How to Check Available Disk Space on Linux

When a program appends data to a file, you need to know how much space remains on the partition containing that file. This determines whether the file can grow or if you’re about to hit a capacity limit.

Quick answer

To find available space for a file, use df:

df -B1 /mnt/logs/app-log.log | tail -1 | awk '{print $4}'

This outputs available space in bytes. Replace /mnt/logs/app-log.log with your target file.

Understanding the output

The df command without the file argument shows all mounted filesystems:

$ df -B1 /mnt/logs/app-log.log
Filesystem     1B-blocks        Used   Available Use% Mounted on
/dev/sda1    1099511627776 549755813888 549755813888  50% /mnt

Breaking down the output:

Filesystem: The actual device or mount
1B-blocks: Total capacity in bytes
Used: Bytes consumed
Available: Bytes remaining (what we care about)
Use%: Percentage of capacity used
Mounted on: The mount point

The tail -1 grabs the last line (skipping headers), and awk '{print $4}' extracts the available space column.

More practical approaches

Human-readable format:

df -h /mnt/logs/app-log.log | tail -1 | awk '{print "Available:", $4}'

Output: Available: 256G

Check if space is sufficient before writing:

required_bytes=10737418240  # 10GB in bytes
available=$(df -B1 /mnt/logs/app-log.log | tail -1 | awk '{print $4}')

if [ "$available" -lt "$required_bytes" ]; then
    echo "Insufficient space: need $required_bytes bytes, have $available"
    exit 1
fi

Get both used and available space:

df -B1 /mnt/logs/app-log.log | tail -1 | awk '{printf "Used: %d bytes\nAvailable: %d bytes\n", $3, $4}'

Using stat for file-specific info

While df shows partition space, stat displays the actual file’s properties:

stat /mnt/logs/app-log.log | grep -E "Access|Modify|Change"

This shows file modification times, useful for understanding when files were last written.

Monitoring disk usage over time

For files that grow continuously, use watch to monitor available space:

watch -n 5 'df -h /mnt/logs/app-log.log | tail -1'

Updates every 5 seconds. Press q to exit.

Common gotchas

Reserved space: Linux reserves 5% of filesystem capacity (typically) for the root user. Non-root users see less available space than the actual free space:

df /mnt/logs/app-log.log  # Shows available to regular user
sudo df /mnt/logs/app-log.log  # Shows reserved space included

Inode limits: Disk space isn’t the only constraint. Systems can run out of inodes before bytes:

df -i /mnt/logs/app-log.log

If “IUse%” approaches 100%, you’ll fail to create files despite available bytes.

Mounted filesystems: The file’s partition is determined by its mount point. Symbolic links report the target file’s partition:

df /path/to/symlink  # Reports space for the target, not the symlink location