Converting CSV to TSV

Python

Add to file named csv2tab, and make it executable

touch csv2tab && chmod u+x csv2tab

Add to it

#!/usr/bin/env python
import csv, sys
csv.writer(sys.stdout, dialect='excel-tab').writerows(csv.reader(sys.stdin))

Test runs

$ echo 'A,,C,"D,E,F","G",I,"K,L,M",Z' | ./csv2tab                     
A       C   D,E,F   G   I   K,L,M   Z

$ ./csv2tab < data.csv > data.tsv && head data.tsv                                                   
1A      C   D,E,F   G   I   K,L,M   Z
2A      C   D,E,F   G   I   K,L,M   Z
3A      C   D,E,F   G   I   K,L,M   Z

Using csvkit (Python), for example:

$ csvformat -T in.csv > out.txt

Does streaming, with correct CSV and TSV quoting and escaping

It's in apt and other package managers

For fun, sed.

sed -E 's/("([^"]*)")?,/\2\t/g' file

If your sed doesn't support -E, try with -r. If your sed doesn't support \t for a literal tab, try putting a literal tab (in many shells, ctrl-v tab) or in Bash, use a $'...' C-style string (in which case the backslash in \2 needs to be doubled). If you want to keep the quotes, use \1 instead of \2 (in which case the inner pair of parentheses is useless, and can be removed).

This makes no attempt to handle escaped double quotes inside double quotes; some CSV dialects support this by doubling the quoted double quote (sic).