The /etc/mtab file is a critical system file in Linux that maintains a real-time record of all currently mounted filesystems. Unlike its counterpart /etc/fstab which contains static mount configurations, /etc/mtab is dynamically updated by the mount command.

# Sample /etc/mtab entry structure
device_path mount_point filesystem_type options dump_freq pass_num

Each line in /etc/mtab contains exactly six fields, separated by whitespace:

1. Device/Source

This column specifies the physical device or virtual filesystem being mounted. Examples include:

• Physical devices: /dev/sda1, /dev/mapper/vg-root
• Virtual filesystems: proc, sysfs, tmpfs
• Network shares: server:/export

2. Mount Point

The directory where the filesystem is attached to the directory hierarchy:

/              # Root filesystem
/boot          # Boot partition
/proc          # Process information

3. Filesystem Type

The format/structure of the filesystem:

• ext4, xfs, btrfs - Common disk filesystems
• proc, sysfs, devpts - Special purpose filesystems
• nfs, cifs - Network filesystems

4. Mount Options

Comma-separated list of mount flags controlling behavior:

rw           # Read-write
ro           # Read-only
noexec       # Disable execution
nosuid       # Disable SUID bits
defaults     # Standard options (rw,suid,dev,exec,auto,nouser,async)

5. Dump Frequency

Historically used by the dump backup utility (0 means ignore):

0 # Most modern systems use 0
1 # Filesystems needing backup

6. Filesystem Check Order

Controls fsck order during boot:

• 0 - Don't check
• 1 - Check first (root filesystem)
• 2+ - Check in numerical order

Let's examine some real-world entries with explanations:

# Physical disk partition
/dev/nvme0n1p2 / ext4 rw,relatime 0 1
# ^ Device     ^ Mount ^ FS  ^ Options      ^ Dump ^ Check

# Temporary memory-based filesystem
tmpfs /run tmpfs rw,nosuid,nodev,size=10%,mode=755 0 0
# ^ Virtual ^ Mount ^ Type ^ Special options for security/size

# Network filesystem
nas:/mnt/backup /mnt/backup nfs rw,hard,intr 0 0
# ^ Server ^ Local ^ Protocol ^ Network options

For developers needing to parse /etc/mtab programmatically:

# Python example
def parse_mtab():
    mounts = []
    with open('/etc/mtab') as f:
        for line in f:
            if line.startswith('#'):
                continue
            parts = line.strip().split()
            if len(parts) >= 6:
                mounts.append({
                    'device': parts[0],
                    'mountpoint': parts[1],
                    'fstype': parts[2],
                    'options': parts[3].split(','),
                    'dump': int(parts[4]),
                    'pass': int(parts[5])
                })
    return mounts

While /etc/mtab serves a similar purpose to /proc/mounts, key differences exist:

• /proc/mounts shows actual kernel view of mounts
• /etc/mtab may include additional user-space options
• /proc/mounts is always up-to-date, while /etc/mtab requires write access

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The /etc/mtab file is a dynamic system file that maintains a real-time record of currently mounted filesystems on a Linux system. Each line represents a mounted filesystem, formatted with six distinct fields separated by whitespace:

device mount_point filesystem_type options dump_freq pass_num

Let's analyze each column using your provided example:

/dev/mapper/VolGroup00-LogVol00 / ext3 rw 0 0

1. Device/Source: The block device or virtual filesystem source (/dev/mapper/VolGroup00-LogVol00)
2. Mount Point: Where the filesystem is mounted in the directory tree (/)
3. Filesystem Type: The format of the filesystem (ext3)
4. Mount Options: Comma-separated mount flags (rw for read-write)
5. Dump Frequency: Obsolete backup flag (typically 0)
6. Pass Number: fsck check order (0 means no checking)

Here's how to programmatically parse mtab in Python:

import re

def parse_mtab():
    with open('/etc/mtab', 'r') as f:
        for line in f:
            if line.startswith('#'):
                continue
            fields = re.split(r'\s+', line.strip())
            if len(fields) == 6:
                yield {
                    'device': fields[0],
                    'mount_point': fields[1],
                    'fs_type': fields[2],
                    'options': fields[3].split(','),
                    'dump': int(fields[4]),
                    'pass_num': int(fields[5])
                }

for mount in parse_mtab():
    print(f"{mount['device']} mounted at {mount['mount_point']}")

Note that on newer systems:

• /etc/mtab may be symlinked to /proc/mounts
• The format remains identical to maintain compatibility
• Some pseudo-filesystems (like proc) show special device names

Common issues and solutions:

# Check for mount conflicts
grep "/mnt/point" /etc/mtab

# Verify mount options
awk '$2 == "/mount/point" {print $4}' /etc/mtab

# Find all read-only mounts
grep "\bro\b" /etc/mtab