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Suppose I want to remove all files in a directory except for one named "notes.txt". I would do this with the pipeline, ls | grep -v "notes.txt" | xargs rm. Why do I need xargs if the output of the second pipe is the input that rm should use?

For the sake of comparison, the pipeline, echo "#include <knowledge.h>" | cat > foo.c inserts the echoed text into the file without the use of xargs. What is the difference between these two pipelines?

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You should not use ls | grep -v "notes.txt" | xargs rm to remove everything except for notes.txt, or in general, never parse ls output. Your command would break if a single file contained a space, for example. The safer way would be rm !(notes.txt) in Bash (with shopt -s extglob set), or rm ^notes.txt in Zsh (with EXTENDED_GLOB) etc. –  slhck May 27 '13 at 6:10
    
To avoid spaces you could do find . -maxdepth 1 -mindepth 1 -print0 | xargs -0 instead of ls | xargs :-) –  flob Jun 27 at 9:41

2 Answers 2

cat takes input from STDIN and rm does not. For such commands you need xargs to iterate through STDIN line by line and execute the commands with command line parameters.

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You are confusing two very different kinds of input: STDIN and arguments. Arguments are a list of strings provided to the command as it starts, usually by specifying them after the command name (e.g. echo these are some arguments or rm file1 file2). STDIN, on the other hand, is a stream of bytes (sometimes text, sometimes not) that the command can (optionally) read after it starts. Here are some examples (note that cat can take either arguments or STDIN, but it does different things with them):

echo file1 file2 | cat    # Prints "file1 file2", since that's the stream of
                          # bytes that echo passed to cat's STDIN
cat file1 file2    # Prints the CONTENTS of file1 and file2
echo file1 file2 | rm    # Prints an error message, since rm expects arguments
                         # and doesn't read from STDIN

xargs can be thought of as converting STDIN-style input to arguments:

echo file1 file2 | cat    # Prints "file1 file2"
echo file1 file2 | xargs cat    # Prints the CONTENTS of file1 and file2

echo actually does more-or-less the opposite: it converts its arguments to STDOUT (which can be piped to some other command's STDIN):

echo file1 file2 | echo    # Prints a blank line, since echo doesn't read from STDIN
echo file1 file2 | xargs echo    # Prints "file1 file2" -- the first echo turns
                                 # them from arguments into STDOUT, xargs turns
                                 # them back into arguments, and the second echo
                                 # turns them back into STDOUT
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