Missing Cs Semester

What is a shell?

A textual interface and command interpreter that lets you run programs, give them input, inspect output, and automate tasks.

What is a terminal?

The visual program that provides an interface where a shell runs.

What is Bash?

The Bourne Again Shell, a widely used Unix shell and scripting language.

Why is the shell useful?

It is fast, composable, scriptable, and lets you combine programs to automate tasks that may be awkward in graphical interfaces.

What is a shell prompt?

The place in a terminal where the shell waits for you to enter a command.

What is a command-line argument?

A string passed to a program when it is executed.

How does the shell normally split a command into a program and arguments?

It splits on whitespace unless characters are quoted or escaped.

How do you pass an argument containing spaces to a shell command?

Quote it with single or double quotes, or escape the spaces with backslashes.

What does man do?

Displays the manual page for a command.

What is tldr useful for?

Showing concise, example-focused help for common command usage.

What does cd do?

Changes the shell's current working directory.

Why must cd be a shell built-in rather than a normal child process?

A child process cannot change the working directory of its parent shell.

What does pwd do?

Prints the current working directory.

What does $PWD contain?

The path of the current working directory.

What is an absolute path?

A path that starts at the filesystem root / and does not depend on the current directory.

What is a relative path?

A path interpreted relative to the current working directory.

What does . mean in a filesystem path?

The current directory.

What does .. mean in a filesystem path?

The parent directory.

What does ~ usually represent in the shell?

The current user's home directory.

What does pressing Tab commonly do in a shell?

Autocompletes commands, filenames, or paths when possible.

What is $PATH?

An environment variable listing directories the shell searches for executable programs.

What does which command do?

Shows the executable path that would be used for a command name.

How can you bypass $PATH when running a program?

Invoke the executable using an explicit path such as /bin/echo or ./program.

What does ls do?

Lists directory contents.

What does cat file do?

Prints the contents of a file.

What does sort file do?

Outputs the file's lines in sorted order.

What does uniq do?

Removes adjacent duplicate lines, so input is commonly sorted first.

What do head and tail do?

Print the first or last portion of input or a file.

What does grep pattern file do?

Prints lines that match the given pattern, which may be a regular expression.

How do you recursively search files with grep?

Use grep -r with a directory or search path.

What is ripgrep (rg)?

A fast, user-friendly recursive search tool commonly used as an alternative to grep.

What is sed primarily used for?

Programmatic text transformation and editing.

What does sed -i 's/pattern/replacement/g' file do?

Replaces all matches of pattern with replacement in the file in place.

What is find used for?

Recursively finding filesystem entries that match conditions such as name, type, size, or modification time.

What does find ~/Downloads -type f -name '*.zip' -mtime +30 find?

Regular files ending in .zip under Downloads that are older than 30 days.

What does {} represent in find -exec?

The pathname of the current matching file.

What is fd?

A more user-friendly alternative to find, though generally less portable.

What is awk especially useful for?

Parsing and processing structured line-oriented text, especially fields or columns.

What does awk '{print $2}' file do?

Prints the second whitespace-separated field from each line.

What does the pipe operator | do?

Connects the stdout of the command on the left to the stdin of the command on the right.

What does > do in the shell?

Redirects stdout to a file, overwriting the file.

What does >> do in the shell?

Redirects stdout to a file by appending.

What does < do in the shell?

Redirects a file into a command's stdin.

What does tee do?

Copies stdin to stdout while also writing it to a file.

What is command substitution?

Running a command and substituting its stdout into the surrounding shell expression using $(command).

Why is $(command) preferred over backticks?

It is clearer and can be nested more easily.

What does a shebang such as #!/bin/bash do?

Specifies the interpreter that should execute the script.

What does set -e do in a Bash script?

Causes the script to exit early when a command fails in applicable contexts.

What does set -u do in a Bash script?

Treats use of an unset variable as an error.

What does set -o pipefail do?

Makes a pipeline fail if any command in the pipeline fails rather than only considering the last command.

In a shell script, what does $0 contain?

The name or path used to invoke the script.

In a shell script, what does $1 contain?

The first positional argument.

What does $@ represent in a shell script?

All positional arguments.

What does $# represent in a shell script?

The number of positional arguments.

What is a CLI flag?

An usually optional argument, commonly beginning with - or --, that modifies program behavior.

What is the common difference between -a and --all style flags?

-a is a short flag and --all is a long flag; programs often provide both for the same option.

What does combining short flags such as -la usually mean?

It is shorthand for separate short flags such as -l -a when the program supports grouping.

What does -- commonly mean when parsing CLI arguments?

Stop parsing options; treat everything after it as positional arguments.

What is globbing?

Shell expansion of wildcard patterns into matching pathnames before the program is executed.

What does * match in a shell glob?

Zero or more characters.

What does ? match in a shell glob?

Exactly one character.

What does {a,b,c} do in shell brace expansion?

Expands into separate alternatives: a, b, and c.

What happens before rm *.py is executed?

The shell expands *.py to matching filenames and passes those filenames as arguments to rm.

What can ** represent in shells such as Zsh when recursive globbing is enabled?

Paths across directory levels, enabling recursive matching.

Do commands in a pipeline normally wait for the previous command to finish before starting?

No. The shell starts the pipeline processes concurrently and connects their streams.

What is stdin?

Standard input, stream number 0, used by a program to receive input.

What is stdout?

Standard output, stream number 1, used for normal program output and pipelines.

What is stderr?

Standard error, stream number 2, used for warnings and errors separately from normal output.

Why is stderr separate from stdout?

So diagnostic messages do not accidentally become data consumed by the next command in a pipeline.

What does 2> errors.txt do?

Redirects stderr to errors.txt.

What does &> output.txt do in Bash?

Redirects both stdout and stderr to the same file.

What does /dev/null do when used as a redirection target?

Discards the data written to it.

What does cmd > /dev/null 2>&1 do?

Discards both stdout and stderr.

What is fzf?

An interactive fuzzy finder that reads candidate lines from stdin and lets you filter and select one.

What is the correct Bash variable assignment syntax?

name=value with no spaces around =.

Why is foo = bar not a Bash variable assignment?

Whitespace causes argument splitting, so the shell interprets it as running command foo with arguments = and bar.

What type are ordinary shell variables conceptually?

Strings.

How do single and double quotes differ in Bash?

Single quotes keep contents literal; double quotes allow expansions such as variables and command substitution.

What does files=$(ls) do?

Stores the stdout of ls in the shell variable files.

What is process substitution?

Using syntax such as <(command) to expose a command's output through a temporary file-like pathname.

When is process substitution useful?

When a command expects filename arguments rather than stdin, such as diff <(cmd1) <(cmd2).

What does printenv show?

Environment variables available to the process.

What is the difference between a shell variable and an exported environment variable?

A shell variable exists in the current shell; an exported variable is also inherited by child processes.

What does TZ=Asia/Tokyo date do?

Runs date with TZ set only in that command's environment.

What does export DEBUG=1 do?

Sets DEBUG in the current shell and marks it for inheritance by child processes.

What does unset DEBUG do?

Removes the variable DEBUG from the current shell environment.

What does exit code 0 conventionally mean?

Success.

What does a nonzero exit code conventionally mean?

Failure or some exceptional condition.

What does $? contain?

The exit status of the most recently executed command.

What does command1 && command2 mean?

Run command2 only if command1 succeeds.

What does command1 || command2 mean?

Run command2 only if command1 fails.

How do Bash if and while conditions determine true or false?

They use the exit status of the condition command; 0 is treated as true/success.

What signal is normally sent when you press Ctrl-C?

SIGINT.

What does Ctrl-Z normally do to the foreground job?

Sends SIGTSTP, suspending the job.

What is SIGTERM intended for?

Requesting that a process terminate gracefully.

Why is SIGKILL special?

It cannot be caught or ignored and immediately terminates the process.

What does fg do?

Resumes a suspended/background job in the foreground.

What does bg do?

Resumes a suspended job in the background.

What does jobs do?

Lists unfinished jobs associated with the current shell session.

What does placing & after a command do?

Starts the command as a background job and returns the shell prompt.