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:

<code>cat >target_file &lt;&lt;EOF
line1
line2
EOF</code>

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.