Managing Output with Pipes¶

Previous Section: Managing Processes

When dealing with processes, the output of those processes is important. In this section, we will discuss how to manage the output of processes in different ways.

The main concepts that we will go over are: * File descriptors and standard input/output * Input/output redirection * Special files * Command pipelining * Useful commands for processing output

Programs often write output to the console, this is called standard output. The shell defines stdin, stdout, and stderr for every program.

File descriptors are integers that are used by processes to attach to open files.

Standard File Descriptors¶

#	Common Name	Description
0	stdin	Attached to programs for input data
1	stdout	Attached to programs for output data
2	stderr	Attached as a secondary output for errors

You can also redirect these file descriptors to attach to files other than the console.

Redirection Notation¶

Notation	Meaning
`< FILE`	Read `stdin` from `FILE`
`> FILE`	Write `stdout` to `FILE`
`>> FILE`	Append `stdout` to `FILE`
`2> FILE`	Write `stderr` to `FILE`
`2>> FILE`	Append `stderr` to `FILE`
`\| PROGRAM`	Join `stdout` to `stdin` of `PROGRAM`

The following example redirects "Ooh, so scary!" from the console into the file message.txt.

$ boo > message.txt

You can concatenate (print) the contents of files with the cat program.

$ cat message.txt
Ooh, so scary!

To reduce output, you can count the number of words, lines, chars, etc. with the wc program.

$ cat message.txt | wc --chars
15

Useful Programs¶

Program	Meaning
`cat`	Concatenate (or slice) one or more files
`head/tail`	Limit output to first/last `N` lines (default 10)
`less/more`	Page output interactively (for ease of viewing)
`cut`	Slice columns from input by delimiter
`sort/uniq`	Reorder and filter input
`grep/sed`	Filter/modify input based on regular expression
`awk`	Streaming programming language (superpowers)
`wc`	Reduce output by counting (words, lines, etc)
`xargs`	Transpose output as positional arguments to next program

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