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Comments on How to run a command on a list of files?

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How to run a command on a list of files?

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Suppose I have a list of files on standard input. These may be the output of find, cat filelist.txt or something else.

How can I run a command on each file in turn?

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  • If I just used find to generate a list of files, then find's -exec argument is usually the way to run some other program on each file found.

    If you pipe the command to xargs, note that -P n will run up to n commands in parallel. The best value of n will depend on the relative usage of your CPU and your storage system.

  • If I have a program (say, generate_lists) that generates a list of files,

    for filename in $(generate_lists); do some_program "$filename" ; done
    

    is usually helpful. Make sure you quote your use of $filename -- more of them have spaces than you'd think.

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3 comment threads

xargs(1) is usually better than looping (1 comment)
for loops on command substitutions have quoting issues (1 comment)
Usually "number of cores" is a good starting value for `n`. (2 comments)
Usually "number of cores" is a good starting value for `n`.
matthewsnyder‭ wrote about 1 year ago

Usually "number of cores" is a good starting value for n.

dsr‭ wrote about 1 year ago

Sometimes? It really depends. Here are some reasons why it wouldn't be:

  • Huge number of cores, low I/O: you have a 192 core box but the filesystem is mounted over a transPacific link. You might want n=1 or 2 or maybe 3.

  • Simple task, great I/O: you're going to stat each file and report on metadata, and they are all stored in a tmpfs. Even if you only have 2 cores, you might want to try n=4.

Those are extremes, of course. Most cases will be somewhere in between, and fairly often this is going to be a one-off where there is no point in re-running for performance gain -- you have the answer!