What happens when you type this command?

$ echo "Hello"

Hello

The echo command accepts the string "Hello" asstandard input, and echoes the string "Hello" back to the terminal as standard output.

Let's learn more about standard input, standard output, and standard error:

1. standard input, abbreviated as stdin, is information inputted into the terminal through the keyboard or input device.

2. standard output, abbreviated as stdout, is the information outputted after a process is run.

3. standard error, abbreviated as stderr, is an error message outputted by a failed process.

Redirection reroutes standard input, standard output, and standard error to or from a different location.

How does redirection work?

$ echo "Hello" > hello.txt

The > command redirects the standard output to a file. Here, "Hello" is entered as the standard input. The standard output "Hello" is redirected by > to the filehello.txt.

$ cat hello.txt

The cat command outputs the contents of a file to the terminal. When you type cat hello.txt, the contents of hello.txt are displayed.

$ cat oceans.txt > continents.txt

> takes the standard output of the command on the left, and redirects it to the file on the right. Here the standard output of cat oceans.txt is redirected tocontinents.txt.

Note that > overwrites all original content incontinents.txt. When you view the output data by typing cat on continents.txt, you will see only the contents of oceans.txt.

$ cat glaciers.txt >> rivers.txt

>> takes the standard output of the command on the left and appends (adds) it to the file on the right. You can view the output data of the file with cat and the filename.

Here, the the output data of rivers.txt will contain

$ cat < lakes.txt

< takes the standard input from the file on the right and inputs it into the program on the left. Here,lakes.txt is the standard input for the cat command. The standard output appears in the terminal.

the original contents of rivers.txt with the content ofglaciers.txt appended to it.

$ cat volcanoes.txt | wc

| is a "pipe". The | takes the standard output of the command on the left, and pipes it as standard input to the command on the right. You can think of this as "command to command" redirection.

Here the output of cat volcanoes.txt is the standard input of wc. in turn, the wc command outputs the number of lines, words, and characters involcanoes.txt, respectively.

$ cat volcanoes.txt | wc | cat > islands.txt

Multiple |s can be chained together. Here the standard output of cat volcanoes.txt is "piped" to thewc command. The standard output of wc is then "piped" to cat. Finally, the standard output of cat is redirected to islands.txt.

You can view the output data of this chain by typingcat islands.txt.

$ sort lakes.txt

sort takes the standard input and orders it alphabetically for the standard output. Here, the lakes insort lakes.txt are listed in alphabetical order.

$ cat lakes.txt | sort > sorted-lakes.txt

Here, the command takes the standard output fromcat lakes.txt and "pipes" it to sort. The standard output of sort is redirected to sorted-lakes.txt.

You can view the output data by typing cat on the filesorted-lakes.txt.

$ uniq deserts.txt

uniq stands for "unique" and filters out adjacent, duplicate lines in a file. Here uniq deserts.txt filters out duplicates of "Sahara Desert", because the duplicate of 'Sahara Desert' directly follows the previous instance. The "Kalahari Desert" duplicates are not adjacent, and thus remain.

$ sort deserts.txt | uniq

A more effective way to call uniq is to call sort to alphabetize a file, and "pipe" the standard output to uniq. Here by pipingsort deserts.txt to uniq, all duplicate lines are alphabetized (and thereby made adjacent) and filtered out.

sort deserts.txt | uniq > uniq-deserts.txt

Here we simply send filtered contents to uniq-deserts.txt, which you can view with the cat command.

$ grep Mount mountains.txt

grep stands for "global regular expression print". It searches files for lines that match a pattern and returns the results. It is also case sensitive. Here, grep searches for "Mount" in mountains.txt.

$ grep -i Mount mountains.txt

grep -i enables the command to be case insensitive. Here, grepsearches for capital or lowercase strings that match Mount inmountains.txt.

The above commands are a great way to get started with grep. If you are familiar with regular expressions, you can use regular expressions to search for patterns in files.

$ grep -R Arctic /home/ccuser/workspace/geography

grep -R searches all files in a directory and outputs filenames and lines containing matched results. -R stands for "recursive". Heregrep -R searches the /home/ccuser/workspace/geographydirectory for the string "Arctic" and outputs filenames and lines with matched results.

$ grep -Rl Arctic /home/ccuser/workspace/geography

grep -Rl searches all files in a directory and outputs only filenames with matched results. -R stands for "recursive" and lstands for "files with matches". Here grep -Rl searches the/home/ccuser/workspace/geography directory for the string "Arctic" and outputs filenames with matched results.

$ sed 's/snow/rain/' forests.txt

$ sed 's/snow/rain/g' forests.txt