Linux | xrags

• xargs（eXtended ARGuments）是給指令傳遞參數的一個篩選器，也是組合多個指令的一個工具。
• xargs 可以將管道或標準輸入（stdin）資料轉換成命令列參數，也能夠從檔案的輸出中讀取資料。
• xargs 也可以將單行或多行文字輸入轉換為其他格式，例如多行變單行，單行變多行。
• xargs 預設的指令是 echo，這表示透過管道傳遞給 xargs 的輸入將會包含換行和空白，不過透過 xargs 的處理，換行和空白將會被空格取代。
• xargs 是一個強而有力的指令，它能夠捕捉一個指令的輸出，然後傳遞給另一個指令。

xargs

How to use xargs with the options -t, -P, and -I:

Options Explained

• -t: Print each command before executing it.
• -P <num>: Run up to <num> processes in parallel.
• -I{}: Replace occurrences of {} in the command with the input items.

Example Usage

1. Create a File: Make a file named items.txt with the following contents:

   file1.txt
   file2.txt
   file3.txt

2. Run xargs:

   cat files.txt | xargs -t -P 2 -I{} echo Processing {}
   cat files.txt | xargs -t -P 3 -I{} sh -c 'cd /path/to/dir && echo Processing {}'

Breakdown of the Command

• cat items.txt: Reads the list of items from the file.
• xargs -t -P 2 -I{}:
  • -t: Prints the command before execution.
  • -P 2: Executes up to 2 commands in parallel.
  • -I {}: Replaces {} with each line from the input.
• echo Processing {}: The command executed, which will print "Processing itemX" for each item.

Notes

• Adjust the number in -P based on your system's capabilities.
• You can replace echo Processing {} with any command you want to execute for each item.

Example: wrf.exe

To use the command xargs -t -P 2 -I{} mpirun -np 2 -wdir {} ./wrf.exe >& log.log, follow these steps:

Explanation of the Command

• xargs -t -P 2 -I{}:
  • -t: Print each command before executing it.
  • -P 2: Run up to 2 processes in parallel.
  • -I{}: Replace occurrences of {} with input items.
• mpirun -np 2 -wdir {}:
  • -np 2: Run 2 MPI processes.
  • -wdir {}: Set the working directory to the value provided by {}.

• ./wrf.exe: The executable you want to run.

• >& log.log: Redirect both standard output and standard error to log.log.

Example Usage

1. Create a File: Prepare a file named dirs.txt containing directory paths:

   /path/to/dir1
   /path/to/dir2

2. Run the Command:

   cat dirs.txt | xargs -t -P 2 -I{} mpirun -np 2 -wdir {} ./wrf.exe >& log.log

Breakdown of the Command Execution

• cat dirs.txt: Reads the list of directories.
• xargs -t -P 2 -I{}: For each directory in dirs.txt, it will:
  • Print the command being executed.
  • Run up to 2 instances of the mpirun command in parallel.
• mpirun -np 2 -wdir {}: Executes the WRF model in the specified directory with 2 MPI processes.
• ./wrf.exe: The executable to run.
• >& log.log: Captures all output and errors into log.log.

Notes

• Ensure that each directory in dirs.txt contains the necessary files for wrf.exe.
• Adjust the parameters as needed based on your system and requirements.

References

1. Linux xargs 命令
2. xargs 命令教程