How to add line number at the beginning of each line

There are many ways to do this, I will try to show some

Sample file '/tmp/file'

Apple  
Apricot  
Avocado  
Banana  
Bilberry  
Blackberry  
Blackcurrant

Method 1

Using 'nl'

# nl /tmp/file  
     1  Apple  
     2  Apricot  
     3  Avocado  
     4  Banana  
     5  Bilberry  
     6  Blackberry  
     7  Blackcurrant

Add a symbol after the numbers

# nl -s ". " /tmp/file  
     1. Apple  
     2. Apricot  
     3. Avocado  
     4. Banana  
     5. Bilberry  
     6. Blackberry  
     7. Blackcurrant

Save the output to a file

# nl -s ". " /tmp/file  > /tmp/newfile

Method 2

Using 'perl'

#  perl -pe 'printf "%u. ", $.' /tmp/file  
1. Apple  
2. Apricot  
3. Avocado  
4. Banana  
5. Bilberry  
6. Blackberry  
7. Blackcurrant

Method 3

Using 'awk'

# awk '{printf "%d. %sn", NR, $0}' /tmp/file  
1. Apple  
2. Apricot  
3. Avocado  
4. Banana  
5. Bilberry  
6. Blackberry  
7. Blackcurrant

# awk '{print NR".",$0}' /tmp/file  
1. Apple  
2. Apricot  
3. Avocado  
4. Banana  
5. Bilberry  
6. Blackberry  
7. Blackcurrant

Method 4

Using 'sed'

# sed '=' /tmp/file | sed '{N;s/n/. /}'  
1. Apple  
2. Apricot  
3. Avocado  
4. Banana  
5. Bilberry  
6. Blackberry  
7. Blackcurrant