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You might find that in your daily computer work, there are some commands that are long and you execute quite often. Sometimes it may also be a series of commands that you execute in a row that you do quite often. It may be worth it to write a custom function that wraps these lines of code into a short, so that you do not have to type all of these commands anymore.

We have already discussed one such function in a previous section:
<code>
# easier viewing of tab separated files
function coltab {
    column -t -s $'\t' $1 | less -S
}
</code>

I will discuss here some more custom functions that I found are very useful in my daily workflow. To add these functions to your system, simply add them to your ''.bashrc''

===Selecting or removing sequences from a FASTA file===

<code>
# fish out a sequence from a fasta file
function grabseq {
    fasta=$3
    to_grab=$2
    case "$1" in
        -s) seqtk subseq $fasta <(grep    "$to_grab" $fasta | sed -e 's/>//' -e 's/ .*//');;
        -l) seqtk subseq $fasta <(grep -f "$to_grab" $fasta | sed -e 's/>//' -e 's/ .*//');;
        *) echo -e "grabseq -s <seqname> <fasta> to grab a single sequence\ngrab_seq -l <seqlist> <fasta> to grab a list of sequences"
    esac
}
</code>

This is essentially a wrapper for the [[https://github.com/lh3/seqtk|seqtk]] tool.

<code>
# select any sequence that has <pattern> in the header
# useful if you want to find a single sequence
$ grabseq -s <pattern> <FASTA> > <NEWFASTA>

# select a particular set of sequences that have <pattern> in their header and are in the <pattern_list> file
# useful if you want to multiple sequences
$ grabseq -l <pattern_list> <FASTA> > <NEWFASTA>
</code>

The next function does the opposite of grabseq. It will remove particular sequences from a FASTA file.

<code>
# remove a particular entry from a fasta file
function rmseq {
    fasta=$3
    to_rmv=$2
    case "$1" in
        -s) seqtk subseq $fasta <( grep ">" $fasta | grep -E -v "$to_rmv" | sed "s/>//" );;
        -l) seqtk subseq $fasta <( grep ">" $fasta | grep -v -f "$to_rmv" | sed "s/>//" );;
        *) echo -e "rmseq -s <seqname> <fasta> to remove a single sequence\nremove_seq -l <seqlist> <fasta> to remove a list of sequences"
    esac
}
</code>

<code>
# remove a particular sequence that has <pattern> in the header
$ rmseq -s <pattern> <FASTA> > <NEWFASTA>

# remove a particular set of sequences that have <pattern> in the header and are in the <pattern_list> file
$ rmseq -l <pattern_list> <FASTA> > <NEWFASTA>
</code>

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