Through redirection you can direct the input and output of a command to and from other files and programs, and chain commands together in a pipeline.
Let's begin by taking a closer look at input and output.
In the terminal, after the shell prompt, type
The In the terminal, after the shell prompt, type
$ echo "Hello"
Hello echo command accepts the string "Hello" as standard input, and echoes the string "Hello" back to the terminal as standard output. 2.stdin, stdout and stderr
- standard input, abbreviated as
stdin, is information inputted into the terminal through the keyboard or input device.
- standard output, abbreviated as
stdout, is the information outputted after a process is run.
- standard error, abbreviated as
stderr, is an error message outputted by a failed process.
3.Your first redirect
$ echo "Hello" > hello.txt
The
Hello > 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 file hello.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.4.>
$ cat oceans.txt > continents.txt
$ cat continents.txt
Arctic Ocean
Atlantic Ocean
Indian Ocean
Pacific Ocean
Southern Ocean > 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 to continents.txt.Note that
> overwrites all original content in continents.txt. When you view the output data by typing cat on continents.txt, you will see only the contents of oceans.txt.5.>>
$ 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 the original contents of rivers.txt with the content of glaciers.txt appended to it.
6.<
$ 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.
$ cat volcanoes.txt | wc
17 26 204
| 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 in volcanoes.txt, respectively.
$ cat volcanoes.txt | wc | cat > islands.txt
$ cat islands.txt
17 26 204
|s can be chained together. Here the standard output of cat volcanoes.txt is "piped" to the wc 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 typing
cat islands.txt. 8.sort
$ sort lakes.txt
Caspian Sea
Great Bear Lake
Lake Baikal
Lake Malawi
Lake Michigan
Lake Nicaragua
Lake Superior
Lake Taganyika
Lake Titicaca
Lake Victoria
sort takes the standard input and orders it alphabetically for the standard output. Here, the lakes in sort lakes.txt are listed in alphabetical order.
$ sort lakes.txt | sort > sorted-lakes.txt
$ cat sorted-lakes.txt
Caspian Sea
Great Bear Lake
Lake Baikal
Lake Malawi
Lake Michigan
Lake Nicaragua
Lake Superior
Lake Taganyika
Lake Titicaca
Lake Victoria
Here, the command takes the standard output from cat 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 file sorted-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
uniq is to call sort to alphabetize a file, and "pipe" the standard output to uniq. Here by piping sort deserts.txt to uniq, all duplicate lines are alphabetized (and thereby made adjacent) and filtered out.
sort deserts.txt | uniq > uniq-deserts.txt
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, grep searches for capital or lowercase strings that match Mount in mountains.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
/home/ccuser/workspace/geography/deserts.txt:Arctic Desert
/home/ccuser/workspace/geography/oceans.txt:Arctic Ocean
/home/ccuser/workspace/geography/uniq-deserts.txt:Arctic Desert
/home/ccuser/workspace/geography/continents.txt:Arctic Ocean
grep -R searches all files in a directory and outputs filenames and lines containing matched results. -R stands for "recursive". Here grep -R searches the /home/ccuser/workspace/geography directory for the string "Arctic" and outputs filenames and lines with matched results.
$ grep -Rl Arctic /home/ccuser/workspace/geography
/home/ccuser/workspace/geography/deserts.txt
/home/ccuser/workspace/geography/oceans.txt
/home/ccuser/workspace/geography/uniq-deserts.txt
/home/ccuser/workspace/geography/continents.txt
grep -Rl searches all files in a directory and outputs only filenames with matched results. -R stands for "recursive" and l stands 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 stands for "stream editor". It accepts standard input and modifies it based on an expression, before displaying it as output data. It is similar to "find and replace". Let's look at the expression
's/snow/rain/':s: stands for "substitution". it is always used when usingsedfor substitution.snow: the search string, the text to find.rain: the replacement string, the text to add in place.
sed searches forests.txt
for the word "snow" and replaces it with "rain." Importantly, the above
command will only replace the first instance of "snow" on a line.
$ sed 's/snow/rain/g' forests.txt
g expression, meaning "global". Here sed searches forests.txt for the word "snow" and replaces it with "rain", globally. All instances of "snow" on a line will be turned to "rain".- Redirection reroutes standard input, standard output, and standard error.
- The common redirection commands are:
>redirects standard output of a command to a file, overwriting previous content.>>redirects standard output of a command to a file, appending new content to old content.<redirects standard input to a command.|redirects standard output of a command to another command.
- A number of other commands are powerful when combined with redirection commands:
sort: sorts lines of text alphabetically.uniq: filters duplicate, adjacent lines of text.grep: searches for a text pattern and outputs it.sed: searches for a text pattern, modifies it, and outputs it.
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