[SOLVED] Redirecting ls output to a file

raphaelabb · 2024-03-24 17:45:09

Hello,

I need to make a list of all files inside a directory.
Let's say file1 file2 file3.

When I use:

 ls > my_list

I end up with this content inside my_list

file1
file2
file3
my_list

How come my_list appears in the file since it does not yet exist when ls is invoked?

Last edited by raphaelabb (2024-03-25 16:54:03)

gxt25 · 2024-03-24 18:09:00

https://www.gnu.org/software/bash/manua … tions.html says the resulting destination for the output is opened for writing before the left side operation, so my_list is created before ls is performed.

Head_on_a_Stick · 2024-03-24 20:24:50

Parsing the output of ls is usually a bad idea: https://mywiki.wooledge.org/ParsingLs

Better:

$ mkdir test ; cd $_
$ touch a b c d e
$ for i in * ; do echo "$i" >> my_list ; done
$ cat my_list                                                                  
a
b
c
d
e
$

Last edited by Head_on_a_Stick (2024-03-24 20:26:25)

raphaelabb · 2024-03-24 20:38:43

I see. But why

ls * > my_list

does not include my_list inside the created file?

Trilby · 2024-03-24 20:42:08

Because the glob is parsed by the shell not by ls - this command line parsing is done before the redirection is handled to create the "my_list" file.

gxt25 · 2024-03-24 20:49:33

https://www.gnu.org/software/bash/manua … nsion.html says the asterisk is replaced by a list of filenames, so in effect here you are listing explicitly the filenames you wish to have operated on by ls and placed into my_list. This replacement happens first in the shell order of operations.

I typed too slow...

Last edited by gxt25 (2024-03-24 20:50:14)

sukolyn · 2024-03-25 02:23:34

Head_on_a_Stick wrote:

$ for i in * ; do echo "$i" >> my_list ; done

printf '%s\n' * >my_list

Trilby · 2024-03-25 02:31:47

This thread is a perfect example of the problem of learning some "rule" without understanding why the rule exists. The OP isn't parsing the output of ls, they're just sending it to a file. And as for all these "alternatives" to `ls`, they don't do anything better than ls itself. Stop over-correcting ... incorrectly.

GerBra · 2024-03-25 06:30:59

a) put the my_list file outside of the scanned directory

ls > ../my_list

b) let ls ignore the filelist

ls --ignore my_list > my_list

c) use tee to generate my_list

ls | tee my_list

seth · 2024-03-25 08:49:48

Trilby wrote:

This thread is a perfect example of the problem of learning some "rule" without understanding why the rule exists.

To some extent.

The OP wrote:

I need to make a list of all files inside a directory.

and while there's no indication of the reasons for that "need", ls will not necessarily do that - it could be a random alias or function¹ or suffer from the "file contains newline" condition.
So to get a (probably) desired and somewhat predictable output (*esp* outside a script) you'd start by "command ls" and somehow put "-b" in there.

¹column is about to get its ANSI bug fixed at which point the default "ls" will ellipt long filenames here

Trilby · 2024-03-25 13:06:12

GerBra, `ls * > my_list` is far simpler and has already been noted in the thread.

Seth, if were looking for portable usage, the -b flag is not a good idea. That is a non-posix extension.

Last edited by Trilby (2024-03-25 13:08:28)

GerBra · 2024-03-25 16:11:05

Trilby wrote:

`ls * > my_list` is far simpler and has already been noted in the thread.

Hey, shell globbing is a lot more expensive ;-)

mkdir /tmp/tt
cd /tmp/tt
$ touch file{1..10000}
$ time ls --ignore my_list > my_list

real	0m0,015s
user	0m0,011s
sys	0m0,004s
$ time ls * > my_list

real	0m0,047s
user	0m0,022s
sys	0m0,025s

And using --ignore foobar will never include foobar in the generated filelist, even on multiple runs (but then the ls output is not exactly what it should be...<g>)

Duck and away ;-)

Btw: using the Pipe with tee doesn't work on a lot of entities, at some time the my_list file would be created by tee and get listed by ls

raphaelabb · 2024-03-25 16:53:25

The list of files is not to be used for further programming, it's just for me to keep track of certain files I will have to back up in my external HDD once I get it back.
So

ls * > my_list

is the simplest way in that case.
Thank you.

Trilby · 2024-03-25 17:49:17

GerBra, globbing in *your shell* is slow

(bash)$ time ls * > my_list
real	0m0.044s
user	0m0.020s
sys	0m0.023s

(bash)$ time ls --ignore my_list > my_list
real	0m0.018s
user	0m0.004s
sys	0m0.014s

(bash)$ exit

(a real shell)$ time ls * >| my_list
real	0m 0.02s
user	0m 0.00s
sys	0m 0.01s

EDIT: in fact, globbing is faster in my shell than letting ls figure it out:

$ time ls >| my_list 
real	0m 0.03s
user	0m 0.02s
sys	0m 0.01s

Last edited by Trilby (2024-03-25 17:50:58)

seth · 2024-03-25 17:55:51

Though even in A SHell "ls *" will handle directories wrong™ and should™ be lucy in the sky with diamonds, no?

Trilby · 2024-03-26 00:27:42

What do you mean by "wrong"? EDIT: if you mean `ls *` would list contents of subdirectories, then just add the -d flag.

Last edited by Trilby (2024-03-26 01:11:51)

sukolyn · 2024-03-26 01:05:49

ls * goes into direct sub-directories.
and ls -R is recursive, as noticed we don't know if ls is an alias and how it's configured.

seth · 2024-03-26 08:14:24

then just add the -d flag

https://en.wikipedia.org/wiki/Lucy_in_t … SD_rumours

Depending on

keep track of certain files I will have to back up

one also might want to put an "-A" in there, resp. "ls -d .* *" while the exact behavior of the globbing depends on the set shell options.

Arch Linux

#1 2024-03-24 17:45:09

[SOLVED] Redirecting ls output to a file

#2 2024-03-24 18:09:00

Re: [SOLVED] Redirecting ls output to a file

#3 2024-03-24 20:24:50

Re: [SOLVED] Redirecting ls output to a file

#4 2024-03-24 20:38:43

Re: [SOLVED] Redirecting ls output to a file

#5 2024-03-24 20:42:08

Re: [SOLVED] Redirecting ls output to a file

#6 2024-03-24 20:49:33

Re: [SOLVED] Redirecting ls output to a file

#7 2024-03-25 02:23:34

Re: [SOLVED] Redirecting ls output to a file

#8 2024-03-25 02:31:47

Re: [SOLVED] Redirecting ls output to a file

#9 2024-03-25 06:30:59

Re: [SOLVED] Redirecting ls output to a file

#10 2024-03-25 08:49:48

Re: [SOLVED] Redirecting ls output to a file

#11 2024-03-25 13:06:12

Re: [SOLVED] Redirecting ls output to a file

#12 2024-03-25 16:11:05

Re: [SOLVED] Redirecting ls output to a file

#13 2024-03-25 16:53:25

Re: [SOLVED] Redirecting ls output to a file

#14 2024-03-25 17:49:17

Re: [SOLVED] Redirecting ls output to a file

#15 2024-03-25 17:55:51

Re: [SOLVED] Redirecting ls output to a file

#16 2024-03-26 00:27:42

Re: [SOLVED] Redirecting ls output to a file

#17 2024-03-26 01:05:49

Re: [SOLVED] Redirecting ls output to a file

#18 2024-03-26 08:14:24

Re: [SOLVED] Redirecting ls output to a file

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