# Convert FASTQ to FASTA
seqtk seq -a IN.fastq > OUT.fasta
# Convert FASTQ to FASTA and set bases of quality lower than 20 to N
seqtk seq -aQ64 -q20 -n N IN.fastq > OUT.fasta
# Download Seqtk
https://github.com/lh3/seqtk
See also
awk, sed, perl one-liners