Giving cat output to rm

Question

I have file names in a file which I need to delete in a different directory.

Let's say I have x and y files in dir a. How do I delete it using cat?

I tried,

rm -f a/{`cat a.txt`}

a.txt has contents x,y,z.

If they are in the same folder, I can put x y z in a.txt and run,

rm -f `cat a.txt`

which works fine.

I have also tried,

rm -f "a/{"`cat a.txt`"}"

This command will go in a dockerfile so I prefer not to use any variables too.

I do not want to put a/x a/y a/z in the file which can be an option, as it is fixed that a will only contain the files. But a should be changed only in the dockerfile. Thanks for all suggestions in advance :)

Answer 1

You should use while loop to read file line by line and then apply each line to rm. This a very common and frequently used approach in scripting

while IFS= read -r line
do
       rm a/"$line"
done < file.txt

Naturally the format of the file should be list of files with one file per line

Answer 2

Assuming your filenames don't contain spaces or any special characters, just reuse your original command with a cd before it:

(cd a; rm -f $(cat a.txt))

Be warned that rm -f `cat a.txt` breaks easily with spaces or any special characters in filenames, you should really use xargs with NUL-delimited filenames.

Answer 3

Don't try to use $(cat file) for this kind of thing - it will break for example if there are spaces in file names e.g. given

$ cat a.txt 
foo
bar baz
bam

and

$ ls -Q a
"bam"  "bar baz"  "foo"  "other file"  "somefile"

then

$ (cd a ; rm $(cat ../a.txt))
rm: cannot remove 'bar': No such file or directory
rm: cannot remove 'baz': No such file or directory

Instead, you can use xargs

$ ls -Q a
"bam"  "bar baz"  "foo"  "other file"  "somefile"
$ xargs -a a.txt -I{} rm a/{}
$ ls -Q a
"other file"  "somefile"

If you really want to use cat, then combine it with xargs:

cat a.txt | xargs -I{} rm a/{}

(although given the -a feature, it's a Useless Use of Cat)

---

Note that -I{} implies -L 1 i.e. rm is invoked once for each line of the input file; if you don't need to prepend a directory path, then you can make the command more efficient by doing away with the -I in which case xargs will pass multiple arguments to rm. However in that case you should explicitly set the input delimiter to newline e.g. xargs -a a.txt -d '\n' rm to prevent breaking on spaces.