Understanding Angle Brackets in Bash

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Understanding Angle Brackets in Bash

Bash provides many important built-in commands, like

Code:

ls

,

Code:

cd

, and

Code:

mv

, as well as regular tools such as

Code:

grep

,

Code:

awk,

and

Code:

sed

. But, it is equally important to know the punctuation marks — the glue in the shape of dots, commas, brackets. and quotes — that allow you to transform and push data from one place to another. Take angle brackets (

Code:

< >

), for example.

Pushing Around

If you are familiar with other programming and scripting languages, you may have used

Code:

<

and

Code:

>

as logical operators to check in a condition whether one value is larger or smaller than another. If you have ever written HTML, you have used angle brackets to enclose tags.

In shell scripting, you can also use brackets to push data from place to place, for example, to a file:

Code:

ls > dir_content.txt

In this example, instead of showing the contents of the directory on the command line,

Code:

>

tells the shell to copy it into a file called dir_content.txt. If dir_content.txt doesn’t exist, Bash will create it for you, but if dir_content.txt already exists and is not empty, you will overwrite whatever it contained, so be careful!

You can avoid overwriting existing content by tacking the new stuff onto the end of the old stuff. For that you use

Code:

>>

(instead of

Code:

>

):

Code:

ls $HOME > dir_content.txt; wc -l dir_content.txt >> dir_content.txt

This line stores the list of contents of your home directory into dir_content.txt. You then count the number of lines in dir_content.txt (which gives you the number of items in the directory) with

Code:

wc -l

and you tack that value onto the end of the file.

After running the command line on my machine, this is what my dir_content.txt file looks like:

Code:

Applications bin cloud Desktop Documents Downloads Games ISOs lib logs Music OpenSCAD Pictures Public Templates test_dir Videos 17 dir_content.txt

The mnemonic here is to look at

Code:

>

and

Code:

>>

as arrows. In fact, the arrows can point the other way, too. Say you have a file called CBActors containing some names of actors and the number of films by the Coen brothers they have been in. Something like this:

Code:

John Goodman 5 John Turturro 3 George Clooney 2 Frances McDormand 6 Steve Buscemi 5 Jon Polito 4 Tony Shalhoub 3 James Gandolfini 1

Something like

Code:

sort < CBActors # Do this Frances McDormand 6 # And you get this George Clooney 2 James Gandolfini 1 John Goodman 5 John Turturro 3 Jon Polito 4 Steve Buscemi 5 Tony Shalhoub 3

Will

Code:

sort

the list alphabetically. But then again, you don’t need

Code:

<

here since

Code:

sort

already expects a file anyway, so

Code:

sort CBActors

will work just as well.

However, if you need to see who is the Coens’ favorite actor, you can check with :

Code:

while read name surname films; do echo $films $name $surname > filmsfirst.txt; done < CBActors

Or, to make that a bit more readable:

Code:

while read name surname films;\ do echo $films $name $surname >> filmsfirst;\ done < CBActors

Let’s break this down, shall we?

• the
  Code:

  while ...; do ... done
  structure is a loop. The instructions between
  Code:

  do
  and
  Code:

  done
  are repeatedly executed while a condition is met, in this case…
  
  
• … the
  Code:

  read
  instruction has lines to read.
  Code:

  read
  reads from the standard input and will continue reading until there is nothing more to read…
  
  
• … And as standard input is fed in via
  Code:

  <
  and comes from CBActors, that means the
  Code:

  while
  loop will loop until the last line of CBActors is piped into the loop.
  
  
• Getting back to
  Code:

  read
  for a sec, the tool is clever enough to see that there are three distinct fields separated by spaces on each line of the file. That allows you to put the first field from each line in the
  Code:

  name
  variable, the second in
  Code:

  surname
  and the third in
  Code:

  films
  . This comes in handy later, on the line that says
  Code:

  echo $films $name $surname >> filmsfirst;\
  , allowing you to reorder the fields and push them into a file called filmsfirst.
  

At the end of all that, you have a file called filmsfirst that looks like this:

Code:

5 John Goodman 3 John Turturro 2 George Clooney 6 Frances McDormand 5 Steve Buscemi 4 Jon Polito 3 Tony Shalhoub 1 James Gandolfini

which you can now use with

Code:

sort

:

Code:

sort -r filmsfirst

to see who is the Coens’ favorite actor. Yes, it is Frances McDormand. (The

Code:

-r

option reverses the sort, so McDormand ends up on top).

We’ll look at more angles on this topic next time!