Working with Files and Pipes

Introduction

Command line tools are small and do one job each. Their real power comes from combining them, like building with blocks.

Reading files

$ cat notes.txt          # print the whole file
$ head -n 3 notes.txt    # first 3 lines
$ tail -n 3 notes.txt    # last 3 lines
$ less notes.txt         # scroll a long file; press q to quit
$ wc -l notes.txt        # count the lines

Copy, move, rename and delete

$ cp notes.txt backup.txt     # copy
$ mv backup.txt old.txt       # move or rename
$ rm old.txt                  # remove (delete)
$ rm -r practice              # remove a folder and everything inside

Warning: rm does not use a recycle bin. Deleted files are gone. Double check the name before pressing Enter, and be especially careful with rm -r.

Redirection

Use > to send output into a file instead of the screen:

$ echo "buy milk" > todo.txt     # create or overwrite
$ echo "call mum" >> todo.txt    # append to the end
$ cat todo.txt
buy milk
call mum

Be careful: a single > replaces the file contents.

Pipes

The pipe | sends the output of one command into the next:

$ ls | wc -l                      # how many items are in this folder?
$ cat todo.txt | sort             # sort the lines
$ history | grep "git"            # find earlier commands containing git

grep searches text for a pattern. For example, grep "milk" todo.txt prints lines that contain the word milk.

Try it yourself

Create a file with three lines using echo and >>. Use sort to show them in order, and wc -l to count them. Then use grep to find one word.

Summary

• cat, head, tail, less and wc read files.
• cp, mv and rm copy, move and delete, and rm is permanent.
• > and >> redirect output to files, and | connects commands together.