How to Replace Multiple Spaces by One Tab

If your character is multiple tabs you can also use tr -s:

-s, --squeeze-repeats   replace each input sequence of a repeated character
                        that is listed in SET1 with a single occurrence

For example:

my_file.txt | tr -s " "

All white spaces will become one.

You can use sed to replace a number of spaces with a tab.:

Example to replace one-or-more-spaces with one tab:

cat spaced-file | sed 's/ \+/\t/g' > tabbed-file

perl -p -i -e 's/\s+/\t/g' *.txt

The easiest answer using only bash is:

while read -r col1 col2 col3 ...; do
    echo -e "$col1\t$col2\t$col3..."
done <file

If there are a variable number of columns, you can do this, but it will only work in bash, not sh:

while read -r -a cols; do
    (
        IFS=$'\t'
        echo "${cols[*]}"
    )
done <file

e.g.

while read -r -a cols; do
    (
        IFS=$'\t'
        echo "${cols[*]}"
    )
done <<EOF
a b   c
d   e    f
  g h i
EOF

produces:

a   b   c
d   e   f
g   h   i

(there is a tab in between each, but it's hard to see when I paste it here)

You could also do it using sed or tr, but notice that the handling of blanks at the start produces different results.

sed:

$ sed 's/  */\t/g' << EOF
a b   c
d   e    f
  g h i
EOF
a       b       c
d       e       f
        g       h       i

tr:

$ tr -s ' ' '\t' <<EOF
a b   c
d   e    f
  g h i
EOF
a       b       c
d       e       f
        g       h       i

Try the following SED script:

 sed 's/  */<TAB>/g' <spaces-file > tabs-file

Where <TAB> is pressing the TAB key.

This is a very simple solution:

    sed -E 's/\s+/\t/g' your_file > new_file

sed basically works in this manner (sed 's/old_pattern/new_pattern/g'). In this case the old pattern is "\s+" which means find space "s" one or more time "+" and the back slash "\" to interpret that as regular expression.
The new pattern is tab "\t" which is written in regular expression format and the "g" is apply the replacement to all lines "globally".

The option -h, --tabs of col command can be useful (it's part of the util-linux package, col reads from standard input and writes to standard output).

cat file.txt | col -h

or redirect the output to a file:

col -h <file.txt >output.txt