Search for a Tab Character with grep in Bash

You need the lines that contain a tab, such as the columns of a tab-separated file.

printf 'a\tb\ntest\n' > t.txt
grep $'\t' t.txt | cat -A
grep -P '\t' t.txt | cat -A
grep '\t' t.txt

a^Ib$
a^Ib$
test

In Bash, ANSI-C quoting (a dollar sign before single quotes) turns the tab escape into a real tab character before grep sees it. grep -P understands the escape itself, and both matched the first line, which cat -A shows as a^Ib$. A plain single-quoted tab escape is different: GNU grep reads it as the letter t, so the third command matched the line test and skipped the tab line. See ANSI-C Quoting and the grep manual for -P.

-P is a GNU extension and is missing from the grep on macOS and BusyBox, and ANSI-C quoting is not POSIX, so in sh put the tab in a variable with printf first. At an interactive prompt, Ctrl-V followed by Tab also inserts a literal tab.