>

$ echo "Hello" > hello.txt

The > command redirects the standard output to a file. Here, "Hello" is enered as the standard input, and is then redirected to the file hello.txt by >.

$ cat milk.txt > coffee.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 milk.txt is redirected to coffee.txt.

Note that > overwrites all original content in coffee.txt.

>>

$ cat sugar.txt >> coffee.txt

>> takes the standard output of the command on the left, and adds it to the file on the right. Here, the output data of coffee.txt will contain the original contents of coffee.txt with the contents of the sugar.txt file appended to it.

| (pipe)

The | takes the standard output of the command on the left, and pipes it as standard input to the command on the right.

$ cat countries.txt | wc

Above, the output of cat countries.txt becomes the standard input of wc. In turn, the wc, (word count) command outputs the number of lines, words and characters in countries.txt.

sort

sort takes the standard input and orders it alphabetically for the standard output (it doesn't change the file itself.)

$ sort countries.txt | wc

Here the countries in countries.txt will be listed in alphabetical order.

uniq

$ uniq articles.txt

uniq stands for unique. It filters out adjacent duplicate lines in a file. And duplicates that are not adjacent remain in the file.

$ sort articles.txt | uniq

So, a more effective way to use uniq is to call sort to alphabetize a file and 'pipe' the standard output to uniq.

$ sort articles.txt | uniq > unique_articles.txt

Above, by piping sort articles.txt to uniq, all duplicate lines are alphabetized (and thereby made adjacent) and filtered out. Then, we send the filtered contents to unique_articles.txt, which you can view with the cat command.

grep

grep stands for global regular expression print. It searches files for lines that match a pattern and then returns the results. It is also case sensitive.

Below, grep searches for anything that matched "React" in libraries.txt.

$ grep React libraries.txt

grep -i enables the grep command to be case insensitive. Below, grep searched for capital or lowercase strings that match "React" in libraries.txt.

$ grep -i React libraries.txt

sed

sed stands for "stram editor". It accepts standard input and modifies it based on an expression, before displaying it as output data. It is similar to "find & replace".

sed 's/Angular/React/' libraries.txt

• s stands for substitution.
• Angular is the search string.
• React is the replacement string

In this case, sed searches libraries.txt for the word "Angular" and replaces it with "React". More importantly, the above command will replace only the first occurence of the word "Angular".

To remove the search string globally, we use the g expression as shown below:

sed 's/Angular/React/g' libraries.txt

sed, as we have used, only rewrites the command line output and the actual file is not changed. In order to rewrite the actual file, we need to use -i at the beginning of the command.

sed -i 's/Angular/React/g' libraries.txt

The above command will rewrite libraries.txt and will replaces all instances of "Angular" with "React".