Bài giảng Quản trị Linux: I/O redirection - Đặng Thanh Bình

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Đặng Thanh Bình

I/O Redirection

SIMPLE REDIRECTIONS

Standard Input and Standard Output

• The keyboard is your standard input (stdin)device, and the screen or a particular terminalwindow is the standard output (stdout) device

• These default settings don't necessarily have to

be applied

• The standard output, for example, on a heavilymonitored server in a large environment may be

a printer

The Redirection Operators

• Output redirection with > and |

– Sends the standard output of one command to another command as standard input.

The Redirection Operators

• Output redirection with > and |

– Truncating

The Redirection Operators

• Output redirection with > and |

– Create a new empty file with the given name

The Redirection Operators

• Output redirection with > and |

– To find a word within some text, display all lines matching "pattern1", and exclude lines also matching

"pattern2" from being displayed

– To display output of a directory listing one page at a time

– To find a file in a directory

The Redirection Operators

• Input redirection using the < operator

– Sending a file to somebody

– Similar to

The Redirection Operators

The Redirection Operators

• Combining redirections

– Redirect the output to a file

The Redirection Operators

• The >> operator

– Instead of overwriting file data, you can also append text to an existing file using >>

ADVANCED REDIRECTION FEATURES

Use Of File Descriptors

• There are three types of I/O, which each havetheir own identifier, called a file descriptor:

– Standard input: 0

– Standard output: 1

– Standard error: 2

Use Of File Descriptors

• If the file descriptor number is omitted, and thefirst character of the redirection operator is <, theredirection refers to the standard input (filedescriptor 0)

• If the first character of the redirection operator is

>, the redirection refers to the standard output(file descriptor 1)

Use Of File Descriptors

• Direct both standard output and standard error

to the file dirlist

• The ampersand & serves as an indication that the number that follows is not a file name, but rather

Use Of File Descriptors

• The > sign should not be separated by spacesfrom the number of the file descriptor

– If it would be separated, we would be pointing the output to a file again

• Analyzing errors

• Separating standard output from standard error

– Constructs like these are often used by programmers,

so that output is displayed in one terminal window, and errors in another

• Writing to output and files simultaneously

– tee command: copy input to standard output and one

or more output files in one move

– Using the -a option to tee results in appending input

• Writing to output and files simultaneously

FILTERS

• When a program performs operations on inputand writes the result to the standard output, it iscalled a filter.

• One of the most common uses of filters is torestructure output

More about grep

• Scan the output line per line, searching formatching patterns

• All lines containing the pattern will be printed tostandard output

• This behavior can be reversed using the -v option

• Recursive grep that searches all subdirectories ofencountered directories using the -r option

Filtering Output

• sort arranges lines in alphabetical order

• Directory content is sorted smallest files first,biggest files last

More about sort

• Use -k for sorting according to column (kolumn)

– Sort by column 3:

sort -k3 filename.txt

• By default sort will take space/tabs as fieldseparators But in /etc/passwd file the fieldseparator is : so we have to mention this onewhen sorting a file using –t option

sort -t: -k6 /etc/passwd

More about sort

• Sort according to number, use -n option

– Sort will not understand numbers by default, we have

to use -n to make sure sort command understand it – Ex: sort /etc/passwd file according to UID

More about sort

• Sort the file and display only unique values

sort -u filename

• Sort a file according to some requirements andsave it to a different file

sort -o temp.txt filename.txt

• Sort accourding to human readable numbers(e.g., 2K 1G)

sort -h filename.txt

• Sort according to month of the year

sort -M filename.txt