I have a command that copies file from one dir to another
FILE_COLLECTOR_PATH="/var/www/";
FILE_BACKUP_PATH='/home/'
ls $FILE_COLLECTOR_PATH | head -${1} | xargs -i basename {} | xargs -t -i cp $FILE_COLLECTOR_PATH{} "${FILE_BACKUP_PATH}{}-`date +%F%H%M%S%N`"
I loop it in a shell script like,
#!/bin/sh
SLEEP=120
FILE_COLLECTOR_PATH="/var/www/";
FILE_BACKUP_PATH='/home/'
while true
do
ls $FILE_COLLECTOR_PATH | head -${1} | xargs -i basename {} | xargs -t -i cp $FILE_COLLECTOR_PATH{} "${FILE_BACKUP_PATH}{}-`date +%F%H%M%S%N`"
sleep ${SLEEP}
done
But it seems to move only 10 files and not all files in the dir, Why? It should suppose to move all files.
In general, don't try to parse the output of ls in a script. You can end up with many different types of subtle problems. There is almost always a better tool for the job. Many times, this tool is find. For example, to generate a list of all of the files in a directory and do something to each of them, you would do something like this:
find <search directory> -maxdepth 1 -type f -print0 | xargs -0i basename {} ...
The -print0 and -0 arguments allow find and xargs to communicate filenames in a way that handles special characters (like spaces) correctly.
The find command has other options that you may find useful in a backup script (which is what it appears you are building). Options like -mmin and -newer will enable you to only back up files that have changed since the last iteration.
Try doing
ls -1
instead of just ls, because ls by default don't displays files on a newline (tail expect newlines) for each files when ls -1 does.
Related
So I have a directory with files and sub-directories in it. I want to get all the files recursively and then list them in long format, sorted by the modified date. Here's what I came up with.
find . -type f | xargs -d "\n" | ls -lt
However this only lists the files in the current directory and not the sub-directories. I don't understand why, given that the following prints out all the files.
find . -type f | xargs -d "\n" | cat
Any help appreciated.
xargs can only start ls if it's passed ls as an argument. When you pipe from xargs into ls, only one copy of ls is started -- by the parent shell -- and it isn't given any of the filenames from find | xargs as arguments -- instead they're on its stdin, but ls never reads its stdin, so it doesn't even know that they're there.
Thus, you need to remove the | character:
# Does what you specified in the common case, but buggy; don't use this
# (filenames can contain newlines!)
# ...also, xargs -d is GNU-only
find . -type f | xargs -d '\n' ls -lt
...or, better:
# uses NUL separators, which cannot exist inside filenames
# also, while a non-POSIX extension, this is supported in both GNU and BSD xargs
find . -type f -print0 | xargs -0 ls -lt
...or, even better than that:
# no need for xargs at all here; find -exec can do the same thing
# -exec ... {} + is POSIX-mandated functionality since 2008
find . -type f -exec ls -lt {} +
Much of the content in this answer is also covered in the Actions, Complex Actions, and Actions in Bulk sections of Using Find, which is well worth reading.
Fundamentally, I want to do something like:
find . -type f -print0 | xargs -0 -P 3 blahblahcommand > `mktemp`.blah
(not sure where the backticks went around mktemp.)
or perhaps blahblahcommand allows the specification of an output file:
find . -type f -print0 | xargs -0 -P 3 blahblahcommand -o `mktemp`.blah
So that as the 3 processes are running (or max of 3 rather), each individual blahblahcommand output is going into a unique file. But mktemp is evaluated once, and of course, the output of blahblahcommand is stomped on.
blahblahcommand does not have support to generate a uniquely defined filename unfortunately.
Is there an easy way to do this in bash?
Individually is trivial, but I am not enough of a shell programmer to figure out how to make this work like I want. Any tips appreciated.
You can use GNU parallel instead of xargs:
readarray -t files < <(find . -type f -print0 | parallel -0 -X -P3 --files blahblahcommand)
will save the output of executions of blahblahcommand file1 file2 file3 ... fileN in temporary files and store their names in the files array for further use.
I am new to bash and I am trying to understand the use of xargs, which is still not clear for me. For example:
history | grep ls
Here I am searching for the command ls in my history. In this command, I did not use xargs and it worked fine.
find /etc - name "*.txt" | xargs ls -l
I this one, I had to use xargs but I still can not understand the difference and I am not able to decide correctly when to use xargs and when not.
xargs can be used when you need to take the output from one command and use it as an argument to another. In your first example, grep takes the data from standard input, rather than as an argument. So, xargs is not needed.
xargs takes data from standard input and executes a command. By default, the data is appended to the end of the command as an argument. It can be inserted anywhere however, using a placeholder for the input. The traditional placeholder is {}; using that, your example command might then be written as:
find /etc -name "*.txt" | xargs -I {} ls -l {}
If you have 3 text files in /etc you'll get a full directory listing of each. Of course, you could just have easily written ls -l /etc/*.txt and saved the trouble.
Another example lets you rename those files, and requires the placeholder {} to be used twice.
find /etc -name "*.txt" | xargs -I {} mv {} {}.bak
These are both bad examples, and will break as soon as you have a filename containing whitespace. You can work around that by telling find to separate filenames with a null character.
find /etc -print0 -name "*.txt" | xargs -I {} -0 mv {} {}.bak
My personal opinion is that there are almost always alternatives to using xargs (such as the -exec argument to find) and you will be better served by learning those.
When you use piping without xargs, the actual data is fed into the next command. On the other hand, when using piping with xargs, the actual data is viewed as a parameter to the next command. To give a concrete example, say you have a folder with a.txt and b.txt. a.txt contains just a single line 'hello world!', and b.txt is just empty.
If you do
ls | grep txt
you would end up getting the output:
a.txt
b.txt
Yet, if you do
ls | xargs grep txt
you would get nothing since neither file a.txt nor b.txt contains the word txt.
If the command is
ls | xargs grep hello
you would get:
hello world!
That's because with xargs, the two filenames given by ls are passed to grep as arguments, rather than the actual content.
Short answer: Avoid xargs for now. Return to xargs when you have written dozens or hundreds of scripts.
Commands can get their input from parameters (like rm bad_example) or can get the input from stdin (not just the y on the question after rm -i is_this_bad_too, but also read answer). Other commands like grep and sed will look for parameters and when the parameters don't show the input, switch to the input.
Your grep example works fine reading from stdin, nothing special needed.
Your ls needs the output of find as a parameter. xargs is just one way to turn things around. Use man xargs for more about xargs. Alternatives:
find /etc -name "*.txt" -exec ls -l {} \;
find /etc -name "*.txt" -ls
ls -l $(find /etc -name "*.txt" )
ls /etc/*.txt
First try to see which of this commands is best when you have a nasty filename with spaces.txt in /etc.
xargs(1) is dangerous (broken, exploitable, etc.) when reading non-NUL-delimited input.
If you're working with filenames, use find's -exec [command] {} + instead.
If you can get NUL-delimited output, use xargs -0.
GNU Parallel can do the same as xargs, but does not have the broken and exploitable "features".
You can learn GNU Parallel by looking at examples http://www.gnu.org/software/parallel/man.html#EXAMPLE:-Working-as-xargs--n1.-Argument-appending and walking through the tutorial http://www.gnu.org/software/parallel/parallel_tutorial.html
I would like to search a directory for any file that matches any of a list of words. If a file matches, I would like to copy that file into a new directory. I created a small batch of test files and got the following code working:
cp `grep -lir 'word\|word2\|word3\|word4\|word5' '/Users/originallocation'` '/Users/newlocation'
Unfortunately, when I run this code on a large folder with a few thousand files it says the argument list is too long for cp. I think I need to loop this or use a xargs but I can't figure out how to make the conversion.
The minimal change from what you have would be:
grep -lir 'word\|word2\|word3\|word4\|word5' '/Users/originallocation' | \
xargs cp -t '/Users/newlocation'
But, don't use that. Because you never know when you will encounter a filename with spaces or newlines in it, null-terminated strings should be used. On linux/GNU, add the -Z option to grep and -0 to xargs:
grep -Zlir 'word\|word2\|word3\|word4\|word5' '/Users/originallocation' | \
xargs -0 cp -t '/Users/newlocation'
On Macs (and AIX, HP-UX, Solaris, *BSD), the grep options change slightly but, more importantly, the GNU cp -t option is not available. A workaround is:
grep -lir --null 'word\|word2\|word3\|word4\|word5' '/Users/originallocation' | \
xargs -0 -I fname cp fname '/Users/newlocation'
This is less efficient because a new instance of cp has to be run for each file to be copied.
Alternative solution for those without grep -r. Using find + egrep + xargs , hope there is no file with same file name in different folders. Secondly, I replaced the ugly style of word\|word2\|word3\|word4\|word5
find . -type f -exec egrep -l 'word|word2|word3|word4|word5' {} \; |xargs -i cp {} /LARGE_FOLDER
So after a lot of searching and trying to interpret others' questions and answers to my needs, I decided to ask for myself.
I'm trying to take a directory structure full of images and place all the images (regardless of extension) in a single folder. In addition to this, I want to be able to remove images matching certain filenames in the process. I have a find command working that outputs all the filepaths for me
find -type f -exec file -i -- {} + | grep -i image | sed 's/\:.*//'
but if I try to use that to copy files, I have trouble with the spaces in the filenames.
cp `find -type f -exec file -i -- {} + | grep -i image | sed 's/\:.*//'` out/
What am I doing wrong, and is there a better way to do this?
With the caveat that it won't work if files have newlines in their names:
find . -type f -exec file -i -- {} + |
awk -vFS=: -vOFS=: '$NF ~ /image/{NF--;printf "%s\0", $0}' |
xargs -0 cp -t out/
(Based on answer by Jonathan Leffler and subsequent comments discussion with him and #devnull.)
The find command works well if none of the file names contain any newlines. Within broad limits, the grep command works OK under the same circumstances. The sed command works fine as long as there are no colons in the file names. However, given that there are spaces in the names, the use of $(...) (command substitution, also indicated by back-ticks `...`) is a disaster. Unfortunately, xargs isn't readily a part of the solution; it splits on spaces by default. Because you have to run file and grep in the middle, you can't easily use the -print0 option to (GNU) find and the -0 option to (GNU) xargs.
In some respects, it is crude, but in many ways, it is easiest if you write an executable shell script that can be invoked by find:
#!/bin/bash
for file in "$#"
do
if file -i -- "$file" | grep -i -q "$file:.*image"
then cp "$file" out/
fi
done
This is a little painful in that it invokes file and grep separately for each name, but it is reliable. The file command is even safe if the file name contains a newline; the grep is probably not.
If that script is called 'copyimage.sh', then the find command becomes:
find . -type f -exec ./copyimage.sh {} +
And, given the way the grep command is written, the copyimage.sh file won't be copied, even though its name contains the magic word 'image'.
Pipe the results of your find command to
xargs -l --replace cp "{}" out/
Example of how this works for me on Ubuntu 10.04:
atomic#atomic-desktop:~/temp$ ls
img.png img space.png
atomic#atomic-desktop:~/temp$ mkdir out
atomic#atomic-desktop:~/temp$ find -type f -exec file -i \{\} \; | grep -i image | sed 's/\:.*//' | xargs -l --replace cp -v "{}" out/
`./img.png' -> `out/img.png'
`./img space.png' -> `out/img space.png'
atomic#atomic-desktop:~/temp$ ls out
img.png img space.png
atomic#atomic-desktop:~/temp$