In unix you can redirect stdout to file:

samtools mpileup bam > out.txt

I suppose you just want to redirect the output of your command into a file. This is not a bioinformatics question, but a UNIX question.

Anyway, you should use the redirection character (>) and your syntax should look like:

samtools_command -andoptions inputfile > outputfile

try to use the pipeline redirection: http://www.codecoffee.com/tipsforlinux/articles2/042.html

samtools depth file.bam > result.txt