There are several ways you can convert fastq to fasta sequences. Some methods are listed below.

Using SED

sed can be used to selectively print the desired lines from a file, so if you print the first and 2rd line of every 4 lines, you get the sequence header and sequence needed for fasta format.

sed -n '1~4s/^@/>/p;2~4p' INFILE.fastq > OUTFILE.fasta

Using PASTE

You can linerize every 4 lines in a tabular format and print first and second field using paste

cat INFILE.fastq | paste - - - - |cut -f 1, 2| sed 's/@/>/'g | tr -s "/t" "/n" > OUTFILE.fasta

EMBOSS:seqret

Standard script that can be used for many purposes. One such use is fastq-fasta conversion

seqret -sequence reads.fastq -outseq reads.fasta

awk can be used for conversion as follows:

Using AWK

cat infile.fq | awk '{if(NR%4==1) {printf(">%s\n",substr($0,2));} else if(NR%4==2) print;}' > file.fa

FASTX-toolkit

fastq_to_fasta is available in the FASTX-toolkit that scales really well with the huge datasets

fastq_to_fasta -h
usage: fastq_to_fasta [-h] [-r] [-n] [-v] [-z] [-i INFILE] [-o OUTFILE]
# Remember to use -Q33 for illumina reads!
version 0.0.6
       [-h]         = This helpful help screen.
       [-r]         = Rename sequence identifiers to numbers.
       [-n]         = keep sequences with unknown (N) nucleotides.
                   Default is to discard such sequences.
       [-v]         = Verbose - report number of sequences.
                   If [-o] is specified,  report will be printed to STDOUT.
                   If [-o] is not specified (and output goes to STDOUT),
                   report will be printed to STDERR.
       [-z]         = Compress output with GZIP.
       [-i INFILE]  = FASTA/Q input file. default is STDIN.
       [-o OUTFILE] = FASTA output file. default is STDOUT.

Bioawk

Another option to convert fastq to fasta format using bioawk

bioawk -c fastx '{print ">"$name"\n"$seq}' input.fastq > output.fasta

Seqtk

From the same developer, there is another option using a tool called seqtk

seqtk seq -a input.fastq > output.fasta

Note that you can use either compressed or uncompressed files for this tool