Mastering sed: Stream Editing Basics, Commands, and Real‑World Examples

1. Introduction to sed sed stands for Stream Editor, a powerful tool for processing text line by line in a stream. 2. How sed works When sed processes a file, it reads each line into the pattern space, applies editing commands, outputs the result, and then discards the pattern space before moving to the next line. 3. Basic syntax sed OPTIONS… [SCRIPT] [INPUTFILE…] Common options: -n, --quiet : suppress automatic printing of the pattern space. -i : edit files in place. -e : allow multiple commands. -f /path/from/sed_script : read commands from a script file. -r : use extended regular expressions. 4. Addressing in the pattern space Addresses specify which lines a command applies to: # : a specific line number. $ : the last line. /regexp/ : lines matching a regular expression. /regexp/I : case‑insensitive match. addr1,addr2 : a range of lines. first~step : start at first and select every step line (e.g., 1~2 selects 1,3,5,…). 5. Common editing commands d : delete the selected line. p : print the pattern space (often used with -n to print only matched lines). a \text : append text after the matched line. i \text : insert text before the matched line. c \text : replace the matched line with text . s/regexp/replacement/flags : substitute regexp with replacement . Flags include g (global), i (case‑insensitive), p (print if substituted), w /path/to/file (write matched lines), r /path/to/file (read file into output). 6. Practical exercises Exercise 1: Print even-numbered lines Use the first~step address to select every second line: sed -n '2~2p' file.txt Exercise 2: Comment lines containing "ftp" sed '/ftp/ s/^/#This is a command/' file.txt Exercise 3: Upper‑case letters in lines ending with /sbin/nologin First select the lines, then apply a case conversion: sed -n '\#/sbin/nologin$# { s/[a-z]/\U&/g; p }' file.txt Exercise 4: Save non‑comment lines from /etc/man.config sed -n -e '/^#/d;w /tmp/sed/man.txt' /etc/man.config Exercise 5: Change the initdefault ID in /tmp/sed/inittab sed -i 's/^id:[0-9]:initdefault:/id:5:initdefault:/' /tmp/sed/inittab 7. Advanced sed concepts Besides the pattern space, sed also has a hold space for temporary storage. Relevant commands include: h : copy pattern space to hold space. H : append pattern space to hold space. g : replace pattern space with hold space. G : append hold space to pattern space. x : exchange pattern and hold spaces. D : delete the first line of a multi‑line pattern space. n : read the next line into pattern space (replace). N : read the next line and append it to pattern space. Example: print even lines using n and p : sed -n 'n;p' file.txt 8. Summary sed is a versatile text‑processing tool that follows the Unix philosophy of combining small, focused commands to accomplish complex tasks. Mastering its basic syntax, addressing, and editing commands provides a solid foundation for efficient shell scripting and data manipulation.