How to Check for a Read-Only File System in Linux

Hi,

A Linux filesystem can sometimes become mounted in read-only (ro) mode due to disk errors, filesystem corruption, storage issues, or administrative configuration changes. When a filesystem is mounted as read-only, users cannot create, modify, or delete files within that mount point.

Common Symptoms of a Read-Only File System

You may notice the following behaviors:

• Unable to create new files using the touch command.

• Unable to edit existing files.

• File deletion operations fail.

• Applications report "Read-only file system" errors.

• Log files stop updating.

Example Error

touch testfile
touch: cannot touch 'testfile': Read-only file system

Method 1: Check Mounted File Systems Using the mount Command

The simplest way to identify a read-only filesystem is to inspect the mount options.

mount

Example Output

/dev/sda1 on /data type ext4 (ro,relatime)

If the mount options contain ro, the filesystem is mounted as read-only.

You can filter the output as follows:

mount | grep "(ro,"

This displays only filesystems currently mounted in read-only mode.

Method 2: Check Using the findmnt Command

findmnt -o TARGET,SOURCE,FSTYPE,OPTIONS

Example:

findmnt | grep " ro,"

This provides a cleaner view of mount options and filesystem status.

Method 3: Test File Creation

Attempt to create a temporary file in the target filesystem.

cd /mountpoint
touch testfile

If the filesystem is read-only, Linux returns:

touch: cannot touch 'testfile': Read-only file system

Automated Check Using a Shell Script

The following script attempts to create a temporary file on each mounted filesystem and reports whether the operation succeeds.

#!/bin/ksh

df -P | awk 'NR>1 {print $6}' > /tmp/mountpoints.lst

for mp in $(cat /tmp/mountpoints.lst)
do
    echo "Checking: $mp"

    touch "$mp/test_rw_$$" 2>/dev/null

    if [ $? -eq 0 ]
    then
        echo "Writable"
        rm -f "$mp/test_rw_$$"
    else
        echo "Read-Only or Permission Denied"
    fi

    echo "--------------------------------"
done

Sample Output

Checking: /
Writable
--------------------------------
Checking: /data
Read-Only or Permission Denied
--------------------------------

Method 4: Check Kernel Messages

Linux often logs filesystem-related issues in system logs.

dmesg | grep -i "read-only"

or

dmesg | grep -i ext4

Example output:

EXT4-fs error detected
Remounting filesystem read-only

This indicates that Linux automatically remounted the filesystem in read-only mode after detecting corruption or I/O errors.

Common Causes of Read-Only Filesystems

• Filesystem corruption

• Disk I/O errors

• Storage device failures

• Unexpected server shutdowns

• Incorrect mount configuration in /etc/fstab

• SAN or network storage issues

Remount a Filesystem as Read-Write

If the issue has been resolved and the filesystem supports it, remount it as read-write:

mount -o remount,rw /mountpoint

Example:

mount -o remount,rw /data

Note: If the filesystem was remounted as read-only because of corruption, run the appropriate filesystem repair utility (such as fsck) before switching it back to read-write mode.

Conclusion

Read-only filesystems are often a warning sign of underlying storage or filesystem issues. Commands such as mount, findmnt, and touch can quickly help identify affected mount points. For enterprise environments, automated scripts can be used to regularly verify filesystem accessibility and detect problems before they impact applications.