Let’s use the tools we’ve added to our tool kit so far, along with a few new ones, to example our SRA metadata file. First, let’s navigate to the correct directory.
$ cd
$ cd shell_data/sra_metadata
This file contains a lot of information about the samples that we submitted for sequencing. We took a look at this file in an earlier lesson. Here we’re going to use the information in this file to answer some questions about our samples.
The samples that we submitted to the sequencing facility were a mix of single and paired end
libraries. We know that we recorded information in our metadata table about which samples used
which library preparation method, but we don’t remember exactly where this data is recorded.
Let’s start by looking at our column headers to see which column might have this information. Our
column headers are in the first row of our data table, so we can use head with a -n flag to
look at just the first row of the file.
$ head -n 1 SraRunTable.txt
BioSample_s InsertSize_l LibraryLayout_s Library_Name_s LoadDate_s MBases_l MBytes_l ReleaseDate_s Run_s SRA_Sample_s Sample_Name_s Assay_Type_s AssemblyName_s BioProject_s Center_Name_s Consent_s Organism_Platform_s SRA_Study_s g1k_analysis_group_s g1k_pop_code_s source_s strain_s
That is only the first line of our file, but because there are a lot of columns, the output
likely wraps around your terminal window and appears as multiple lines. Once we figure out which
column our data is in, we can use a command called cut to extract the column of interest.
Because this is pretty hard to read, we can look at just a few column header names at a time by combining the | redirect and cut.
$ head -n 1 SraRunTable.txt | cut -f1-4
cut takes a -f flag, which stands for “field”. This flag accepts a list of field numbers,
in our case, column numbers. Here we are extracting the first four column names.
BioSample_s InsertSize_l LibraryLayout_s Library_Name_s
The LibraryLayout_s column looks like it should have the information we want. Let’s look at some
of the data from that column. We can use cut to extract only the 3rd column from the file and
then use the | operator with head to look at just the first few lines of data in that column.
$ cut -f3 SraRunTable.txt | head -n 10
LibraryLayout_s
SINGLE
SINGLE
SINGLE
SINGLE
SINGLE
SINGLE
SINGLE
SINGLE
PAIRED
We can see that there are (at least) two categories, SINGLE and PAIRED. We want to search all entries in this column
for just PAIRED and count the number of matches. For this, we will use the | operator twice
to combine cut (to extract the column we want), grep (to find matches) and wc (to count matches).
$ cut -f3 SraRunTable.txt | grep PAIRED | wc -l
2
We can see from this that we have only two paired-end libraries in the samples we submitted for sequencing.
Exercise
How many single-end libraries are in our samples?
Solution
$ cut -f3 SraRunTable.txt | grep SINGLE | wc -l35
We can extract even more information from our metadata table if we add in some new tools: sort and uniq. The sort command will sort the lines of a text file and the uniq command will
filter out repeated neighboring lines in a file. You might expect uniq to
extract all of the unique lines in a file. This isn’t what it does, however, for reasons
involving computer memory and speed. If we want to extract all unique lines, we
can do so by combining uniq with sort. We’ll see how to do this soon.
For example, if we want to know how many samples of each library type are recorded in our table,
we can extract the third column (with cut), and pipe that output into sort.
$ cut -f3 SraRunTable.txt | sort
If you look closely, you might see that we have one line that reads “LibraryLayout_s”. This is the
header of our column. We can discard this information using the -v flag in grep, which means
return all the lines that do not match the search pattern.
$ cut -f3 SraRunTable.txt | grep -v LibraryLayout_s | sort
This command returns a sorted list (too long to show here) of PAIRED and SINGLE values. We can use
the uniq command to see a list of all the different categories that are present. If we do this,
we see that the only two types of libraries we have present are labelled PAIRED and SINGLE. There
aren’t any other types in our file.
$ cut -f3 SraRunTable.txt | grep -v LibraryLayout_s | sort | uniq
PAIRED
SINGLE
If we want to count how many of each we have, we can use the -c (count) flag for uniq.
$ cut -f3 SraRunTable.txt | grep -v LibraryLayout_s | sort | uniq -c
2 PAIRED
35 SINGLE
Exercise
- How many different sample load dates are there?
- How many samples were loaded on each date?
Solution
There are two different sample load dates.
cut -f5 SraRunTable.txt | grep -v LoadDate_s | sort | uniq25-Jul-12 29-May-14Six samples were loaded on one date and 31 were loaded on the other.
cut -f5 SraRunTable.txt | grep -v LoadDate_s | sort | uniq -c6 25-Jul-12 31 29-May-14
We might want to re-order our entire metadata table so that all of the paired-end samples appear
together and all of the single-end samples appear together. We can use the -k (key) flag for sort to
sort based on a particular column. This is similar to the -f flag for cut.
Let’s sort based on the third column (-k3) and redirect our output to a new file.
$ sort -k3 SraRunTable.txt > SraRunTable_sorted_by_layout.txt
We also might want to extract the information for all samples that meet a specific criterion
(for example, are paired-end) and put those lines of our table in a new file. First, we need
to check to make sure that the pattern we’re searching for (“PAIRED”) only appears in the column
where we expect it to occur (column 3). We know from earlier that there are only two paired-end
samples in the file, so we can grep for “PAIRED” and see how many results we get.
$ grep PAIRED SraRunTable.txt | wc -l
2
There are only two results, so we can use “PAIRED” as our search term to extract the paired-end samples to a new file.
$ grep PAIRED SraRunTable.txt > SraRunTable_only_paired_end.txt
Exercise
Sort samples by load date and export each of those sets to a new file (one new file per unique load date).
Solution
$ grep 25-Jul-12 SraRunTable.txt > SraRunTable_25-Jul-12.txt $ grep 29-May-14 SraRunTable.txt > SraRunTable_29-May-14.txt