Viewing and Reading File Contents

Why read files in the terminal?

Configuration files, logs and scripts are all plain text. On a server you often have no graphical editor, so you need commands that show the contents of a file directly in the terminal.

Make a practice file

First create a small file to experiment with:

printf "apple\nbanana\ncherry\ndate\nelderberry\n" > fruits.txt

printf prints text, and \n means "start a new line". The > symbol sends the output into a file instead of showing it on the screen. This is called redirection and has its own lesson later. After this command, fruits.txt contains five lines, one fruit on each.

cat: print the whole file

cat fruits.txt
cat -n fruits.txt
cat a.txt b.txt

cat prints the entire file. The name comes from "concatenate" because it can join several files one after another. -n adds line numbers. cat is ideal for short files. For long ones the text rushes past before you can read it.

less: read page by page

less /var/log/syslog

less opens a file in a viewer where you can move around. It does not load the whole file at once, so it handles very large logs easily. Keys inside less:

• Space: next page. b: previous page.
• Arrow keys: one line at a time.
• /word: search forward for "word". Then press n for the next match.
• G: jump to the end. g: jump to the start.
• q: quit.

head and tail: the start and end

head fruits.txt
head -n 3 fruits.txt
tail fruits.txt
tail -n 2 fruits.txt

head prints the first 10 lines and tail prints the last 10. The -n option sets how many lines you want. These are useful for logs, where the newest entries are at the end.

tail -f: follow a file

tail -f /var/log/syslog

-f means follow. tail prints the last lines, then keeps running and prints any new line the moment it is added to the file. This is how you watch a log live. Press Ctrl + C to stop.

wc: count lines, words and bytes

wc fruits.txt
wc -l fruits.txt

wc ("word count") prints the number of lines, words and bytes. -l shows only the line count, which answers "how many lines are in this file?".

file: check what a file really is

On Linux a file extension is only a hint in the name. The file command looks at the content and tells you what the file really is.

file fruits.txt
file /bin/ls
file photo.jpg

Typical answers are ASCII text, ELF executable and JPEG image data.

diff: compare two files

diff old.txt new.txt
diff -u old.txt new.txt

diff shows the lines that differ. A line starting with < comes from the first file and a line starting with > comes from the second. If nothing is printed, the files are identical. -u prints the "unified" format, which shows a few lines of context and is easier to read.

Choosing the right tool

• Small file, quick look: cat
• Big file you want to scroll or search: less
• Only the beginning or the end: head or tail
• Watch a log as it grows: tail -f
• How many lines: wc -l
• What kind of file is it: file
• How do two files differ: diff