Redirect stderr to file

Split stdout and stderr into separate files

command > /tmp/out.log 2> /tmp/err.log

More linux commands

Capture both streams

command 2>&1 | tee /tmp/all.log

Redirect stdout+stderr to file and screen simultaneously
Replace string in files recursively

grep -rl 'old_string' /etc/ | xargs sed -i 's/old_string/new_string/g'

Find files containing text, then replace in-place
Print specific columns with awk

awk '{print $1, $3}' /etc/passwd

Print 1st and 3rd fields of a space-delimited file
Sum a column with awk

awk '{sum += $2} END {print sum}' file.txt

Sum values in column 2 of a text file
Delete lines matching pattern

sed -i '/^#/d' config.conf

Remove all comment lines starting with #