Chaining: ;, &&, ||, subshells and grouping

Lesson 89 introduced && and || inside scripts. They are just as useful at the prompt, where a well-built one-liner can replace a small script. Every operator in this lesson relies on the exit status of the command on its left: 0 is success, anything else is failure.

~% mkdir lab/l96/build && cd lab/l96/build && echo "ready"
ready
build% cd /nope && rm -rf *
sh: cd: can't cd to /nope: No such file or directory
build% cp custom.conf app.conf 2>/dev/null || cp default.conf app.conf
build% grep -q root /etc/passwd && echo "root exists" || echo "no root"
root exists

Look at the second line: with &&, the dangerous rm -rf * never runs because the cd failed. Had it been written with ;, it would have deleted everything in the current directory. Whenever a command only makes sense if the previous one worked, join them with &&. Use || to provide a plan B: copy the custom file, or if that fails, the default one.

Subshells: ( … )

Commands inside parentheses run in a child shell. Whatever they change, the working directory, variables, umask, is thrown away when the parentheses close. That makes them perfect for "go there, do something, and come back" without having to remember where you were.

~% pwd
/root
~% (cd /etc && ls | wc -l)
24
~% pwd
/root
~% x=1; (x=2; echo "inside: $x"); echo "outside: $x"
inside: 2
outside: 1

Grouping: { …; }

Braces also group commands, but they run in the current shell, so a cd inside them sticks. Their main use is to treat several commands as one for a redirection or a pipe: one > captures the output of all of them. The syntax is picky: a space after { and a ; (or newline) before }, because { and } are reserved words, not symbols.

~% { hostname; uname -r; date +%Y; } > lab/l96/info.txt
~% cat lab/l96/info.txt
(none)
4.16.13
2026
~% { echo "== users"; cut -d: -f1 /etc/passwd; } | head -3
== users
root
daemon

Without the braces, hostname; uname -r; date > info.txt would redirect only date. Two more details round off the topic. The exit status of a pipeline is that of its last command, so grep x f | sort && echo ok tests sort, not grep. And $( … ), which you used for command substitution, is itself a subshell: a cd inside it never affects you.