Chapter 4: Linux Advanced Commands
4.3 Text processing: awk and sed
awk — column-based processing
awk splits each line into fields $1, $2, ... (space-separated by default; -F sets the separator).
awk '{print $1}' /var/log/nginx/access.log | sort | uniq -c | sort -rn | head
# ^ top 10 client IPs hitting your web server
awk -F: '{print $1, $7}' /etc/passwd # user name and shell
awk -F: '$3 >= 1000 {print $1}' /etc/passwd # normal (non-system) users
df -h | awk 'NR>1 {print $5, $6}' # usage % and mount point
awk '{sum += $10} END {print sum/1024/1024 " MB"}' /var/log/nginx/access.log # bytes served
sed — stream editor (find & replace)
sed 's/http/https/' file.txt # replace first match per line (prints result)
sed 's/http/https/g' file.txt # replace all matches
sed -i 's/Listen 80/Listen 8080/' /etc/httpd/conf/httpd.conf # edit file in place
sed -i.bak 's/old/new/g' config.ini # in place, keep backup config.ini.bak
sed -n '10,20p' file.txt # print only lines 10-20
sed '/^#/d' file.txt # delete comment lines
Test sed before using -i
Run the sed command without -i first and check the output. Once you are happy, add -i (or -i.bak to keep a backup).