Read length distribution from FASTA file
If you want something quick and dirty you could rapidly index the FASTA with samtools faidx and then put the lengths column through R (other languages are available) on the command line.

samtools faidx $fasta
cut -f2 $fasta.fai | Rscript -e 'data &lt;- as.numeric (readLines ("stdin")); summary(data); hist(data)'


This outputs a statistical summary, and creates a PDF in the current directory called Rplots.pdf, containing a histogram.