linuxexpert-practice-labs/labs/linux-plus/4-2-3-loops-and-functions/README.md

Linux+ 4.2.3: Loops and functions

Reading a file line by line into fields, the pipe subshell trap, and a script with functions and local variables.

Before you start

Work as root, so ~ is /root. The lab creates user alice, so there is at least one normal account to find, and checks that ~/hosts.sh doesn't exist yet. Write answers to the files in /root/lab-answers/.

Tasks

1. With a while IFS=: read -r ... loop over /etc/passwd, print USER -> SHELL for every account with a UID from 1000 up to (not including) 65534. Save the output in /root/lab-answers/1.
2. Count the entries of ls /etc twice into a variable c: once in a while read loop at the end of a pipe, printing pipe: $c, and once with the loop reading from process substitution, printing process substitution: $c. Save both lines in /root/lab-answers/2, and work out why the first count is 0.
3. Write ~/hosts.sh: it sets msg="global", has a function check with a local msg="checking HOST" that it prints before pinging the host once (one-second timeout), loops over its arguments printing up or down after each check, and ends by printing msg is still: $msg. Run it with localhost 192.0.2.1.

Hints

• while IFS=: read -r user _ uid _ _ home shell; do ...; done < /etc/passwd.
• ls /etc | while read -r x; do c=$((c+1)); done runs the loop in a subshell; done < <(ls /etc) doesn't.
• check() { local msg="checking $1"; ...; }, for h in "$@", check "$h" && echo " up" || echo " down".
• The chapter's step 2 (until and a second terminal) isn't graded; try it anyway.