George Adams Blog - Bash: How to Loop Through Lines in a File

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24 June 2010, 23:29

Bash: How to Loop Through Lines in a File
This is a post that I've wanted to do for a long time. It's not something that is obvious, but it is very necessary for a lot of different scripting scenarios. For instance, in one recent project, I had a text file where each line was a sudoku puzzle generated by gnome-sudoku. I wanted to use each line of that file to provide input for a sudoku solver I was writing.

The first thing that I thought to do was use a for-loop similar to how you would read files in a directory:
```
for line in $(cat $filename); do
    echo "$line"
done
```
And that might work for you. But it will not work if you have any spaces in a line, like I did. Basically, spaces of any kind are used as the delimiter. There's a better way that will read the entire line every time.
```
while read line; do
    echo "$line"
done < "$filename"
```
It might seem strange, but you can pipe a file into a while loop. This while loop will use the read command to read lines of input from $filename and assign the current line to the aptly named $line variable. This will continue until EOF is read.
- bash
- tutorial
—geowa4