Shell Scripting

Bash scripting turns sequences of commands into repeatable, automatable scripts. This section covers the bash features beyond interactive use: functions, arrays, process substitution, redirection tricks, here-docs, exit code handling, and the various shell expansion modes.

Quick Reference

bash cheatsheet — A dense quick reference covering all bash syntax: variables, parameter expansions, conditionals, loops, functions, arrays, case statements, globbing, history, debugging. Modeled on devhints.io/bash — bookmark this one.

Input/Output

Shell Redirection — The complete guide to file descriptors: stdin (0), stdout (1), stderr (2). Redirecting and combining them: >, >>, 2>, &>, &>>. Redirecting stderr to stdout or vice versa. The /dev/null special file. Why 2>&1 must follow the redirect it’s merging.

here-docs and here-strings — Feeding multi-line input to a command with <<EOF. Indented here-docs with <<-. Preventing variable expansion with <<'EOF'. Here-strings (<<<) for single-line input. Practical uses: generating config files inline, feeding multi-line strings to cat or grep, sudo tee pattern for writing to protected files.

Process Substitution — <(command) and >(command) — treating command output as a file. Common uses: diffing directory listings (diff <(ls dir1) <(ls dir2)), feeding output of a command as input to another (while read < <(cmd)), the tee >(gzip > out.gz) pattern.

Shell Expansion

Shell Expansion Order — The eight types of expansion bash applies, in order: brace expansion → tilde expansion → parameter/variable expansion → command substitution → arithmetic expansion → word splitting → pathname expansion → quote removal. Why ${var:-default} and $((i++)) behave the way they do.

Exit Codes and Error Handling

Exit Codes and set -e — Every command returns an exit code: 0 for success, 1-255 for failure. How $? captures it. set -e (exit on error), set -u (error on undefined variable), set -o pipefail (fail if any command in a pipe fails), and set -x (trace execution). trap for running cleanup on exit or signal.

Concepts Reference

O Redirection — Part of the beginner curriculum, covers the foundational concept of stdin/stdout/stderr, pipes, tee, and xargs.

Shell Basics — Part of the beginner curriculum, covers environment variables, PATH, aliases, history, and job control.