Standard streams & arguments

Introduction to Bash Scripting

Alex Scriven

Data Scientist

STDIN-STDOUT-STDERR

In Bash scripting, there are three 'streams' for your program:

STDIN (standard input). A stream of data into the program
STDOUT (standard output). A stream of data out of the program
STDERR (standard error). Errors in your program

By default, these streams will come from and write out to the terminal.

Though you may see 2> /dev/null in script calls; redirecting STDERR to be deleted. (1> /dev/null would be STDOUT)

STDIN-STDOUT graphically

Here is a graphical representation of the standard streams, using the pipeline created previously:

Standard Streams diagram

STDIN example

Consider a text file (sports.txt) with 3 lines of data.

football
basketball
swimming

The cat sports.txt 1> new_sports.txt command is an example of taking data from the file and writing STDOUT to a new file. See what happens if you cat new_sports.txt

football
basketball
swimming

STDIN vs ARGV

A key concept in Bash scripting is arguments

Bash scripts can take arguments to be used inside by adding a space after the script execution call.

ARGV is the array of all the arguments given to the program.
Each argument can be accessed via the $ notation. The first as $1, the second as $2 etc.
$@ and $* give all the arguments in ARGV
$# gives the length (number) of arguments

ARGV example

Consider an example script (args.sh):

#!/usr/bash
echo $1
echo $2
echo $@
echo "There are " $# "arguments"

Running the ARGV example

Now running bash args.sh one two three four five

one
two
one two three four five
There are 5 arguments

#!/usr/bash
echo $1
echo $2
echo $@
echo "There are " $# "arguments"

Let's practice!

Introduction to Bash Scripting