Supress specific output lines of a command

Question

I wanted to search all the files starting with chrom in /var directory. So I executed this

$ ls -R | grep "^chrom"

To my astonishment, It showed this. Few starting lines look like this :

amit@ElMaverick:/var$ ls -R | grep "^chromium"

chromium-browser_55.0.2883.87-0ubuntu0.16.04.1263_amd64.deb
chromium-browser-l10n_55.0.2883.87-0ubuntu0.16.04.1263_all.deb
chromium-codecs-ffmpeg-extra_55.0.2883.87-0ubuntu0.16.04.1263_amd64.deb
ls: cannot open directory './cache/apt/archives/partial': Permission denied
ls: cannot open directory './cache/cups': Permission denied
ls: cannot open directory './cache/ldconfig': Permission denied
chromium-browser_chromium-browser.png
chromium-browser_chromium-browser.png
chromium-bsu_chromium-bsu.png
chromium-browser_chromium-browser.png
ls: cannot open directory './lib/apt/lists/partial': Permission denied
ls: cannot open directory './lib/bluetooth/78:0C:B8:46:10:D5': Permission denied
chromium-browser.conffiles
chromium-browser-l10n.list
chromium-browser-l10n.md5sums
chromium-browser.list
chromium-browser.md5sums
chromium-browser.postinst
chromium-browser.prerm
chromium-codecs-ffmpeg-extra.list
chromium-codecs-ffmpeg-extra.md5sums
ls: cannot open directory './lib/mysql': Permission denied
ls: cannot open directory './lib/mysql-files': Permission denied

I don't want to see these lines in the output.

ls: cannot open directory

So, I have 2 questions here :

1. How to filter only those lines which have no error of any type ?
2. Is there a way to supress the specific lines of output ?

Please provide a general approach for the problem, not just specific to this question.

Answer 1

1. Redirect the standard error stream of ls to /dev/null:

   ls -R 2>/dev/null | grep '^chrom'
   

2. Do further grepping:

   ls -R 2>/dev/null | grep '^chrom' | grep -v 'some additional pattern'
   

You may also consider using find (Why *not* parse `ls`?):

find . -name 'chrom*' ! -name 'some additional pattern' 2>/dev/null

Note that find uses filename globbing patterns while grep uses regular expressions. With find you may also look specifically for files (with -type f) or for directories (-type d) etc., e.g.

find . -type f -name 'chrom*' ! -name 'some additional pattern' 2>/dev/null

Each part of the find command acts like a "test" against the found pathname. -type tests against the file type while -name tests against the final filename component of the path. The tests are logically AND-ed together (unless -o is used). An exclamation point inverts the sense of a test.

Answer 2

Better use find it's better suites for this kind of tasks :

As simple user (because you don't have permission to read files as non root):

$ find /var -type f -name 'chrom*' 2>/dev/null

Or as root :

# find /var -type f -name 'chrom*'

And if you want to filter out a specific error in bash, use :

command 2> >(grep -v 'specific error')

Warning

Never parse ls output !