Mastering the `truncate` Command (with examples)

The truncate command is a useful utility in Unix-like operating systems that allows users to adjust the size of a file to a specified amount. By either expanding or shrinking the file, truncate can create space, remove unnecessary data, or even initialize files of a certain size without editing their contents manually. It is especially helpful in cases where the precise control of file size is required, whether for testing, data management, or troubleshooting purposes.

Set a size of 10 GB to an existing file, or create a new file with the specified size

Code:

truncate --size 10G path/to/file

Motivation:

This use case is particularly relevant when a user needs to allocate or reserve disk space for a virtual machine, a database, or any application that necessitates a specific amount of storage capacity. It could also be desired for testing and development purposes, where a file of a certain size is necessary to replicate specific conditions or behaviors without requiring it to contain actual associated data.

Explanation:

• truncate: The command used to alter the file size.
• --size 10G: Specifies the new desired size of the file. The size is set to 10 gigabytes (GB), and the G suffix signifies the unit is gigabytes. This argument will either change the size of an existing file to 10 GB or create a new file of that size.
• path/to/file: Specifies the file path to the target file. If the file does not exist, truncate will create a new one with the specified size.

Example output:

After executing the command, if the file did not previously exist, a new 10 GB file will appear in the specified location. If it did exist, its size will now reflect 10 GB. This can be confirmed using file management utilities or a command like ls -lh.

Extend the file size by 50 MiB, filling with holes (which reads as zero bytes)

Code:

truncate --size +50M path/to/file

Motivation:

This scenario is vital when incrementally increasing the storage size of a file without needing to append actual data. By extending a file’s size in this manner, gaps (or “holes”) are introduced—a feature especially useful in sparse files, frequently used in database systems or specific engineering applications, optimizing space usage by not physically recording zero-byte data.

Explanation:

• truncate: This command is being used to modify the current size of a file.
• --size +50M: Instructs truncate to increase the file size by 50 megabytes (MB), with + indicating a size increment. The M suffix signifies that the unit is megabytes.
• path/to/file: Indicates the file whose size is to be extended by the specified amount.

Example output:

Upon completion, the file has increased by 50 MB in apparent size, which can be observed using commands like ls -lh. Despite the increase, these additional bytes are “holes” and, though they appear to be present, they do not occupy physical disk space until written with actual data.

Shrink the file by 2 GiB, by removing data from the end of the file

Code:

truncate --size -2G path/to/file

Motivation:

This operation is especially useful for file cleanup or when needing to enforce data retention policies by trimming unnecessary or outdated data from the end of large log files, databases, or binary blobs. It can be crucial in efficiently managing disk space and ensuring files do not exceed certain size limits in production environments.

Explanation:

• truncate: The command used to modify the file size.
• --size -2G: Instructs truncate to reduce the file size by 2 gibibytes (GiB), where - indicates a size decrement. The G suffix denotes that the size unit is gibibytes.
• path/to/file: Denotes the file whose size is meant to be reduced.

Example output:

Following execution, the target file has decreased by 2 GiB from its end. This change is evident when examining the file’s properties, showing its new reduced size, accomplished without manipulating any data at the beginning or middle sections of the file.

Empty the file’s content

Code:

truncate --size 0 path/to/file

Motivation:

Truncating a file to zero size is often applied when resetting log files or clearing out content that’s no longer needed without deleting the file itself. This might be critical for avoiding disruption to systems or applications expecting the presence of the file while ensuring the contained data is removed.

Explanation:

• truncate: The pivotal command to modify the size of the file.
• --size 0: Configures the file’s new size to zero bytes, effectively emptying its content.
• path/to/file: Specified file whose content is being emptied.

Example output:

Upon execution, the file exists but is empty, containing 0 bytes of data. This can be verified using file information commands, reflecting no remaining content within the file.

Empty the file’s content, do not create the file if it does not exist

Code:

truncate --no-create --size 0 path/to/file

Motivation:

This method is beneficial when clearing contents of an existing file while avoiding unintended creation of new files. Particularly useful in system scripts or automated processes, it ensures only pre-existing files are affected, preventing accidental alterations to the directory structure or the introduction of files that complicate management and monitoring.

Explanation:

• truncate: Used here to change the file size.
• --no-create: Instructs truncate to not create the file if it is absent, ensuring only existing files are impacted.
• --size 0: Sets the file’s size to zero bytes, thus clearing its contents without affecting the file’s presence.
• path/to/file: Indicates which file’s content is being cleared.

Example output:

After running the command, if the specified file exists, it becomes empty. If the file is not present, the command does nothing; hence, no new file is created or affected. This behavior ensures a controlled content-clearing operation, ascertainable via file inspection commands.

Conclusion

The truncate command provides powerful control over file sizes, ranging from creation and extension to reduction and total clearing. These functionalities are indispensable tools for both developers and system administrators aiming for efficient file handling and resource management across various applications and environments. Through practical examples, this article illustrates truncate’s versatility and utility in managing data space effectively.