Shell Scripts, Decisions, Loops, Arguments and Functions

shell script

A shell script is a text file of shell commands executed by an interpreter, with behavior determined by arguments, environment, working directory.

shebang

The shebang at the first line names the interpreter the kernel should run when an executable text script is invoked directly.

positional parameters

Positional parameters hold a script or function's arguments as $1, $2 and so on; "$@" preserves each argument as a separate word.

test expression

A shell test expression evaluates file properties, string relations or arithmetic relations and communicates its Boolean result through exit status.

if and case

Shell if selects commands by exit status, while case chooses a branch by matching one word against ordered shell patterns.

for while and until

for iterates over words, while repeats while a command succeeds and until repeats while a command fails

break and continue

break leaves a loop or switch, while continue skips to the next loop iteration; labels can target an enclosing Java statement.

shell function

A shell function is a named group of commands executed in the current shell context, receiving its own positional parameters while sharing most shell state.

trap and cleanup

trap registers shell code for signals or shell events so a script can remove temporary resources, preserve a status or terminate predictably.

script reliability

Predictable script behavior achieved through validated inputs, quoted expansions, checked failures, cleanup and idempotent operations.

Script Control Flow

A script begins with a shebang that selects its interpreter. Positional parameters are $1, $2 and so on. $# is the argument count, quoted "$@" preserves all arguments as separate words and $? is the latest exit status. shift removes the first parameter. if command; then tests the command’s status directly. test and [[ ... ]] evaluate file, string and numeric conditions. A case statement matches a value against patterns.

A for loop iterates over a list. A while loop repeats while a command succeeds. while IFS= read -r line safely reads complete lines without trimming whitespace or interpreting backslashes. break leaves a loop while continue begins its next iteration. Functions receive their own positional parameters and return an exit status. Computed text is normally written to standard output. Variables should be declared local when their scope belongs inside a Bash function.

Reliable Scripts

Quoting expansions prevents unintended splitting and wildcard expansion. Temporary files should be created safely and removed through a trap. External input should not be passed to eval. Diagnostics belong on standard error so standard output remains usable in a pipeline. A script should validate required files and arguments before changing state, stop when a required step fails and leave an exit status that explains success or failure to its caller.

Input Validation and File Processing

A script should display usage when required arguments are absent or invalid. File tests include -e for existence, -f for a regular file, -d for a directory, -r for readability and -s for nonempty size. Numeric input can be checked before arithmetic. Option parsing separates flags from positional operands and should reject unknown options rather than silently ignoring them.

When iterating over files, for file in "$dir"/* preserves spaces because the expanded path is quoted when used. Parsing ls output is unsafe because filenames can contain whitespace and control characters. find with null-delimited output and a matching reader handles arbitrary names. A pipeline may cause a loop to execute in a subshell in some shells, so variables changed inside it may not remain afterward.

Traps respond to signals or shell exit. A common pattern creates a temporary directory with mktemp -d and registers an EXIT trap that removes it. The cleanup handler should tolerate partial initialization. Signals such as INT and TERM should lead to a consistent exit without publishing incomplete output.

Strict-mode options can expose unset variables, failed commands and pipeline failures but they have detailed shell semantics. They do not replace explicit error handling around expected failures. Commands inside conditions, retries and cleanup frequently need deliberate status handling.