Replace string in files recursively

Find files containing text, then replace in-place

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

More linux commands

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 #
Multi-line sed replacement

sed -n '/START/,/END/p' file.txt

Print block of text between START and END markers
Unique sorted values

sort file.txt | uniq -c | sort -rn

Count occurrences of each unique line