[Linuxcommand-announce] [LinuxCommand.org: Tips, News And Rants] Tip: Redirecting Multiple Command Ou...

[William S. <bs...@pa...>]

Let's imagine a simple script:

#!/bin/bash

echo 1
echo 2
echo 3

Simple enough. It produces three lines of output:

1
2
3

Now let's say we wanted to redirect the output of the commands to a
file named foo.txt $F

Again, pretty straightforward, but what if we wanted to pipe the output
of all three echo commands into less? We would soon discover that this
won't work:


#!/bin/bash

F=foo.txt

echo 1 | less
echo 2 | less
echo 3 | less

This causes less to be executed three times. Not what we want. We want
a single instance of less to input the results of all three echo
commands. There are four approaches to this:

Make A Separate Script
script1:


#!/bin/bash

echo 1
echo 2
echo 3

script2:


#!/bin/bash

script1 | less

By running script2, script1 is also executed and its output is piped
into less. This works but it's a little clumsy.

Write A Shell Function
We could take the basic idea of the separate script and incorporate it
into a single script by making script1 into a shell function:


#!/bin/bash

# shell function
run_echoes () {
echo 1
echo 2
echo 3
}

# call shell function and redirect
run_echoes | less

This works too, but it's not the simplest way to do it.

Make A List
We could construct a compound command using {} characters to enclose a
list of commands:


#!/bin/bash

{ echo 1; echo 2; echo 3; } | less

The {} characters allow us to group the three commands into a single
output stream. Note that the spaces between the {} and the commands, as
well as the trailing semicolon after the third echo, are required.

Launch A Subshell foo.txt &

This doesn't make much sense for our echo commands (they execute too
quickly to bother with), but if we have commands that take a long time
to run, this technique can come in handy.

Enjoy!

--
Posted By William Shotts to LinuxCommand.org: Tips, News And Rants at
4/03/2009 03:02:00 PM