day_11.md

Day 11

Notes

• updatedb needs to be run because I had made changes to the config files under /etc/logrotate.d which have configs for apache2, apt, ufw, etc. The configs affect the files under /var/log which has files with substring access.log under apache2/access.log. Now running locate access.log will list all such files which have access.log as a substring.

• I cannot seem to fing how updatedb command runs (by cron or systemd?). There doesn't seem to be a reasonable entry in /etc/cron.weekly or /etc/cron.daily and the files under /run/systemd are complex for me to understand at the moment.

• There are 3 kind of "timestamps" (output of stat <filename>) -

  Access - the last time the file was read

  Modify - the last time the file was modified (content has been modified)

  Change - the last time meta data of the file was changed (e.g. permissions)

• find uses the permissions of the logged-in user you’ll get “permission denied” messages for many directories if you search the whole system.

• The output (stdout) produced by find and the "Permission denied" error (stderr) produced by it go on two different streams. Thus, what grep filters is the output (stdout) only and not the errors (stderr). They (errors) show up as they are. Therefore, we need to do this,

find /home -mtime 0  2>&1 | grep -vi "Permission denied"

This takes in stderr stream to the stdout then grep can remove the unwanted things. This syntax should be used with find to filter through errors and outputs.

Source, nixCraft

• On the above lines, we can use grep in the same way, but this now searches for a substring delaycompress in the content of a text file by reading recursively all files under /etc/.

grep -R -i "delaycompress" /etc/* 2>&1 | grep -vi "Permission denied"

• Because grep -R only works on plain text files, it's most useful for the /etc and /var/log folders.

• Know to use -exec option of find command.

Inspiration

My pull request (on the above discussion of filtering out errors from find with grep) was merged. I am so happy. 🤩

Readings

• 25 find command examples...
• 10 Tips for using "find"
• Five simple recipes for "grep"