Exit codes, conditionals, and tests

The exit code is the universal decision signal in shell. This lesson shows the constructs that read it.

Conditional operators

The two shortest conditional constructs:

$ mkdir -p /tmp/example && cd /tmp/example && pwd

/tmp/example

$ test -f /etc/shadow && echo exists || echo missing

exists

The if statement

if condition; then
    :   # success branch
elif other_condition; then
    :   # second branch
else
    :   # failure branch
fi

The : is bash’s null command — it does nothing and returns 0. It is here because a branch containing only a comment is a syntax error: bash requires at least one command per branch, and shellcheck reports SC1073: Couldn't parse this then clause. : is the idiomatic placeholder, and it is also how you write a deliberately empty branch in real code.

The condition is any command. Its exit code is what if reads.

$ if grep -q '^root:' /etc/passwd; then echo found; else echo missing; fi

found

Tests: [, [[, and test

Bash has three ways to write a test expression:

• test expression — the POSIX-portable form.
• [ expression ] — the same form, with the brackets as syntax sugar. Note the spaces inside the brackets — [ foo = bar ] is a test; [foo=bar] is something else.
• [[ expression ]] — the bash extension. Adds pattern matching, regex matching (=~), and removes some word-splitting hazards.

$ x=10; if [[ "$x" -ge 5 ]]; then echo big; else echo small; fi

big

Common test operators

Reading command output

The right way to capture command output in a variable:

$ kernel=$(uname -r); echo "running kernel: $kernel"

running kernel: 6.6.31-linuxkit

The wrong way:

$ kernel=`uname -r`; echo "running kernel: $kernel"

running kernel: 6.6.31-linuxkit

The “no matches” trap

grep, find -print, test -f, and many other commands return non-zero when they find nothing. This is not an error — it is the documented way to signal “no results”. Scripts that treat it as an error need explicit handling:

$ if grep -q 'definitely-not-present' /etc/passwd; then echo found; else echo "exit=$? — no matches is exit 1, not an error"; fi

exit=1 — no matches is exit 1, not an error

Knowledge check