Command Substitution and Chaining: $( ), ;, && and ||

$( ): put a command's output inside another command

Bash runs the command inside $( ) first and puts its output in its place:

echo "I am $(whoami) on $(hostname)"
echo "/etc has $(ls /etc | wc -l) entries"

In the terminal

student@vulntech:~$ echo "I am $(whoami) on $(hostname)"
I am student on vulntech
student@vulntech:~$ echo "/etc has $(ls /etc | wc -l) entries"
/etc has 64 entries

It's often used with variables. date +%F prints today's date as year-month-day, perfect for naming backups:

today=$(date +%F)
mkdir "backup-$today"
ls

In the terminal

student@vulntech:~$ today=$(date +%F)
student@vulntech:~$ mkdir "backup-$today"
student@vulntech:~$ ls
backup-2026-10-17

Tip: Old scripts use backticks for the same thing: `date`. $( ) is easier to read and can be nested.

Exit status: did it work?

Every command finishes with an exit status: 0 means success, anything else means something went wrong. $? holds the status of the last command:

ls /etc/hostname
echo $?
ls /etc/nothing-here
echo $?

In the terminal

student@vulntech:~$ ls /etc/hostname
/etc/hostname
student@vulntech:~$ echo $?
0
student@vulntech:~$ ls /etc/nothing-here
ls: cannot access '/etc/nothing-here': No such file or directory
student@vulntech:~$ echo $?
2

Running several commands

mkdir projects && cd projects && pwd
cd /etc/nothing-here || echo "No such folder, staying here"

In the terminal

student@vulntech:~$ mkdir projects && cd projects && pwd
/home/student/projects
student@vulntech:~/projects$ cd /etc/nothing-here || echo "No such folder, staying here"
-bash: cd: /etc/nothing-here: No such file or directory
No such folder, staying here

Think of it like this: Read && as and then and || as or else: make the folder and then go in; go there, or else complain.

Common mistake: Use && when the next command depends on the last one. If the folder doesn't exist, cd old-logs && rm *.log deletes nothing, but cd old-logs ; rm *.log deletes the logs in whatever folder you're still in!

Remember: $(command) inserts the output. && = only if it worked, || = only if it failed, $? = the last exit status.