Comprehensive Guide to the Linux df Command

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1. Introduction

The df command (full English: display free disk space) is used to display or view the space usage of file systems (or disks), including total capacity, used space, available space, usage rate, and mount points.

2. Syntax

Syntax: df [options]… [file]…

Parameter Description:

Parameter	Parameter Description
-a, –all	Display all file systems, including virtual file systems (such as proc, sysfs)
-B, –block-size=SIZE	Specify the display unit (e.g., -BK=KB, -BM=MB, -BG=GB)
-h, –human-readable	Display in a human-readable format (automatically converts units: K, M, G, T based on 1024)
-H, –si	Similar to -h, but uses 1000 as the conversion unit (according to SI standards)
-i, –inodes	Display inode usage (instead of disk space)
-k	Display in 1KB units (default unit)
-m	Display in 1MB units (supported by some systems)
-l, –local	Only display local file systems (exclude network file systems such as NFS)
–no-sync	Do not call sync before obtaining information (default behavior)
–sync	Call sync before obtaining information (ensures data is up to date)
–total	Display total information
-T, –print-type	Display file system type
-t, –type=TYPE	Only display file systems of the specified type (e.g., ext4, xfs)
-P, –portability	Use a POSIX compatible output format (avoids line break issues)
-x, –exclude-type=TYPE	Exclude specified file system types
–output[=FIELD_LIST]	Customize output fields (e.g., source,fstype,size,pcent)
–help	Display this help information and exit
–version	Display version information and exit

3. Practical Examples

[root@app01 ~]# df
Filesystem                      1K-blocks    Used     Available Use% Mounted on
devtmpfs                     914424       0   914424    0% /dev
tmpfs                        931500       0   931500    0% /dev/shm
tmpfs                        931500   10432   921068    2% /run
tmpfs                        931500       0   931500    0% /sys/fs/cgroup
/dev/mapper/rootvg-lv_root 26193412 4573168 21620244   18% /
/dev/sda1                   1038336  189248   849088   19% /boot
/dev/mapper/rootvg-lv_app  31441920 3723864 27718056   12% /app
tmpfs                        186304      12   186292    1% /run/user/42
tmpfs                        186304       0   186304    0% /run/user/0

Description:

Filesystem: The name or identifier of the file system.

1K-blocks: The total capacity of the file system, in 1KB units, i.e., the total size of the file system.

Used: The capacity used in the file system, in 1KB units.

Available: The capacity available in the file system, in 1KB units.

Use%: The percentage of used capacity relative to the total capacity.

Mounted on: The directory or location where the file system is mounted.

1. The -h option displays disk space usage in a human-readable format.

[root@app01 ~]# df -h
Filesystem                    Size  Used Avail Use% Mounted on
devtmpfs                    893M     0  893M    0% /dev
tmpfs                       910M     0  910M    0% /dev/shm
tmpfs                       910M   11M  900M    2% /run
tmpfs                       910M     0  910M    0% /sys/fs/cgroup
/dev/mapper/rootvg-lv_root   25G  4.4G   21G   18% /
/dev/sda1                  1014M  185M  830M   19% /boot
/dev/mapper/rootvg-lv_app    30G  3.6G   27G   12% /app
tmpfs                       182M   12K  182M    1% /run/user/42
tmpfs                       182M     0  182M    0% /run/user/0

This command parameter displays in a human-readable format, making it easier to read, with capacity units automatically converted (K, M, G, T, based on 1024).

2. The df command can also display the file system information for a specific file system or directory.

[root@app01 ~]# df /app
Filesystem                     1K-blocks    Used     Available Use% Mounted on
/dev/mapper/rootvg-lv_app 31441920 3723864 27718056   12% /app
[root@app01 app]# df /app/jdk
Filesystem                     1K-blocks    Used     Available Use% Mounted on
/dev/mapper/rootvg-lv_app 31441920 3723864 27718056   12% /app

3. The -i option displays inode information instead of block usage.

[root@app01 app]# df -i
Filesystem                      Inodes   IUsed  IFree IUse% Mounted on
devtmpfs                     228606     398  228208       1% /dev
tmpfs                        232875       1  232874       1% /dev/shm
tmpfs                        232875     910  231965       1% /run
tmpfs                        232875      16  232859       1% /sys/fs/cgroup
/dev/mapper/rootvg-lv_root 13103104  160828 12942276       2% /
/dev/sda1                    524288     341  523947       1% /boot
/dev/mapper/rootvg-lv_app  15728640    1231 15727409       1% /app
tmpfs                        232875       9  232866       1% /run/user/42
tmpfs                        232875       1  232874       1% /run/user/0

Note:Sometimes when using the df -Th command to check the file system, you may find that there is sufficient available space, but files cannot be written. This may be due to the inode value being full.

Inodes contain metadata about files, specifically including the following:

* The number of bytes in the file

* The User ID of the file owner

* The Group ID of the file

* The read, write, and execute permissions of the file

* The timestamps, which include three types:

ctime indicates the last time the inode was modified,

mtime indicates the last time the file content was modified,

atime indicates the last time the file was accessed.

* The number of links, i.e., how many file names point to this inode

* The location of the file data blocks

4. Display all information

[root@app01 app]# df --total
Filesystem                      1K-blocks    Used     Available Use% Mounted on
devtmpfs                     914424       0   914424    0% /dev
tmpfs                        931500       0   931500    0% /dev/shm
tmpfs                        931500   10432   921068    2% /run
tmpfs                        931500       0   931500    0% /sys/fs/cgroup
/dev/mapper/rootvg-lv_root 26193412 4573168 21620244   18% /
/dev/sda1                   1038336  189248   849088   19% /boot
/dev/mapper/rootvg-lv_app  31441920 3723864 27718056   12% /app
tmpfs                        186304      12   186292    1% /run/user/42
tmpfs                        186304       0   186304    0% /run/user/0
total                      62755200 8496724 54258476   14% -

Using the –total parameter to view the file system, we see an additional line at the end of the output (total), which shows the sum of each column.

5. Check the file system type

[root@app01 app]# df -T
Filesystem                   Type        1K-blocks    Used     Available Use% Mounted on
devtmpfs                   devtmpfs   914424       0   914424    0% /dev
tmpfs                      tmpfs      931500       0   931500    0% /dev/shm
tmpfs                      tmpfs      931500   10432   921068    2% /run
tmpfs                      tmpfs      931500       0   931500    0% /sys/fs/cgroup
/dev/mapper/rootvg-lv_root xfs      26193412 4573168 21620244   18% /
/dev/sda1                  xfs       1038336  189248   849088   19% /boot
/dev/mapper/rootvg-lv_app  xfs      31441920 3723864 27718056   12% /app
tmpfs                      tmpfs      186304      12   186292    1% /run/user/42
tmpfs                      tmpfs      186304       0   186304    0% /run/user/0

Using the -T parameter, you can see the type of each file system, which is the data in the second column (Type).

Note: When using the df command, you can combine parameters to see different output results; this is not shown here, but you can try it yourself to see the different outputs.

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