Reading a file line by line using bash

Welcome to this weeks edition of practical shell scripting. This week I'm going to give a small simple example of a how to read a file one line at a time using bash. As with my other previous and hopefully future blog entries I'm trying to keep this down to just a single small topic and try to give a good example of it without clouding it with a lot of other things.

What prompted this weeks entry was a side project I had over the weekend to update one of my servers at home that required me to download a large number of files from an http server. However, I did not want to go to each file one at a time and click on it and download through the web so I viewed the source of the html file, saved it, and stripped out the html until I had a file that was nothing more than a list of all the files that I wanted to download. Once I had that file I fed it into a much more complicated script that retrieved those files from the http server using wget.

For the sake of this example we're going to be looking at just a tiny part of that and I've turned it into a generic example to show how bas reads line by line from a file called grocery.list.

First, here's what's in the file called grocery.list :

apples
oranges
bananas
pears
juice

Now here's the bash script to read it:

#!/bin/bash
###-------------------------------------------------###
#
# this is a simple bash script to show
# how to read from a file
#
###-------------------------------------------------###

INPUT_FILE="$1"

while read curline; do
echo $curline

done < "$INPUT_FILE"



....and that's all there is to it. So what we have here is a simple looping construct that goes one line at a time and steps through the input file that is given as the first argument or $1. In his example "curline" is the variable that the data is read into. This can be confusing because "curline" does not need to be declared a head of time, and in fact in all the shell scripts I have seen I can not recall a single instance of ever being declared up front. The other thing that has always been a little counter intuitive to me is that file being read from is always at the bottom of the while loop which is taking it's input in the done line. Most of us who come from a more formal language like C or C++ are used to having the file declared and read from someplace closer to the top, and this always seems odd to me.


So to execute this example you would type:
./ReadLineExample.sh grocery.list

And the screen should show exactly what is in the file. While there are a great many tools in bash for parsing and working with files this is one of the most versatile since it allows simple usage of a looping construct.

So that's it for this week. As always if you have any questions or comments please feel free to contact me. Happy coding.