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Command line tools for large tabular data files.

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Tips and Tricks


Useful bash aliases and shell scripts

Bash aliases

A bash alias is a keystroke shortcut known by the shell. They are setup in the user's ~/.bashrc or another shell init file. A convenient alias when working with TSV files is tsv-header which lists the field numbers for each field in a TSV file. To define it, put the following in ~/.bashrc or other init file:

tsv-header () { head -n 1 "$@" | tr $'\t' $'\n' | nl -ba ; }

Once this is defined, use it as follows:

$ tsv-header worldcitiespop.tsv
     1	Country
     2	City
     3	AccentCity
     4	Region
     5	Population
     6	Latitude
     7	Longitude

A similar alias can be setup for CSV files. Here are two. The first uses csv2tsv to interpret the CSV header line, including CSV escape characters. The second uses only standard Unix tools. It won't interpret CSV escapes, but many header lines don't use escapes. (Define only one):

csv-header () { csv2tsv "$@" | head -n 1 | tr $'\t' $'\n' | nl -ba ; }
csv-header () { head -n 1 "$@" | tr $',' $'\n' | nl -ba ; }

Many useful aliases can be defined. Here is another the author finds useful with TSV files. It prints a file excluding the first line (the header line):

but-first () { tail -n +2 "$@" ; }

These aliases can be created in most shells. Non-bash shells may have a different syntax though.

Shell scripts

Shell scripts are an alternative to bash aliases. They function similarly, but shell scripts are preferred for longer scripts or commands that need to be invoked by other programs. Here's an example of the tsv-header command written as a script rather than an alias. It extends the alias by printing help when invoked with the -h or --help options.

if [ "$1" == "-h" ] || [ "$1" == "--h" ] || [ "$1" = "--help" ]; then
    program_filename=$(basename $0)
    echo "synopsis: $program_filename <tsv-file>"
    echo ""
    echo "Print field numbers and header text from the first line of <tsv-file>."
    head -n 1 "$@" | tr $'\t' $'\n' | nl -ba

Put the above in the file tsv-header somewhere on the PATH. A common location is the ~/bin directory, but this is not required. Run the command $ chmod a+x tsv-header to make it executable. Now it can be invoked just like the alias version of tsv-header.

The are a couple of simple sample scripts in the Customize the sort command section.

macOS: Install GNU versions of Unix command line tools

If you're using a Mac, one of best things you can do is install GNU versions of the typical Unix text processing tools. cat, cut, grep, awk, etc. The versions shipped with macOS are older and quite slow compared to the newer GNU versions, which are often more than five times faster. The 2017 Comparative Benchmarks includes several benchmarks showing these deltas.

The Homebrew and MacPorts package managers are good ways to install these tools and many others. Useful packages for data processing include:

Note that in many cases the default installation process will install the tools with alternative names to avoid overriding the built-in versions. This is often done by adding a leading g. For example, gawk, gsort, ggrep, gwc, etc. Each package manager provides instructions for installing using the standard names.

Customize the sort command

The standard Unix sort utility works quite well on TSV files. The syntax for sorting on individual fields (-k|--key option) takes getting used to, but once learned sort becomes a very capable tool. However, there are few simple tweaks that can improve convenience and performance.

Install an updated sort utility (especially on macOS)

Installing an up-to-date utility is a worthwhile step on all platforms. This is especially so on macOS, as the the default sort program is a bit slow. The sort utility available as part of GNU Core Utils is typically quite a bit faster. As of late 2019, the current GNU sort (version 8.31) is often more than twice as fast as the sort utility shipped with OS X Mojave.

Use your system's package manager to upgrade to the latest sort utility and consider installing GNU sort if it's not currently on your system. (Two popular package managers on the Mac are Homebrew and MacPorts.) Note that in some cases the GNU sort routine may be installed under a different name than the built-in sort utility, typically gsort.

Specify TAB as a delimiter in a shell command

Unix sort utilities are able to sort using fields as keys. To use this feature on TSV files the TAB character must be passed as the field delimiter. This is easy enough, but specifying it on every sort invocation is a nuisance.

The way to fix this is to create either a bash alias or a shell script. A shell script is a better fit for sort, as shell commands can be invoked by other programs. This is convenient when using tools like keep-header.

Put the lines below in a file, eg. tsv-sort. Run $ chmod a+x tsv-sort and add the file to the PATH:

file: tsv-sort

sort -t $'\t' "$@"

Now tsv-sort will run sort with TAB as the delimiter. The following command sorts on field 2:

$ tsv-sort worldcitiespop.tsv -k2,2

Set the buffer size for reading from standard input

GNU sort uses a small buffer by default when reading from standard input. This causes it to run much more slowly than when reading files directly. On the author's system the delta is about 2-3x. This will happen when using Unix pipelines. The keep-header tool uses a pipe internally, so it is affected as well. Examples:

$ grep green file.txt | sort
$ keep-header file.txt -- sort

Most of the performance of direct file reads can be regained by specifying a buffer size in the sort command invocation. The author has had good results with a 2 GB buffer on machines having 16 to 64 GB of RAM, and a 1 GB buffer obtains most of improvement. The change to the above commands:

$ grep green file.txt | sort --buffer-size=2G
$ keep-header file.txt -- sort --buffer-size=2G

These can be added to the shell script shown earlier. The revised shell script:

file: tsv-sort

sort  -t $'\t' --buffer-size=2G "$@"

Now the commands are once again simple and have good performance:

$ grep green file.txt | tsv-sort
$ keep-header file.txt -- tsv-sort

Remember to use the correct sort program name if an updated version has been installed under a different name. This may be gsort on some systems.

A sample implementation of this script can be found in the extras/scripts directory in the tsv-utils GitHub repository. This sample script is also included in the prebuilt binaries package.

More details: The --buffer-size option may affect sort programs differently depending on whether input is being read from files or standard input. This is the case for GNU sort, perhaps the most common sort program available. This is because by default sort uses different methods to choose an internal buffer size when reading from files and when reading from standard input. --buffer-size controls both. On a machine with large amounts of RAM, e.g. 64 GB, picking a 1 or 2 GB buffer size may actually slow sort down when reading from files while speeding it up when reading from standard input. The author has not experimented with enough systems to make a universal recommendation, but a bit of experimentation on any specific system should help. GNU sort has additional options when optimum performance is needed.

Turn off locale sensitive sorting when not needed

GNU sort performs locale sensitive sorting, obeying the locale setting of the shell. Locale sensitive sorting is designed to produce standard dictionary sort orders across all languages and character sets. However, it is quite a bit slower than sorting using byte values for comparisons, in some cases by an order of magnitude.

This affects shells set to a non-default locale ("C" or "POSIX"). Setting the locale is normally preferred and is especially useful when working with Unicode data. Run the locale command to check the settings.

Locale sensitive sorting can be turned off when not needed. This is done by setting environment variable LC_ALL=C for the duration of the sort command. Here is a version of the sort shell script that does this:

file: tsv-sort-fast

(LC_ALL=C sort -t $'\t' --buffer-size=2G "$@")

The tsv-sort-fast script can be used the same way as the tsv-sort script shown earlier.

A sample implementation of this script can be found in the extras/scripts directory in the tsv-utils GitHub repository. This sample script is also included in the prebuilt binaries package.

Enable bash-completion

Bash command completion is quite handy for command line tools. Command names complete by default, already useful. Adding completion of command options is even better. As an example, with bash completion turned on, enter the command name, then a single dash (-):

$ tsv-select -

Now type a TAB (or pair of TABs depending on setup). A list of possible completions is printed and the command line restored for continued entry.

$ tsv-select -
--delimiter  --fields     --header     --help       --rest
$ tsv-select --

Now type 'r', then TAB, and the command will complete up to $ tsv-select --rest.

Enabling bash completion is a bit more involved than other packages, but still not too hard. It will often be necessary to install a package. The way to do this is system specific. A good source of instructions can be found at the bash-completion GitHub repository. Mac users may find the MacPorts How to use bash-completion guide useful. Procedures for Homebrew are similar, but the details differ a bit.

After enabling bash-completion, add completions for the tsv-utils package. Completions are available in the tsv-utils file in the bash_completion directory in the tsv-utils GitHub repository. This file is also included with the prebuilt binary release packages. One way to add them is to 'source' the file from the ~/.bash_completion file. A line like the following will achieve this:

if [ -r ~/tsv-utils/bash_completion/tsv-utils ]; then
    . ~/tsv-utils/bash_completion/tsv-utils

The file can also be added to the bash completions system directory on your system. The location is system specific, see the bash-completion installation instructions for details.

Convert newline format and character encoding with dos2unix and iconv

The TSV Utilities expect input data to be utf-8 encoded and on Unix, to use Unix newlines. The dos2unix and iconv command line tools are useful when conversion is required.

Needing to convert newlines from DOS/Windows format to Unix is relatively common. Data files may have been prepared for Windows, and a number of spreadsheet programs generate Windows line feeds when exporting data. The csv2tsv tool converts Windows newlines as part of its operation. The other TSV Utilities detect Window newlines when running on a Unix platform, including macOS. The following dos2unix commands convert files to use Unix newlines:

$ # In-place conversion.
$ dos2unix file.tsv

$ # Conversion writing to a new file. The existing file is not modified.
$ dos2unix -n file_dos.tsv file_unix.tsv

$ # Reading from standard input writes to standard output
$ cat file_dos.tsv | dos2unix | tsv-select -f 1-3 > newfile.tsv

Most applications and databases will export data in utf-8 encoding, but it can still be necessary to convert to utf-8. iconv serves this purpose nicely. An example converting Windows Latin-1 (code page 1252) to utf-8:

$ iconv -f CP1252 -t UTF-8 file_latin1.tsv > file_utf8.tsv

The above can be combined with dos2unix to perform both conversions at once:

$ iconv -f CP1252 -t UTF-8 file_window.tsv | dos2unix > file_unix.tsv

See the dos2unix and iconv man pages for more details.

Using grep and tsv-filter together

tsv-filter is fast, but a quality Unix grep implementation is faster. There are good reasons for this, notably, grep can ignore line boundaries during initial matching (see "why GNU grep is fast", Mike Haertel).

Much of the time this won't matter, as tsv-filter can process gigabyte files in a couple seconds. However, when working with much larger files or slow I/O devices, the wait may be longer. In these cases, it may be possible to speed things up using grep as a first pass filter. This will work if there is a string, preferably several characters long, that is found on every line expected in the output, but also filtering out a fair number of non-matching lines.

An example, using a set of files from the Google Books Ngram Viewer data-sets. In these files, each line holds stats for an ngram, field 2 is the year the stats apply to. In this test, ngram_*.tsv consists of 1.2 billion lines, 23 GB of data in 26 files. To get the lines for the year 1850, this command would be run:

$ tsv-filter --str-eq 2:1850 ngram_*.tsv

This took 72 seconds on a Mac Mini (6-core, 64 GB RAM, SSD drives) and output 2,493,403 records. Grep can also be used:

$ grep 1850 ngram_*.tsv

This took 38 seconds, quite a bit faster, but produced too many records (2,504,846), as "1850" appears in places other than the year. But the correct result can generated by using grep and tsv-filter together:

$ grep 1850 ngram_*.tsv | tsv-filter --str-eq 2:1850

This took 39 seconds, nearly as fast grep by itself. grep and tsv-filter run in parallel, and tsv-filter keeps up easily as it is processing fewer records.

Using grep as a pre-filter won't always be helpful, that will depend on the specific case, but on occasion it can be quite handy.

Using ripgrep and tsv-filter together

ripgrep is a popular alternative to grep, and one of the fastest grep style programs available. It can be used with tsv-filter in the same way as grep. It has built-in support for parallel processing when operating on multiple files. This creates an interesting comparison point, one useful in conjunction with the next topic, Faster processing using GNU parallel.

This experiment uses the same Google ngram files, but a more complex expression. We'll find stats lines for years 1850 through 1950 on ngrams tagged as verbs. The tsv-filter expression:

tsv-filter --str-in-fld 1:_VERB --ge 2:1850 --le 2:1950

This produces 26,956,517 lines. Prefiltering with grep/ripgrep using _VERB reduces the set passed to tsv-filter to 81,626,466 (from the original 1,194,956,817).

For the first test the data is piped through standard input rather read directly from files. This has the effect of forcing ripgrep (rg command) to run in single threaded mode. This is similar to running against a single large file. The command lines:

$ cat ngram-*.tsv | tsv-filter --str-in-fld 1:_VERB --ge 2:1850 --le 2:1950
$ cat ngram-*.tsv | grep _VERB | tsv-filter --str-in-fld 1:_VERB --ge 2:1850 --le 2:1950
$ cat ngram-*.tsv | rg _VERB | tsv-filter --str-in-fld 1:_VERB --ge 2:1850 --le 2:1950

Timing results, standard input test (Mac Mini; 6-cores; 64 GB RAM; SSD drive):

Command Elapsed User System CPU
tsv-filter 79.08 75.49 8.97 106%
grep & tsv-filter 25.56 32.98 7.16 157%
ripgrep & tsv-filter 14.27 16.29 11.46 194%

The ripgrep version is materially faster on this test. A larger set of tests on different types of files would be needed to determine if this holds generally, but the result is certainly promising for ripgrep. In these tests, tsv-filter was able to keep up with grep, but not ripgrep, which on its own finishes in about 12 seconds. Of course, both grep and ripgrep as prefilters are material improvements over tsv-filter standalone.

The next test runs against the 26 files directly, allowing ripgrep's parallel capabilities to be used. Runs with combining GNU parallel with grep and tsv-filter standalone are included for comparison. The commands:

$ tsv-filter ngram-*.tsv --str-in-fld 1:_VERB --ge 2:1850 --le 2:1950
$ grep _VERB ngram-*.tsv | tsv-filter --ge 2:1850 --le 2:195
$ rg _VERB ngram-*.tsv | tsv-filter --str-in-fld 1:_VERB --ge 2:1850 --le 2:1950
$ parallel tsv-filter --str-in-fld 1:_VERB --ge 2:1850 --le 2:1950 ::: ngram-*.tsv
$ parallel grep _VERB ::: ngram-*.tsv | tsv-filter --str-in-fld 1:_VERB --ge 2:1850 --le 2:1950

Timing results, multiple files test:

  Elapsed User System CPU
tsv-filter 76.06 74.00 3.21 101%
grep & tsv-filter 30.65 37.22 4.05 134%
ripgrep & tsv-filter 11.04 20.94 10.32 283%
parallel grep & tsv-filter 16.48 134.54 6.27 854%
parallel tsv-filter 16.36 134.39 6.12 858%

The ripgrep version, using multiple threads, is the fastest. For these tests, the single threaded use of tsv-filter is the limitation in both ripgrep and parallelized grep cases. By themselves, parallel grep and ripgrep finish the prefiltering steps in 6.3 and 3.5 seconds respectively. Very nicely, GNU parallel with tsv-filter standalone is a nice improvement.

These results show promise for ripgrep and using GNU parallel. Actual results will depend on the specific data files and tasks. The machine configuration will matter for multi-threading cases. See the next section, Faster processing using GNU parallel, for more info about GNU parallel.

Version information for the timing tests:

Faster processing using GNU parallel

The TSV Utilities tools are singled threaded. Multiple cores available on today's processors are utilized primarily when the tools are run in a Unix command pipeline. The example shown using in Using grep and tsv-filter together uses the Unix pipeline approach to some gain parallelism.

This often leaves processing power on the table, power that can be used to run commands considerably faster. This is especially true when reading from fast IO devices such as the newer generations of SSD drives. These fast devices often read much faster than a single CPU core can keep up with.

GNU parallel provides a convenient way to parallelize many Unix command line tools and take advantage of multiple CPU cores. TSV Utilities tools can use GNU parallel as well, several examples are given in this section. The techniques shown can applied to many other command line tools as well. If you are using a machine with multiple cores you may gain performance benefit from using GNU parallel.

GNU parallel may need to be installed, use your system's package manager to do this. GNU parallel provides a large feature set, only a subset is shown in these examples. See the GNU parallel documentation for more details.

GNU parallel and TSV Utilities

The simplest uses of GNU parallel involve processing multiple files that do not contain header lines. In these scenarios, parallel is used to start multiple instances of a command in parallel, each command invocation run against a different file. The results from each command are written to standard output.

Line counting (wc -l) will be used to illustrate the process. The same Google ngram files used in the examples in Using grep and tsv-filter together will be used here (26 files, 1.2 billion lines, 23 GB). All the examples in this section were timed on a 6-core Mac Mini with 64 GB RAM and SSD drives.

The standalone command to count lines in each file is below (output truncated for brevity):

$ wc -l ngram-*.tsv
   86618505 ngram-a.tsv
   61551917 ngram-b.tsv
   97689325 ngram-c.tsv
   ... more files ...
    3929235 ngram-x.tsv
    6869307 ngram-y.tsv
 1194956817 Total

parallel is invoked by passing both the list of files and the command to run. The file names can be provided in standard input or by using the ::: operator. The following command lines show these two methods:

$ ls ngram-*.tsv | parallel wc -l
$ parallel wc -l ::: ngram-*.tsv

Here are the results with parallel:

$ parallel wc -l ::: ngram-*.tsv
17008269 ngram-j.tsv
27279767 ngram-k.tsv
39861295 ngram-g.tsv
... more files ...
88562873 ngram-p.tsv
110075424 ngram-s.tsv

Notice that there is no summary line. That is because wc produces the summary when processing multiple files, but using parallel wc is invoked once per file. Also, the result order has changed. This is because results are output in the order they finish rather than the order the files are listed in. The input order can be preserved using the --keep-order (or -k) option:

`$ parallel --keep-order wc -l ::: ngram-*.tsv
86618505 ngram-a.tsv
61551917 ngram-b.tsv
97689325 ngram-c.tsv
... more files ...
3929235 ngram-x.tsv
6869307 ngram-y.tsv

Timing info from these runs shows substantial performance gains using parallel:

Command Elapsed User System CPU
wc -l ngram-*.tsv 11.95 8.26 3.55 98%
parallel wc -l ::: ngram-*.tsv 2.07 9.88 5.33 734%
parallel -k wc -l ::: ngram-*.tsv 2.03 9.88 5.27 743%

Now for some examples using TSV Utilities.

GNU parallel and tsv-filter

An example using parallel on tsv-filter was shown earlier in the section Using ripgrep and tsv-filter together. That example was for a case where a grep program could be used as a prefilter. But parallel and tsv-filter will also work in cases where a grep style prefilter is not appropriate. Repeating the earlier example:

$ tsv-filter --str-in-fld 1:_VERB --ge 2:1850 --le 2:1950 ngram-*.tsv
$ parallel tsv-filter --str-in-fld 1:_VERB --ge 2:1850 --le 2:1950 ::: ngram-*.tsv
  Elapsed User System CPU
tsv-filter 76.06 74.00 3.21 101%
parallel tsv-filter 16.36 134.39 6.12 858%

This works the same way as the wc -l example. The output from all the individual invocations of tsv-filter are concatenated together, just as in the standalone invocation of tsv-filter. (Use the --keep-order option to preserve the input order.)

It's a valuable performance gain for a minor change in the command structure. Notice though that if the files had header lines additional steps would be needed, as the above commands would do nothing to suppress repeated header lines from the multiple files.

GNU parallel and tsv-select

tsv-select can be parallelized in the same fashion as tsv-filter. This example selects the first, second, and fourth fields from the ngram files. The cut utility is shown as well (cut from GNU coreutils 8.31). The commands and timing results:

$ cut -f 1,2,4 ngram-*.tsv
$ tsv-select -f 1,2,4 ngram-*.tsv
$ parallel -k cut -f 1,2,4 ::: ngram-*.tsv
$ parallel -k tsv-select -f 1,2,4 ::: ngram-*.tsv
  Elapsed User System CPU
cut 158.78 153.82 4.95 99%
tsv-select 100.42 98.44 3.04 101%
parallel cut 41.07 278.71 66.65 840%
parallel tsv-select 29.33 179.39 63.78 828%

GNU parallel and tsv-sample

Bernoulli sampling is relatively easy to parallelize. Let's say we want to take a 0.1% sample from the ngram files. The standalone and parallelized versions of the command are similar:

$ tsv-sample -p 0.001 ngram-*.tsv
$ parallel -k tsv-sample -p 0.001 ::: ngram-*.tsv

The standalone version takes about 46 seconds to complete, the parallelized version less than 9.

  Elapsed User System CPU
tsv-sample 46.08 44.16 2.47 101%
parallel tsv-sample 8.57 67.09 3.56 833%

Suppose we want to do this with simple random sampling? In simple random sampling, a specified number of records are chosen at random. Picking a 1 million random set would be done with a command like:

$ tsv-sample -n 1000000 ngram-*.tsv

We can't parallelize the tsv-sample -n command itself. However, a trick that can be played is to over-sample using Bernoulli sampling, then get the desired number of records with random sampling. Our earlier formula for the Bernoulli sample produces on average about 1.19 million records, a reasonable over-sampling for the 1 million records desired. (It is possible for the Bernoulli sample to produce less than 1 million records, but that would be exceptionally rare with this over-sampling rate.) The resulting formula:

$ tsv-sample -p 0.001 ngram-*.tsv | tsv-sample -n 1000000

The initial Bernoulli stage can be parallelized, just as before:

$ parallel -k tsv-sample -p 0.001 ::: ngram-*.tsv | tsv-sample -n 1000000

The timing results:

  Elapsed User System CPU
random sampling 60.14 58.20 2.68 101%
Bernoulli & random sampling 47.11 45.37 2.95 102%
Parallel Bernoulli & random sampling 8.96 70.86 3.79 832%

Bernoulli sampling is a bit faster than simple random sampling, so there is some benefit from using this technique in single process mode. The real win is when the Bernoulli sampling stage is parallelized.

GNU parallel and tsv-summarize

Many tsv-summary calculations require seeing all the data all at once and cannot be readily parallelized. Computations like mean, median, stdev, and quantile fall into this bucket. However, there are operations that can parallelized. Operations like sum, min and max. We'll use max to show an example of how this works. First, we'll tsv-summarize to find the largest occurrence count (3rd column) in the ngram files:

$ tsv-summarize --max 3 ngram-*.tsv

That works, but took 85 seconds. Here's a version that parallelizes the invocations:

$ parallel tsv-summarize --max 3 ::: ngram-*.tsv

That worked, but produced a result line for each file. To get the final results we need to make another tsv-summarize call aggregating the intermediate results:

$ parallel tsv-summarize --max 3 ::: ngram-*.tsv | tsv-summarize --max 1

This produced the correct result and finished in 18 seconds, much more palatable than the 85 seconds in the single process version.

We could find the maximim occurrence count for each year (column 2) in a similar fashion. This example sorts the results by year for good measure.

$ tsv-summarize --group-by 2 --max 3 ngram-*.tsv | tsv-sort-fast -k1,1n
1505	1267
1507	1938
1515	19549
2005	658292423
2006	703340664
2007	749205383
2008	927838975

This took 110 seconds. Here's the parallel version. It produces the same results, but finishes in 23 seconds, nearly 5 times faster.

$ parallel tsv-summarize --group-by 2 --max 3 ::: ngram-*.tsv | tsv-summarize --group-by 1 --max 2 | tsv-sort-fast -k1,1n

Notice that in the "group-by year" example, the second tsv-summarize pass is necessary because the entries for each year occur in multiple files. If the files are organized by the group-by key, then the second pass is not necessary. The google ngram files are organized by first letter of the ngram (a file for "a", a file for "b", etc.), so "group-by" operations on the ngram field would not need the second pass. The tsv-split tool's "random assignment by key" feature can be used to split a data set into files sharded by key. This is especially helpful when the number of unique keys in the data set is very large.

Using GNU Parallel on files with header lines

All the examples shown so far involve multiple files without header lines. Correctly handling header lines is a more involved. The main issue is that results from multiple files get concatenated together when results are reassembled. One way to deal with this is to drop the headers from each file as they are processed, arranging to have the header from the first file preserved if necessary. Other methods are possible, but more involved than will be discussed here.

GNU Parallel on standard input or a single large file

All the examples shown so far run against multiple files. This is a natural fit for parallel's capabilities. parallel also has facilities for automatically splitting up individual files as well as standard input into smaller chunks and invoking commands on these smaller chunks in parallel.

Unfortunately, performance when using these facilities is more variable than when running against multiple input files. This is because the work to split up the file or input stream is itself single threaded and can be a bottleneck. Performance gains are unlikely to match the gains seen on multiple files, and performance may actually get worse. Performance appears dependent on the specific task, the nature of the files or input stream, and the computation being performed. Some experimentation may needed to identify the best parameter tuning.

For these reasons, paralellizing tasks on standard input or against single files may be most appropriate for repeated tasks. Tasks run on a regular basis against similar data sets, where time invested in performance tuning gets paid back over multiple runs.

Of the two cases, performance gains are more likely when running against a single file than when running against standard input. That is because the mechanism used to split a file (--pipepart) is much faster than the mechanism used to split up standard input (--pipe).

Here are examples of the command syntax. The Bernoulli sampling example used earlier is shown:

$ # Standalone invocation
$ tsv-sample -p 0.001 bigfile.txt

$ # Reading from standard input and splitting via --pipe
$ cat bigfile.txt | parallel --pipe --blocksize=64M tsv-sample -p 0.001

$ # Reading from a single file and splitting via --pipepart
$ parallel -a bigfile.txt --pipepart --blocksize=64M tsv-sample -p 0.001

Consult the GNU parallel documentation for more information about these features. Experiment with them in your environment to see what works for your use cases.

Reading data in R

It's common to perform transformations on data prior to loading into applications like R or Pandas. This especially useful when data sets are large and loading entirely into memory is undesirable. One approach is to create modified files and load those. In R it can also be done as part of the different read routines, most of which allow reading from a shell command. This enables filtering rows, selecting, sampling, etc. This will work with any command line tool. Some examples below. These use read.table from the base R package, read_tsv from the tidyverse/readr package, and fread from the data.table package:

> df1 = read.table(pipe("tsv-select -f 1,2,7 data.tsv | tsv-sample -H -n 50000"), sep="\t", header=TRUE, quote="")
> df2 = read_tsv(pipe("tsv-select -f 1,2,7 data.tsv | tsv-sample -H -n 50000"))
> df3 = fread("tsv-select -f 1,2,7 train.tsv | tsv-sample -H -n 50000")

The first two use the pipe function to create the shell command. fread does this automatically.

Note: One common issue is not having the PATH environment setup correctly. Depending on setup, the R application might not have the full path normally available in a command shell. See the R documentation for details.

Shuffling large files

Line order randomization, or "shuffling", is one of the operations supported by tsv-sample. Most tsv-sample operations can be performed with limited system memory. However, system memory becomes a limitation when shuffling very large data sets, as the entire data set must be loaded into memory. (GNU shuf has the same limitation.)

In many cases the most effective solution is simply to get more memory, or find a machine with enough memory. However, when more memory is not an option, another solution to consider is disk-based shuffling. This is approach described here.

One option for disk-based shuffling is GNU sort's random sort feature (sort --random-sort). This can be used for unweighted randomization. However, there are couple of downsides. One is that it places duplicates lines next to each other, a problem for many shuffling use cases. Another is that it is rather slow.

An better approach is to combine tsv-sample --gen-random-inorder with disk-based sorting. GNU sort serves the latter purpose well. A random value is generated for each input line, the lines are sorted, and the random values removed. GNU sort will use disk if necessary. This technique can be used for both weighted and unweighted line order randomization. There is a catch: GNU sort is dramatically faster when sorting numbers written in decimal notation, without exponents. However, random value generation may generate values with exponents in some cases. This is discussed in more detail at the end of this section.

Here's an example.

$ # In-memory version
$ tsv-sample file.txt > randomized-file.txt

$ # Using disk-based sorting
$ tsv-sample --gen-random-inorder file.txt | tsv-sort-fast -k1,1nr | cut -f 2- > randomized-file.txt

(Note: These examples uses the tsv-sort-fast shell script described earlier, under Customize the Unix sort command. Substitute tsv-sort-fast -k1,1nr with (LC_ALL=C sort -t $'\t' --buffer-size=2G -k1,1nr) to use the sort command directly.)

The above example prepends a random value to each line, sorts, and removes the random values. Now available disk space is the limiting factor, not memory.

This can be done with weighted sampling when the weights are integer values. This is shown in the next example, using a weight from field 3.

$ # In-memory version
$ tsv-sample -w 3 file.tsv > randomized-file.tsv

$ # Using disk-based sampling, with integer weights
$ tsv-sample -w 3 --gen-random-inorder file.tsv | tsv-sort-fast -k1,1nr | cut -f 2- > randomized-file.tsv

The examples so far use "numeric" sorting. When values contain exponents "general numeric" sorting should be used. This is specified using -k1,1gr rather than -k1,1nr. Here's an example:

$ # Using disk-based sampling, with floating point weights
$ tsv-sample -w 3 --gen-random-inorder file.tsv | tsv-sort-fast -k1,1gr | cut -f 2- > randomized-file.tsv

Performance of the approaches described will vary considerably based on the hardware and data sets. As one comparison point the author ran both sort --random-sort and the unweighted, disk based approach shown above on a 294 million line, 5.8 GB data set. A 16 GB macOS box with SSD disk storage was used. This data set fits in memory on this machine, so the in-memory approach was tested as well. Both tsv-sample and GNU shuf were used. The sort --random-sort metric was run with locale sensitive sorting both on and off to show the difference.

The in-memory versions are of course faster. But if disk is necessary, combining tsv-sample --gen-random-inorder with sort is about twice as fast as sort --random-sort and doesn't have the undesirable behavior of grouping duplicate lines.

Command/Method Disk? Time
tsv-sample file.txt > out.txt No 1 min, 52 sec
shuf file.txt > out.txt No 3 min, 9 sec
Method: tsv-sample –gen-random-inorder, cut, sort Yes 13 min, 24 sec
tsv-sort-fast --random-sort file.txt > out.txt Yes 27 min, 44 sec
tsv-sort --random-sort file.txt > out.txt Yes 4 hrs, 55 min


Regarding exponential notation: The faster "numeric" sort will incorrectly order lines where the random value contains an exponent. In version 1.3.2, tsv-sample changed random number printing to limit exponent printing. This was done by using exponents only when numbers are smaller than 1e-12. Though not guaranteed, this does not occur in practice with unweighted sampling or weighted sampling with integer weights. The author has run more than a billion trials without an occurrence. (It may be a property of the random number generator used.) It will occur if floating point weights are used. Use "general numeric" ('g') form when using floating point weights or if a guarantee is needed. However, in many cases regular "numeric" sort ('n') will suffice, and be dramatically faster.

Regarding unweighted shuffling: A faster version of unweighted shuffling appears very doable. One possibility would be to read all input lines and write each to a randomly chosen temporary file. Then read and shuffle each temporary file in-memory and write it to the final output, appending each shuffled file. This would replace sorting with a faster shuffling operation. It would also avoid printing random numbers, which is slow. See Daniel Lemire's blog post External-memory shuffling in linear time? for a more detailed discussion.