How to use the command 'ar' (with examples)
The ‘ar’ command in Unix is used to create, modify, and extract from Unix archives. It is typically used for static libraries (.a) and Debian packages (.deb). It is similar to the ’tar’ command but specifically designed for archives. In this article, we will explore the different use cases of the ‘ar’ command and provide examples for each.
Use case 1: Extract all members from an archive
Code:
ar x path/to/file.a
Motivation: Extract all members from an archive is useful when we want to extract the individual files from a static library or a Debian package. This allows us to access and manipulate the files within the archive separately.
Explanation:
- ‘ar’: The command itself.
- ‘x’: The option for extracting all members from the archive.
- ‘path/to/file.a’: The path to the archive file from which we want to extract the members.
Example output:
file1.o
file2.o
file3.o
Use case 2: List contents in a specific archive
Code:
ar t path/to/file.ar
Motivation: Listing contents in a specific archive can be helpful to get an overview of the files included in the archive. This allows us to verify the contents of the archive before performing any further actions.
Explanation:
- ‘ar’: The command itself.
- ’t’: The option for listing contents.
- ‘path/to/file.ar’: The path to the archive file for which we want to list the contents.
Example output:
file1.o
file2.o
file3.o
Use case 3: Replace or add specific files to an archive
Code:
ar r path/to/file.deb path/to/debian-binary path/to/control.tar.gz path/to/data.tar.xz ...
Motivation: Replacing or adding specific files to an archive is useful when we need to update an existing archive by adding or replacing specific files. This allows us to modify the archive without having to recreate it entirely.
Explanation:
- ‘ar’: The command itself.
- ‘r’: The option for replacing or adding specific files.
- ‘path/to/file.deb’: The path to the archive file to which we want to replace or add files.
- ‘path/to/debian-binary’, ‘path/to/control.tar.gz’, ‘path/to/data.tar.xz’, … : The paths to the files that we want to replace or add to the archive.
Example output: No output is displayed when replacing or adding files to an archive. If the operation is successful, the archive will be updated with the specified files.
Use case 4: Insert an object file index (equivalent to using ‘ranlib’)
Code:
ar s path/to/file.a
Motivation: Inserting an object file index is necessary for libraries created with the ‘ar’ command to be recognized as such and be able to provide symbol resolution during the linking phase. This allows the library to be used by other programs that depend on it.
Explanation:
- ‘ar’: The command itself.
- ’s’: The option for inserting an object file index.
- ‘path/to/file.a’: The path to the archive file for which we want to insert the object file index.
Example output: No output is displayed when inserting an object file index. If the operation is successful, the archive file will be updated with the object file index.
Use case 5: Create an archive with specific files and an accompanying object file index
Code:
ar rs path/to/file.a path/to/file1.o path/to/file2.o ...
Motivation: Creating an archive with specific files and an accompanying object file index is useful when we want to package multiple object files into a single library. The object file index allows the library to be recognized by the linker and can provide symbol resolution.
Explanation:
- ‘ar’: The command itself.
- ‘rs’: The options for creating an archive with specific files and inserting an object file index.
- ‘path/to/file.a’: The path to the archive file we want to create.
- ‘path/to/file1.o’, ‘path/to/file2.o’, …: The paths to the object files that we want to include in the archive.
Example output: No output is displayed when creating an archive with specific files and an accompanying object file index. If the operation is successful, the archive file will be created with the specified files and an object file index.
