Mastering sed and expect: Powerful One‑Liner Tricks for Linux Automation

sed

sed works on lines of a file and can modify file contents without opening the file.

Typical syntax: sed [options] ... {script} filename Key options: -i: edit files in place. -i.bak: edit in place and create a backup with a .bak extension. -n: suppress automatic printing. -r: use extended regular expressions. -p: print. -q: quit early (e.g., 3q prints up to line 3 then exits). -d: delete (e.g., 3d deletes line 3). -a: append after a line (e.g., 3a text). -i: insert before a line (e.g., 3i text). -c: change a line (e.g., 3c new). =: print line numbers.

Examples:

Extract odd lines: sed -n '1~2p' or sed -n 'p;N;D'.

Extract even lines: sed -n '2~2p'.

Print lines between two patterns (e.g., between lines starting with b and f).

Replace text: s/old/new/g (or using alternate delimiters s#old#new#g, s@old@new@g).

Modifiers: g (global), p (print matched lines), w file (write matched lines to a file), i / I (ignore case).

Backup before editing: sed -i.bak 's/SELINUX=enforcing/SELINUX=enable/' file.

Search and replace

Standard form: s/old/new/flags. Alternate delimiters ( #, @) can be used to avoid escaping characters.

Replacement exercise

Replace SELINUX=enforcing with SELINUX=enable in a file, remembering to create a backup with -i.bak.

Group reference

Only extended regular expressions support grouping.

Example pattern to capture an IP address: /inet[[:space:]]+([0-9.]+)/ where the part in parentheses is the captured group.

Exercise 1

Extract the IP address from network interface output using a pattern like /inet[[:space:]]+([0-9.]+)/.

Exercise 2

Extract the numeric part of a version string by using - as a delimiter and capturing the portion before .jar.

Exercise 3

Extract the numeric permission bits from a file permission string using a suitable regular expression.

Non‑interactive input

Multi‑line redirection example:

cat >target_file <<EOF
line1
line2
EOF

This writes multiple lines to target_file in one command.

expect

Expect solves interactive prompts in shell scripts by automating input.

Installation: yum install expect -y Key keywords: spawn: start a program. expect: wait for a pattern. exp_continue: continue expecting after a match. send: send input to the spawned program. interact: hand control back to the user. set: define variables.

Expect scripts must start with #!/usr/bin/expect, not a bash shebang.