We’ve seen how to redirect input from a file and
output to a file. You can also connect two
together so that the output from one
program becomes the input of the next program. Two or more programs
connected in this way form a
To make a pipe, put a vertical bar
) on the command line between two
commands. When a pipe is set up between two commands, the standard
output of the command to the left of the pipe symbol becomes the
standard input of the command to the right of the pipe symbol. Any
two commands can form a pipe as long as the first program writes to
standard output and the second program reads from standard input.
When a program takes its input from another program, performs some operation on that input, and writes the result to the standard output (which may be piped to yet another program), it is referred to as a filter . A common use of filters is to modify output. Just as a common filter culls unwanted items, Unix filters can restructure output.
Most Unix programs can be used to form pipes. Some programs that are commonly used as filters are described in the next sections. Note that these programs aren’t used only as filters or parts of pipes. They’re also useful on their own.
program searches a file or files for
lines that have a certain pattern. The syntax is:
The name “grep” is derived from the
ed (a Unix line editor) command
g/re/p, which means
"globally search for a regular expression and print ...