UNIX Filters Reference

A file stream is a stream of bytes coming from a file; think of it as a "Netflix" view of a file. Both stdin and stdoout are file streams. The file stream stdin is an input file stream and the file stream stdout is an output file stream.

UNIX filters are commands that create or transform file streams. They have as input a file stream and as output a file stream. File streams can be captured into a file; by default they are put to stdout. All UNIX filters have these common features.

They expect an input. The default is stdin, but they often get input from a pipe (|). If you pass them a file or a wildcard as an argument, a file stream is created from that file.
Filters "filter in" data from their input to produce their output.
They produce output. The default is for the output to go to stdout (the screen). Their output can be sent to a pipe (|) or to a file using > or >>.

The figure below illustrates the action of a UNIX filter.


                               ----------------
input       ----------->       |  FILTER      | ----------> output
                               ----------------              

This is the input                                            This is the filter's
file stream.                                                 file output stream.

By default, this is                                          By default, this is
stdin.                                                       stdout.

This can also come from                                      This can go to a pipe,
a pipe, which makes the                                      in which case it 
preceding filter's output stream                             becomes the next
this filter's input stream                                   filter's input stream.

Filter Cheat Sheet
Filter	Option	Action
`cat`		This is the non-filter filter; it shows the entire file. You can give it one or more files as arguments and they will be placed in a file stream `in seriatum`.
`cat`	-n	shows output with line numbers
`head`		This filters in lines at the top of a file. By default, it shows ten lines. Use `head -n` to show the first `n` lines of a file
`tail`		This filters in lines at the bottom of a file. By default, it shows ten lines. Use `tail -n` to show the last `n` lines of a file
`grep`		This filters in lines containing its first argument as a substring. The second argument should be the file you want to filter. You can rummage through several files in seriatum if you pass them as arguments. You can specify these files with a wildcard such as `*.java` (all files ending with `.java` Usage: `grep searchString file(s).`
	-c	prints the number of lines found from grep.
	-i	ignore case
	-l	prints the file name (only) if `grep` makes a match in the file
	-n	prints line numbers along with lines from file
	-v	prints all lines NOT containing the search string you are grepping for.
	-w	searches for a whole word only
`sort`		This sorts the lines that are its input asciicographially, case insensitive.
	-b	ignore leading blanks
	-n	sort numerically (use for files with numbers)
	-r	reverse sorting order
`uniq`		removes neighboring duplicate lines. You almost always want ot sort first.
	-c	Precede each line with the number of times it was duplicated in the input file.
	-d	Only output duplicated lines
	-i	case-insensitive comparison of adjacent lines
`wc`		counts words, lines, and characters of its input
	-c	count only characters
	-w	count only words
	-l	count only lines
`tr`		transforms strings Two arguments are required, strings of equal length. Each character the input stream matching a character in the first string is translated to the corresponding character in the second string. For example `tr abc xyz` changes a to x, b to y, and c to z. You can pass a range such as a-z as an argument. For example, tr a-z A-Z uppercases all alphas.
	-d	When using this option, give `tr` a string as an argument and all characters in that string are deleted.
	-cd	When using this option, give `tr` a string as an argument and all characters not in that string are deleted.

Connector Cheat Sheet
Connector	Usage	Action
>	`command > file`	redirects `command`'s output to the file `file`, performing a destructive overwrite
>>	`command >> file`	appends the output of `command` the file `file`
\|	`command1 \| command2`	redirects the output of `command1` to be the input of `command2`