xargs command

Yes, you can use a placeholder with xargs by using the -I option. This allows you to specify a placeholder (e.g., {}) that will be replaced by the arguments from the input.

Syntax with Placeholder

xargs -I placeholder command placeholder

Here’s how it works:

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Examples

1. Renaming Files

Suppose you have a list of files and want to add a prefix to their names:

ls | xargs -I {} mv {} new_{}

• -I {} specifies {} as the placeholder.
• mv {} new_{} replaces {} with each filename.

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2. Finding and Copying Files

Combine find and xargs to copy files to another directory:

find /source -name "*.txt" | xargs -I {} cp {} /destination

• {} is replaced with the full path of each .txt file found.

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3. Making Directories

Create directories with specific names read from a file:

cat dirnames.txt | xargs -I {} mkdir {}

• {} is replaced with each line from dirnames.txt.

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4. Using Multiple Placeholders

You can use the placeholder multiple times in the command:

echo "file1 file2 file3" | xargs -I {} echo "Processing file: {} and archiving it as {}.bak"

Output:

Processing file: file1 and archiving it as file1.bak
Processing file: file2 and archiving it as file2.bak
Processing file: file3 and archiving it as file3.bak

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Key Points About -I:

1. Performance Note: When using -I, xargs runs the command once for each input argument, which can be slower for large datasets compared to batch processing (xargs -n).
2. Default Placeholder: {} is commonly used, but you can specify any placeholder, like % or $, as long as it’s unique in your command.

Using placeholders is especially useful for complex commands where the arguments need to appear in specific places.

In your command:

find /source -name "*.txt" | xargs -I {} cp {} /destination

The {} is used as a placeholder for the input arguments, but it only replaces occurrences of {} in the command itself. Here’s how it works:

• find /source -name "*.txt" lists files that match the pattern *.txt in the /source directory.
• xargs -I {} replaces each file name (output of the find command) with {} in the command.
• In the command cp {} /destination, the {} is replaced with the full file path returned by find.

Alternative for Multiple Occurrences

If you wanted to use {} in multiple places within the command, it would replace all occurrences of {}. For example:

find /source -name "*.txt" | xargs -I {} cp {} /destination/{}-backup

This would replace {} in both places:

• The first {} is replaced by the file name to copy the file.
• The second {} is replaced by the file name as part of the destination name (/destination/{file}-backup).

So the xargs -I {} syntax works wherever {} appears in the command you provide to xargs.

The issue arises because of how xargs handles the && operator. The && operator runs the second command only if the first one is successful. However, in your command, you’re running echo "convert" {} and then appending && echo "translate" {}, but xargs treats the && and the second echo as separate entities, executing the second one only once after the whole xargs command finishes.

To print “translate” for each line, you need to include both commands inside the same bash command, or modify the logic slightly to ensure they run for each item.

Here’s an updated version of your command:

ls *.mp4 | grep -P "lesson[1,2][0-9]\.mp4" | xargs -I {} bash -c 'echo "convert" {}; echo "translate" {}'

Explanation:

• xargs -I {} is used to substitute {} with each file.
• bash -c '...' runs both echo commands inside the same shell, ensuring that both “convert” and “translate” are printed for each filename.

Output:

This should now produce the expected output:

convert lesson10.mp4
translate lesson10.mp4
convert lesson11.mp4
translate lesson11.mp4
...

This way, both “convert” and “translate” will be printed for each file as intended. Let me know if that works!