You need to know how many lines, words, or characters are in a given file.

Use the wc (word count) command with awk in a command substitution.

The normal output of wc is something like this:

$ wc data_file
       5     15       60 data_file

# Lines only
$ wc -l data_file
       5 data_file

# Words only
$ wc -w data_file
      15 data_file

# Characters (often the same as bytes) only
$ wc -c data_file
      60 data_file

# Note 60B
$ ls -l data_file
-rw-r--r--  1 jp  users   60B Dec    6 03:18 data_file

You may be tempted to just do something like this:

data_file_lines=$(wc -l "$data_file")

That won’t do what you expect, since you’ll get something like "5 data_file" as the value. Instead, try this:

data_file_lines=$(wc -l "$data_file" | awk '{print $1}')

If your version of wc is locale aware, the number of characters will not equal the number of bytes in some character sets.