Bash Builtins & Shell Syntax

Conditional branching based on the exit status of a command (0 = true, non-zero = false). Runs the then-branch when the tested command succeeds; supports optional elif and else clauses and must be terminated with fi.

Iterates a variable over a word list, glob, or command substitution; the C-style form iterates using arithmetic. Executes the body once per iteration and exits with the status of the last command run.

Repeats the body while (or until) the condition command exits 0. 'while' loops while the condition succeeds; 'until' is the inverse and is idiomatic for polling until a service becomes ready.

Pattern-matches a word against glob-style patterns (not regex) and runs the first matching branch. Terminators control fall-through: ;; ends, ;& falls through, ;;& tries next pattern.

Loop-control builtins for for/while/until/select loops. 'break' exits the enclosing loop; 'continue' skips to the next iteration; both accept an integer N to affect N nested loops at once.

Exits a shell function or sourced script and returns exit status N (0-255). Without N, returns the exit status of the last command executed in the function.

Writes arguments to stdout separated by spaces and terminated with a newline. Behavior across systems is inconsistent (especially escape processing) — prefer printf for anything with formatting or portability requirements.

Formats and prints arguments according to a C-style format string. Portable, precise, and reuses the format string if extra arguments are provided — the preferred replacement for echo when you need formatting.

Reads a single line from stdin (or a file descriptor) and splits it into the given variables using IFS. The workhorse for line-by-line input processing and interactive prompts.

Replaces the current shell process with COMMAND (no new process is created). Without a command, applies redirections permanently to the current shell — a common way to open long-lived file descriptors.

Executes FILE in the CURRENT shell (no subshell), so variables, functions, and aliases defined in it persist. Contrast with 'bash file' which runs in a child process — its side effects vanish when it exits.

Marks a shell variable (or function with -f) for export to the environment of subsequently executed commands. Without arguments, prints all exported variables in a re-importable format.

Removes a variable or function from the shell — different from setting it to empty. Without a flag it targets variables first, then functions, unless -f or -v is specified.

Declares variables with attributes: arrays, associative arrays, integers, readonly, lowercase/uppercase, exported. 'readonly' is a shortcut for 'declare -r' that locks a variable so it cannot be reassigned or unset for the lifetime of the shell.

Declares a variable scoped to the current function, hiding any global of the same name during the function's execution. Accepts the same attribute flags as declare (-a, -A, -i, -r, -n, ...).

Renames positional parameters: $2 becomes $1, $3 becomes $2, and so on, decrementing $#. Optional N shifts by N positions and is common in manual argument parsing loops.

POSIX conditional expression evaluator — tests file attributes, string comparisons, and integer comparisons, returning exit status 0 (true) or 1 (false). [ is a command (with a required closing ]), not syntax — arguments must be separated by spaces.

Bash-specific conditional expression with safer parsing, glob and regex matching, and short-circuit logical operators. No word splitting or filename expansion happens on the arguments, so quoting is far less error-prone than [ ] — but [[ ]] is not POSIX.

Bash arithmetic evaluation — treats operands as integers and supports C-style operators (+, -, *, /, %, **, <<, >>, &, |, ^, &&, ||, !, ==, !=, <, >, ternary). (( )) returns exit status 0 if EXPR is non-zero, 1 if zero; $(( )) substitutes the result as a string.

Registers a command to run when the shell receives a signal or special event (EXIT, ERR, DEBUG, RETURN). The workhorse for cleanup handlers, temp-file removal, and error diagnostics in scripts.

Blocks until specified background jobs or PIDs complete, returning the exit status of the last one waited for. Essential for scripts that launch parallel background work and need to synchronize.

Interactive job control: jobs lists background/stopped jobs, bg resumes a stopped job in the background, fg brings it to the foreground. Jobs are addressed with %N, %+, %-, or %string.

Concatenates its arguments into a string and executes it as a shell command, going through parsing, expansion, and execution twice. Powerful for building dynamic commands, dangerous with untrusted input.

Changes shell option flags and/or replaces positional parameters. The 'safer bash' idiom 'set -euo pipefail' is the most common use — it turns on error-exit, unset-variable-error, and pipeline failure propagation.