Code:

## Read first line from file:
read line < FILENAME

If the line contained leading or trailing spaces, they would be lost with that command. To prevent that, the internal field separator is cleared:

Code:

IFS="" read line < FILENAME

If there are any backslashes in the line, they will be removed and the literal character following will be used. If the last character on the line is a backslash, read will continue reading the next line.

To disable backslash escaping, use the -r option:

Code:

IFS="" read -r line < FILENAME

To read an entire file (if the file is long, sed or awk may be a better tool to use):

Code:

while IFS="" read -r line
do
    ## Do something with the line here
done < FILENAME

The input of other commands can also be redirected from a file. For example, to change all uppercase As to Zs:

Code:

tr A Z < FILENAME