How to use the command "sponge" (with examples)

“Sponge” is a command-line utility that reads from standard input and writes to a file, but with a twist. It first soaks up the entire input before writing the output file, making it useful in situations where you need to modify a file but want to avoid issues related to reading and writing to the same file at the same time.

Use case 1: Append file content to the source file

Code:

cat path/to/file | sponge -a path/to/file

Motivation: Appending file content to the source file can be useful in various scenarios, such as when you want to add new lines or data to an existing file without having to determine the exact location manually.

Explanation:

• cat path/to/file: The cat command is used to read the content of the file specified by the given path.
• |: The pipe symbol (|) is used to redirect the output of the cat command as the input for the sponge command.
• sponge -a path/to/file: The sponge command with the -a option appends the input content received from the cat command to the file specified by the given path.

Example output: Let’s say the file at path/to/file initially contains the following content:

Hello, World!

Running the provided code with input content “This is a new line.” would result in the file now containing:

Hello, World!
This is a new line.

Use case 2: Remove all lines starting with # in a file

Code:

grep -v '^#' path/to/file | sponge path/to/file

Motivation: Removing lines starting with the hash symbol (#) from a file can be beneficial when dealing with configuration files or scripts where commented lines need to be removed to improve readability or functionality.

Explanation:

• grep -v '^#' path/to/file: The grep command with the -v option searches for lines in the file specified by the given path that do not match the regular expression ^#, i.e., lines that do not start with a hash symbol (#).
• |: The pipe symbol (|) is used to redirect the output of the grep command as the input for the sponge command.
• sponge path/to/file: The sponge command writes the input received from the grep command to the file specified by the given path, effectively replacing the original file with the filtered content.

Example output: Let’s say the file at path/to/file initially contains the following content:

# This is a comment.
Line 1
# Another comment.
Line 2

Running the provided code would result in the file now containing:

Line 1
Line 2

Conclusion:

The “sponge” command is a handy tool when you need to manipulate files without worrying about reading and writing conflicts. It can be used to append content to a file or modify it by removing specific lines. By understanding these use cases and examples, you can leverage the power of the “sponge” command in your daily sysadmin or scripting tasks.