When I redirect the output of ls to a file, the filename is included in that file. How can I avoid this?
Solution 1
As you've noticed, the file is created before ls
is run. This is due to how the shell handles its order of operations. In order to do
ls > file
the shell needs to create file
and then set stdout to point to that and the finally run the ls
program.
So you have some options.
- Create the file in another directory (eg
/tmp
) and thenmv
it to the final directory - Create it as a hidden file (
.file
) and rename it - Use
grep
to remove the file from the output - Cheat :-)
The cheat would be something like
x=$(ls) ; printf "%s\n" "$x" > file
This causes the output of ls
to be held in a variable, and then we write that out.
Solution 2
The output file is created by the shell before ls
begins. You can get around this by using tee
:
ls | tee list
To thoroughly defeat any race condition, there is always
ls | grep -vx 'list' > list
Or if you like that tee
displays the results as well:
ls | grep -vx 'list' | tee list
However, as pointed out in comments, things like this often break when filenames contain strange characters. Unix filenames can generally contain any characters except for NUL
and /
, so parsing the output of ls
is extremely difficult:
- Assigning to a shell variable can fail if a filename ends in one or more
\n
. - Filtering with
grep
fails when the search term lies between\n
. - You can separate filenames with
NUL
instead of\n
usingfind
, but it can be difficult to convert this into something resembling the traditional sorted, newline-separated output ofls
. - Removing the output filename from the list may be incorrect if it already exists.
So the only truly effective way to do this is to create the output file somewhere else, and move it into place. If you will never use ls -a
, then this works:
ls > .list && mv .list list
If you might be using ls -a
, then .list
could appear in your output but no longer exist in the directory. So then you would use a different directory, like /tmp
to store the intermediate result. Of course, if you always use /tmp
you run into trouble there, so you can write a script:
#!/bin/sh
OUTDIR='/tmp'
if [ "${PWD}" = '/tmp' ]; then
OUTDIR="${HOME}"
fi
ls > "${OUTDIR}/list" && mv "${OUTDIR}/list" list
This seems overly complicated for the task, though.
But the entire cause of the issue is that the shell is creating the output file before the command begins. We can take that into consideration and just have the shell list the files for us. Then we don't even need ls
at all!
printf '%s\n' * > list
This will work until you have too many files in the directory to fit into an argument list.
Solution 3
You can use moreutils
sponge
:
ls | sponge list
Or with zsh
:
cp =(ls) list
With GNU ls
:
ls -I list > list
(though if there had been a file called list
before, that means it won't be listed).
Since ls
output is sorted anyway, you can also use (assuming your filenames don't contain newline characters):
ls | sort -o list
Or to avoid the double sorting, if your ls
supports -U
for U
nsorted (beware some ls
implementations have a -U
for something else):
ls -U | sort -o list
Solution 4
You can make the filename temporarily hidden:
ls >.list && mv .list list
Solution 5
Partial/most credit goes to @StephenHarris...
echo "`ls`" > list
equivalent to
echo "$(ls)" > list
Related videos on Youtube
![Steven Lu](https://i.stack.imgur.com/D0CKc.jpg?s=256&g=1)
Steven Lu
Updated on September 18, 2022Comments
-
Steven Lu almost 2 years
Observe:
$ ls $ ls > list $ cat list list
This appears to indicate that when
ls
is executed that the redirection into filelist
has already begun and thelist
file is already created. A fine enough explanation at any rate, but the question is this: How can I prevent this from happening? What I expected to happen wasls
would execute and its output dumped intolist
and that is what I want. -
Steven Lu almost 8 yearsOK i hadn't thought of this yet. It touches the filesystem a few times but it's somewhat intelligible, which is a big plus... Yes I'm looking for something concise and not too convoluted. Thanks
-
Steven Lu almost 8 yearsVery neat! so here we are using the tee program to write to the file rather than leaving that task to the shell to do (which the shell ends up being overeager with for this particular situation)
-
Steven Lu almost 8 yearsI actually love that it shows the output on stdout... tee is really useful.
-
Stephen Harris almost 8 yearsThis fails to solve the problem if there are too many files in the directory. There's an inherent race condition. In an empty (or few files) directory it works, but with a lot of files the
tee
process can create the file before thels
has had a chance to read the whole directory. -
Stephen Harris almost 8 yearsI did it with 10,000 files and saw the race condition :-) I was in the middle of writing it as an answer until I spotted it. We can kludge with something nasty like
(sleep 1 ; tee file)
, but it's nasty. -
Stephen Harris almost 8 yearsYup,
grep
is one of the options I listed as well. Mostly I try to avoid this problem by using hidden files or other directories 'cos it's just nasty :-) -
Steven Lu almost 8 yearsNice. Your last paragraph prompted me to try
echo `ls` > ls
which I love for being so concise, but this doesnt put an entry on each line which my particular situation has issues with (as I very much do need to deal with directories with spaces in them today)... -
Stephen Harris almost 8 years
ls -1
will force single-column output -
Steven Lu almost 8 years@StephenHarris Yeah I think i might as well give you the accept so somebody gets points, this answer is 99% of the way there. Do you think there are any race conditions or failure modes of
echo "$(ls -1)" > file
? -
Stephen Harris almost 8 yearsYes, there are potential race conditions. If you want to use this sort of cheat you really should split the
ls
and the file creation into two steps. Also beware ofecho
potentially interpreting the output of thels
; (what if you have a file called-ne
?). That's why I usedprintf
in my example. -
Stéphane Chazelas almost 8 years
grep -v '^list$'
or more simply:grep -vx list
removes the line corresponding to the file calledlist
, but also all those coming from filenames containing<newline>list<newline>
-
Stéphane Chazelas almost 8 yearsNote that
x=$(ls)
removes all the trailing newline characters from the output ofls
while yourprintf '%s\n'
adds only one back. -
Overmind Jiang almost 8 years@StevenLu: you could use echo with double quotes —
echo "`ls`" > ls
orecho "$(ls)"
. -
Gilles 'SO- stop being evil' almost 8 yearsYou don't need the
-1
option. It's implicit wheneverls
isn't writing to a terminal (such as here, where it's writing to a pipe that the shell reads from). -
Gilles 'SO- stop being evil' almost 8 years@StevenLu There are no race conditions, the order of evaluation is well-defined: the command substitution
$(ls)
is evaluated before the redirection>file
. The reason you aren't getting one file per line is that you didn't use double quotes:echo "$(ls)" >file
-
Steven Lu almost 8 years@Gilles Right you are. I started trying it with
-1
in hopes it would fix it and it stuck around. I've edited the answer.