SC2189

< file.txt | grep foo

< file.txt grep foo   # or more canonically: grep foo < file.txt

ShellCheck has found a stage in a pipeline that consists of a redirection but no command. This doesn't make sense because a redirection without a command will not read or write any data.

This is most likely to occur when deleting a command that had a redirection, but leaving a | behind instead of moving the redirection to a different command:

# Match lines with line numbers
nl < foo.txt | grep bar

# Incorrect attempt at removing line numbers. grep now has no input:
< foo.txt | grep bar

# Line numbers correctly removed. grep now reads foo.txt as intended.
grep bar < foo.txt

It's technically valid to do e.g. echo foo | > "$(cat)" to truncate a file called "foo", but please consider rewriting such code.