Intro To 'expand' Command In Linux

2023-06-09 - By Robert Elder

I use the 'expand' command to replace tabs with spaces:

echo -n $'\t' | expand | xxd

00000000: 2020 2020 2020 2020

Expand Tabs Into Spaces

Here, I have an 'echo' statement that prints out a single tab character between the letters 'a' and 'b':

echo -en "a\tb"

echo -en "a\tb" | xxd

00000000: 6109 62                                  a.b

If I pipe this 'echo' statement into the 'expand' command, the single tab character will be replaced by eight space characters:

echo -en "a\tb" | expand | xxd

00000000: 6120 2020 2020 2020 62                   a       b

Default Use Case & Custom Tab Width

Here, I have a file called 'numbers.txt' that contains several columns of random numbers:

	29303	32454	7898	4022
	11773	16085	19539	32199
	16335	23913	17824	14969
	25557	15589	2375	8899

With the 'expand' command, the default number of spaces to replace each tab with is 8:

cat numbers.txt | expand

        29303   32454   7898    4022
        11773   16085   19539   32199
        16335   23913   17824   14969
        25557   15589   2375    8899

I can specify a different number like 12 with the '-t' flag:

cat numbers.txt | expand -t 12

            29303       32454       7898        4022
            11773       16085       19539       32199
            16335       23913       17824       14969
            25557       15589       2375        8899

Initial Tabs

The 'expand' command can treat 'initial' tabs differently when they start at the beginning of a line before any non-blank characters. Let's add one extra line to the file above:

echo -e "1234\t5678" >> numbers.txt

so now it looks like this:

	29303	32454	7898	4022
	11773	16085	19539	32199
	16335	23913	17824	14969
	25557	15589	2375	8899
1234	5678

Now, I can include the '-i' flag to cause only the initial tabs to be replaced:

cat numbers.txt | expand -i -t 12

            29303	32454	7898	4022
            11773	16085	19539	32199
            16335	23913	17824	14969
            25557	15589	2375	8899
1234	5678

And here is the hexadecimal output from above:

cat numbers.txt | expand -i -t 12 | xxd

00000000: 2020 2020 2020 2020 2020 2020 3239 3330              2930
00000010: 3309 3332 3435 3409 3738 3938 0934 3032  3.32454.7898.402
00000020: 320a 2020 2020 2020 2020 2020 2020 3131  2.            11
00000030: 3737 3309 3136 3038 3509 3139 3533 3909  773.16085.19539.
00000040: 3332 3139 390a 2020 2020 2020 2020 2020  32199.
00000050: 2020 3136 3333 3509 3233 3931 3309 3137    16335.23913.17
00000060: 3832 3409 3134 3936 390a 2020 2020 2020  824.14969.
00000070: 2020 2020 2020 3235 3535 3709 3135 3538        25557.1558
00000080: 3909 3233 3735 0938 3839 390a 3132 3334  9.2375.8899.1234
00000090: 0935 3637 380a                           .5678.

A Practical Example

Here, I have a tab separated document called 'living-expenses.txt':

	Category	Jan	Feb	Mar

	Food	$102.78	$97.34	$112.42
	Rent	$380.00	$380.00	$380.00
	Linux Weekly News Subscription	$9.00	$9.00	$9.00
	Artistic Anime Figurines	$1,749.38	$2,109.87	$3,829.48

This document is hard to read because some of the entries are much longer than eight characters.

I can provide a list of column offsets to the '-t' flag so the tab characters will be expanded with as many spaces as necessary to reach the specified offset for each column.

cat living-expenses.txt | expand -t 1,33,43,53,63

 Category                        Jan       Feb       Mar

 Food                            $102.78   $97.34    $112.42
 Rent                            $380.00   $380.00   $380.00
 Linux Weekly News Subscription  $9.00     $9.00     $9.00
 Artistic Anime Figurines        $1,749.38 $2,109.87 $3,829.48

And that's why the 'expand' command is my favourite Linux command.

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