1.Deleting files older than N number of days ? 
find . -name '*.trc' -mtime +[N in days]  -exec rm {} \;  
Command will delete files older then N days in that directory

2.Copying file keeping timestamp preserve
cp -ip file.txt file_txt_bkp

3.Listing files modified in last N days
find . -mtime -<ndays> -exec ls -lt {} \;

4.Sorting files based on Size of file ?
ls -l | sort -nrk 5  | more
du -sm *|sort -nr|head -10

5.Finding CPU & Memory detail 

->cat /proc/cpuinfo  (CPU)
->cat /proc/meminfo (Memory)

6.Finding if any service is listening on particular port or not ?
netstat -an | grep {port no} 

7.Finding Process ID (PID) associated with any port ?
lsof | grep {port no.} 
lsof should be installed and in path.Many times it will be installed eith root user.Make sure you have that permission.

9.Finding a pattern in some file in a directory ?
grep pattern  file_name  ( find pattern in particular file )
grep -i pattern  file_name (find pattern ignoring the case)

10.Create symbolic link to a file ?
ln -s  pointing_to_original_file  symbolic_link_name

 ln -s /home/text.txt test.txt

11.History of command executed in the unix box
fc -l
fc -e - ls ( Would execute the last ls command.) 

History command will also do the same

12. Compressing a bigfile
zip -r new.zip new
compress file_name

13.Creating a tar file
tar -cvwf file.tar myfile.txt

14.Extracting the files from a tar file
tar -xvwf myfile.tar(System would unarchive (untar) the myfile.tar file into the current directory.)

tar -xvwzf myfile.tar.gz(System would unarchive (untar) the myfile.tar.gz file in the current directory.)

14.Extracting a gz file?
Use guzip command as follows:
gunzip file.gz
gzip -d file.gz

15.Extract a tar.gz or .tgz file?
Files with extension tar.gz or .tgz are tar files compressed with gzip. On Unix system extract them with following command:
gunzip < file.tar.gz | tar xvf -
gunzip < file.tgz | tar xvf -

If you have GNU tar (Linux system) you can use the z option directly:
tar xvzf file.tar.gz
tar xvzf file.tgz

16.Using mailx command
echo "This is going to be body of the mail" |mailx -s "Subject:Testing" "false@gmail.com"

17.Checking the file system free and used space
df -gt