Exercise: Scripts that decide and repeat

Scripts that decide and repeat

• Validate arguments and exit with distinct statuses.
• Use if/elif/else, case, for and while.
• Parse /etc/passwd line by line.

Start the exercise

On workstation, start the exercise. It removes the scripts of an earlier run from student's home on servera.

[student@workstation ~]$ lab start sa-conditions

Later tasks assume that you are logged in to the server they name: ssh student@servera from workstation, then sudo -i for a root shell.

check.sh: validate and decide

On servera as student, create ~/scripts/check.sh: with exactly one argument, it reports whether it is a directory (with the number of entries), a file (with its size in bytes) or missing. With no argument or too many it prints Usage: SCRIPT PATH to standard error and exits with status 2; a missing path exits with status 1.

[student@workstation ~]$ ssh student@servera
[student@servera ~]$ mkdir -p ~/scripts && cd ~/scripts
[student@servera scripts]$ cat > check.sh <<'EOT'
#!/bin/bash
# Report on a path given as the first argument
if [ $# -ne 1 ]; then
  echo "Usage: $0 PATH" >&2
  exit 2
fi
path=$1
if [ -d "$path" ]; then
  echo "$path is a directory with $(ls "$path" | wc -l) entries"
elif [ -f "$path" ]; then
  echo "$path is a file of $(stat -c %s "$path") bytes"
else
  echo "$path does not exist" >&2
  exit 1
fi
EOT
[student@servera scripts]$ chmod +x check.sh

Test every branch

Run it with no argument, with a directory, with a file, and with a path that does not exist. Print the exit status after each.

[student@servera scripts]$ ./check.sh; echo "rc=$?"
Usage: ./check.sh PATH
rc=2
[student@servera scripts]$ ./check.sh /etc; echo "rc=$?"
/etc is a directory with 172 entries
rc=0
[student@servera scripts]$ ./check.sh /etc/hostname; echo "rc=$?"
/etc/hostname is a file of 24 bytes
rc=0
[student@servera scripts]$ ./check.sh /nope; echo "rc=$?"
/nope does not exist
rc=1

A good script is one where each path through it has been run at least once.

loops.sh

Create loops.sh that prints the squares of 1, 2 and 3 with a for loop and arithmetic, counts down from 3 with a while loop, and then reports ok or MISSING for each of /etc/hostname, /etc/hosts and /etc/missing.

[student@servera scripts]$ cat > loops.sh <<'EOT'
#!/bin/bash
for n in 1 2 3; do
  echo "square of $n is $((n * n))"
done
count=3
while [ $count -gt 0 ]; do
  echo "countdown $count"
  count=$((count - 1))
done
for f in /etc/hostname /etc/hosts /etc/missing; do
  if [ -e "$f" ]; then echo "$f ok"; else echo "$f MISSING"; fi
done
EOT
[student@servera scripts]$ chmod +x loops.sh && ./loops.sh
square of 1 is 1
square of 2 is 4
square of 3 is 9
countdown 3
countdown 2
countdown 1
/etc/hostname ok
/etc/hosts ok
/etc/missing MISSING

svc.sh with case

Write svc.sh that accepts start, stop, or status (also state as a synonym), prints a message, and for anything else prints a usage line to standard error and exits with 2.

[student@servera scripts]$ cat > svc.sh <<'EOT'
#!/bin/bash
case "$1" in
  start)  echo "starting" ;;
  stop)   echo "stopping" ;;
  status|state) echo "all fine" ;;
  *)      echo "Usage: $0 {start|stop|status}" >&2; exit 2 ;;
esac
EOT
[student@servera scripts]$ chmod +x svc.sh
[student@servera scripts]$ ./svc.sh start
starting
[student@servera scripts]$ ./svc.sh state
all fine
[student@servera scripts]$ ./svc.sh bogus; echo "rc=$?"
Usage: ./svc.sh {start|stop|status}
rc=2

users.sh: read a file line by line

Write users.sh that prints the name and the login shell of every user whose UID is 1000 or more but below 60000, using while IFS=: read on /etc/passwd.

[student@servera scripts]$ cat > users.sh <<'EOT'
#!/bin/bash
# List regular users (UID 1000 and up) and their shells
while IFS=: read -r user _ uid _ _ _ shell; do
  if [ "$uid" -ge 1000 ] && [ "$uid" -lt 60000 ]; then
    printf '%-10s %s\n' "$user" "$shell"
  fi
done < /etc/passwd
EOT
[student@servera scripts]$ chmod +x users.sh && ./users.sh
student    /bin/bash

On a machine with more users you would see one line each.

Trace a script

Run check.sh /etc/hostname under bash -x and read how each line is expanded.

[student@servera scripts]$ bash -x ./check.sh /etc/hostname
+ '[' 1 -ne 1 ']'
+ path=/etc/hostname
+ '[' -d /etc/hostname ']'
+ '[' -f /etc/hostname ']'
++ stat -c %s /etc/hostname
+ echo '/etc/hostname is a file of 24 bytes'
/etc/hostname is a file of 24 bytes
[student@servera ~]$ exit

Each + line is a command as bash ran it, after variables were expanded. This is the first tool to reach for when a script misbehaves.

Grade and finish

Check your work with lab grade sa-conditions, then put the exercise away with lab finish sa-conditions.