file systems with the command df. The parameter -h (or --human-readable) transforms the output into a form understandable for common users.

tux@mercury:~> df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda3              11G  3.2G  6.9G  32% /
udev                  252M  104K  252M   1% /dev
/dev/sda1              16M  6.6M  7.8M  46% /boot
/dev/sda4              27G   34M   27G   1% /local

Display the total size of all the files in a given directory and its subdirectories with the command du. The parameter -s suppresses the output of detailed information. -h again transforms the data into a human-readable form:

tux@mercury:~> du -sh /local
1.7M    /local

11.2.3 Additional Information about ELF Binaries

Read the content of binaries with the readelf utility. This even works with ELF files that were built for other hardware architectures:

tux@mercury:~> readelf --file-header /bin/ls
ELF Header:
  Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           Advanced Micro Devices X86-64
  Version:                           0x1
  Entry point address:               0x402430
  Start of program headers:          64 (bytes into file)
  Start of section headers:          98616 (bytes into file)
  Flags:                             0x0
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         9
  Size of section headers:           64 (bytes)
  Number of section headers:         31
  Section header string table index: 30

11.2.4 File Properties: stat

The command stat displays file properties:

tux@mercury:~> stat /etc/profile
  File: '/etc/profile'
  Size: 8080            Blocks: 16         IO Block: 4096   regular file
Device: 806h/2054d      Inode: 64942       Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2007-07-16 23:28:18.000000000 +0200
Modify: 2006-09-19 14:45:01.000000000 +0200
Change: 2006-12-05 14:54:55.000000000 +0100

The parameter --filesystem produces details of the properties of the file system in which the specified file is located:

tux@mercury:~> stat /etc/profile --filesystem
  File: "/etc/profile"
    ID: 0        Namelen: 255     Type: reiserfs
Block size: 4096       Fundamental block size: 4096
Blocks: Total: 2622526    Free: 1809771    Available: 1809771
Inodes: Total: 0          Free: 0