Viewing Contents — cat / head / tail / wc

Show the Whole File — cat

cat is the command that prints a file's entire contents to the screen.

Use it when you want to see the whole thing at a glance, like a short config file or memo.

Just pass the filename and it outputs from the first line to the last as-is.

For very long files, on Ubuntu and similar you'd use less to scroll one screen at a time and q to quit.

In this course's console you display the whole file with cat, and for long files you take just the part you need with the head and tail you'll learn next.

echo one > memo.txt      # write line 1
echo two >> memo.txt     # append line 2
echo three >> memo.txt   # append line 3
cat memo.txt             # show every line

① Write line 1 with echo a > sample.txt, then append with echo b >> sample.txt and echo c >> sample.txt to make a 3-line file.

② Show the whole contents with cat sample.txt and confirm the three lines a b c appear. (Run it correctly and an explanation will appear.)

View the Start — head

head takes just the given number of lines from the start of a file.

With no option it shows the first 10 lines, and writing a number after -n limits it to that many.

It's handy for checking the beginning of a long log or the first few records of data.

echo l1 > log.txt     # create material
echo l2 >> log.txt
echo l3 >> log.txt
head -n 2 log.txt     # first 2 lines (l1 l2)

① Create a 3-line file with echo a > data.txt, echo b >> data.txt, and echo c >> data.txt.

② Run head on data.txt with no -n and confirm it shows the first 10 lines by default (all 3 lines, since the file has 3).

③ Then show just the first 2 lines with head (write the line count after -n, with a space).

View the End — tail

tail takes just the given number of lines from the end of a file.

With no option it shows the last 10 lines, and -n sets the number of lines.

It's often used to check the latest lines of a log.

echo l1 > log.txt     # create material
echo l2 >> log.txt
echo l3 >> log.txt
tail -n 2 log.txt     # last 2 lines (l2 l3)

① Create a 3-line file with echo a > data.txt, echo b >> data.txt, and echo c >> data.txt.

② Show just the last 2 lines with tail (write the line count after -n, with a space).

Count Lines — wc

wc is the command that counts words and lines.

Add -l to show just the line count of a file.

You can quickly see how many lines a log has piled up or how many records there are without reading the whole thing.

echo a > data.txt     # create material
echo b >> data.txt
echo c >> data.txt
wc -l data.txt        # line count (3)

① Create a 3-line file with echo a > data.txt, echo b >> data.txt, and echo c >> data.txt.

② Use wc with the line option to show how many lines data.txt has (add the option that means 'line').

Specify the line count with `-n`

When you know how many lines you want with head or tail, specifying it with -n takes exactly the part you want.