How to Pipe Output from Grep to Cp

How to pipe output from grep to cp?

grep -l -r "TWL" --exclude=*.csv* | xargs cp -t ~/data/lidar/tmp-ajp2/

Explanation:

• grep -l option to output file names only
• xargs to convert file list from the standard input to command line arguments
• cp -t option to specify target directory (and avoid using placeholders)

Is it possible to pipe the results of FIND to a COPY command CP?

Good question!

1. why cant you just use | pipe? isn't that what its for?

You can pipe, of course, xargs is done for these cases:

find . -iname "*.SomeExt" | xargs cp Destination_Directory/

2. Why does everyone recommend the -exec

The -exec is good because it provides more control of exactly what you are executing. Whenever you pipe there may be problems with corner cases: file names containing spaces or new lines, etc.

3. how do I know when to use that (exec) over pipe | ?

It is really up to you and there can be many cases. I would use -exec whenever the action to perform is simple. I am not a very good friend of xargs, I tend to prefer an approach in which the find output is provided to a while loop, such as:

while IFS= read -r result
do
    # do things with "$result"
done < <(find ...)

pass output as an argument for cp in bash

It would be:

cp `ls -SF | grep -v / | head -5` Directory

assuming that the pipeline is correct. The backticks substitute in the line the output of the commands inside it.

You can also make your tests:

cp `echo a b c` Directory

will copy all a, b, and c into Directory.

how to send output of 'ls-l' to cp command using pipes?

Output to file "test.txt":

ls -l > text.txt

Sumary of pipes:

• > Save output to a file.

• >> Append output to a file.

• < Read input from a file.

• 2> Redirect error messages.

• | Send the output from one program as input to another program.

How to copy files found with grep

Try this:

find . -type f -exec grep -q '^beginString' {} \; -exec cp -t /home/user/DestinationFolder {} +

or

grep -lir '^beginString' . | xargs cp -t /home/user/DestinationFolder

But if you want to keep directory structure, you could:

grep -lir '^beginString' . | tar -T - -c | tar -xpC /home/user/DestinationFolder

or if like myself, you prefer to be sure about kind of file you store (only file, no symlinks), you could:

find . -type f -exec grep -l '^beginString' {} + | tar -T - -c |
    tar -xpC /home/user/DestinationFolder

and if your files names could countain spaces and/or special characters, use null terminated strings for passing grep -l output (arg -Z) to tar -T (arg --null -T):

grep -Zlir '^beginString' . | xargs --null cp -t /home/user/DestinationFolder

or

find . -type f -exec grep -lZ '^beginString' {} + | tar --null  -T - -c |
    tar -xpC /home/user/DestinationFolder

How to copy files found with grep on OSX

Less efficient than cp -t, but this works:

grep -lr "foo" --include=*.txt * 2>/dev/null |
  xargs -I{} cp "{}" /path/to/targetdir

Explanation:

For filenames | xargs cp -t destination, xargs changes the incoming filenames into this format:

cp -t destination filename1 ... filenameN

i.e., it only runs cp once (actually, once for every few thousand filenames -- xargs breaks the command line up if it would be too long for the shell).

For filenames | xargs -I{} cp "{}" destination, on the other hand, xargs changes the incoming filenames into this format:

cp "filename1" destination
...
cp "filenameN" destination

i.e., it runs cp once for each incoming filename, which is much slower. For a large number (e.g., >10k) of very small (e.g., <10k) files, I'd guess it could even be thousands of times slower. But it does work :)

PS: Another popular technique is use find's exec function instead of xargs, e.g., https://stackoverflow.com/a/5241677/1563960

perform an operation for *each* item listed by grep

If I understand your specification, you want:

grep --null -l '<pattern>' directory/*.extension1 | \
    xargs -n 1 -0 -I{} bash -c 'rm "$1" "${1%.*}.extension2"' -- {}

This is essentially the same as what @triplee's comment describes, except that it's newline-safe.

What's going on here?

grep with --null will return output delimited with nulls instead of newline. Since file names can have newlines in them delimiting with newline makes it impossible to parse the output of grep safely, but null is not a valid character in a file name and thus makes a nice delimiter.

xargs will take a stream of newline-delimited items and execute a given command, passing as many of those items (one as each parameter) to a given command (or to echo if no command is given). Thus if you said:

printf 'one\ntwo three \nfour\n' | xargs echo

xargs would execute echo one 'two three' four. This is not safe for file names because, again, file names might contain embedded newlines.

The -0 switch to xargs changes it from looking for a newline delimiter to a null delimiter. This makes it match the output we got from grep --null and makes it safe for processing a list of file names.

Normally xargs simply appends the input to the end of a command. The -I switch to xargs changes this to substitution the specified replacement string with the input. To get the idea try this experiment:

printf 'one\ntwo three \nfour\n' | xargs -I{} echo foo {} bar

And note the difference from the earlier printf | xargs command.

In the case of my solution the command I execute is bash, to which I pass -c. The -c switch causes bash to execute the commands in the following argument (and then terminate) instead of starting an interactive shell. The next block 'rm "$1" "${1%.*}.extension2"' is the first argument to -c and is the script which will be executed by bash. Any arguments following the script argument to -c are assigned as the arguments to the script. This, if I were to say:

bash -c 'echo $0' "Hello, world"

Then Hello, world would be assigned to $0 (the first argument to the script) and inside the script I could echo it back.

Since $0 is normally reserved for the script name I pass a dummy value (in this case --) as the first argument and, then, in place of the second argument I write {}, which is the replacement string I specified for xargs. This will be replaced by xargs with each file name parsed from grep's output before bash is executed.

The mini shell script might look complicated but it's rather trivial. First, the entire script is single-quoted to prevent the calling shell from interpreting it. Inside the script I invoke rm and pass it two file names to remove: the $1 argument, which was the file name passed when the replacement string was substituted above, and ${1%.*}.extension2. This latter is a parameter substitution on the $1 variable. The important part is %.* which says

• % "Match from the end of the variable and remove the shortest string matching the pattern.
• .* The pattern is a single period followed by anything.

This effectively strips the extension, if any, from the file name. You can observe the effect yourself:

foo='my file.txt'
bar='this.is.a.file.txt'
baz='no extension'
printf '%s\n'"${foo%.*}" "${bar%.*}" "${baz%.*}"

Since the extension has been stripped I concatenate the desired alternate extension .extension2 to the stripped file name to obtain the alternate file name.

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