gbget tutorial

Table of Contents

Introduction to gbget

The special utility gbget is used to perform basic manipulation of ASCII data files. It takes a file or a list of files containing tabular data and extract command line specified parts of these data. The result of the extraction can be further manipulated with a list of possible transformations.

Essentially, the user provides a list of data specifications at the command line. The program analyze each specification in turns and, according to it, extract and print data to standard output.

Each data specification has the following structure



is the name of a regular (ASCII) file
identify a given data-block inside the file; data-blocks are separated by 2 or more empty lines (format chosen for consistency with gnuplot specifications). Notice that comment lines (beginning with a "#" symbol) DO NOT count as empty lines for this purpose.
the range of required columns, specified as begin:end:skip. Negative values are counted from the end. Default is 1:-1:1 i.e. all columns. If begin>end the columns are read in reversed order.
the range of required rows, specified as begin:end:skip. Negative values are counted from the end. Default is 1:-1:1 i.e. all rows. If begin>end the rows are read in reversed order.
is a list of single letter options that identify successive transformations to be applied to data.

The list of options includes

transpose the matrix
flatten the data column-wise, reducing them to a single column
take the log of all fields
take the column-wise difference: substract column 1 from 2, column 2 from 3, etc.
remove all lines containing at least one NAN entry
remove the mean to each column
reduce each column to zscores, that is remove the mean and divide by the standard deviation


A few examples can help to understand the gbget syntax. Consider the file test.dat with the following content

10 20 30
11 21 31
12 22 32
13 23 33
14 24 34

15 25 35
16 26 36
17 27 37
18 28 38
19 29 39

i.e. two data blocks, separated by two blank lines, each made of three columns and 5 rows. This file should already exists in the gbutils source directory. If the support for zlib has been found on your system, in the examples below you can equivalently use the compressed file test.dat.gz.

Now if you type:

# gbget 'test.dat[1](1:2)'

(the ' are normally required to protect the content of the string from shell expansion) you obtain

1.000000e+01  2.000000e+01
1.100000e+01  2.100000e+01
1.200000e+01  2.200000e+01
1.300000e+01  2.300000e+01
1.400000e+01  2.400000e+01

i.e. the first two column (1:2) of the first datablock [1]. By default, the output is in scientific notation, but more on this below.

If instead you type

# gbget 'test.dat[2](-2:,2:3)'

you obtain

2.600000e+01  3.600000e+01
2.700000e+01  3.700000e+01

i.e. the rows from 2 to 3 (included) of the last two columns of the second data block [2]. Notice that a negative entry in column or row specification means "count from the end".

You can also print each second column of the second block with

# gbget 'test.dat[2](::2)'

that gives you

1.500000e+01  3.500000e+01
1.600000e+01  3.600000e+01
1.700000e+01  3.700000e+01
1.800000e+01  3.800000e+01
1.900000e+01  3.900000e+01

or each third row of the whole file

# gbget 'test.dat(,::3)'

to have

1.000000e+01  2.000000e+01  3.000000e+01
1.300000e+01  2.300000e+01  3.300000e+01
1.600000e+01  2.600000e+01  3.600000e+01
1.900000e+01  2.900000e+01  3.900000e+01

If the initial and final positions in a slice specification are reversed, the columns or the rows are printed in reverse order. For instance

# gbget 'test.dat(,3:1)'


1.200000e+01  2.200000e+01  3.200000e+01
1.100000e+01  2.100000e+01  3.100000e+01
1.000000e+01  2.000000e+01  3.000000e+01

Several different transformations can be applied to the chosen matrix of data. For instance, the matrix can be flattened column-wise, i.e. each column can be put after the previous one in a single, long, column using the flag f, or you can transpose it using the option t. Let see some examples. Consider

# gbget 'test.dat[1](,2:3)'

which gives

1.100000e+01  2.100000e+01  3.100000e+01
1.200000e+01  2.200000e+01  3.200000e+01

now you can flatten the output

# gbget 'test.dat[1](,2:3)f'

or transpose it

# gbget 'test.dat[1](,2:3)t'
1.100000e+01  1.200000e+01
2.100000e+01  2.200000e+01
3.100000e+01  3.200000e+01

Finally, the output format can be customized using options -o and -e. To fully exploit these options you need to know the syntax of the printf command implemented in the standard C libraries. However, the type of output can be easily chosen with a single flag. If instead of the scientific notation you prefer a fixed point notation for your output, you have to use the option -o with the value %f

# gbget 'test.dat[1](,2:3)t' -o ' %f'
11.000000 12.000000
21.000000 22.000000
31.000000 32.000000

while in order to have an integer output you need -o with the value %d

# gbget 'test.dat[1](,2:3)t' -o ' %d'
11 12
21 22
31 32

in this case, however, be aware of the truncations: gbget rounds a non-integer value down to the nearest integer!

Created: 2023-07-06 Thu 18:13