Linux Command Line Flashcards: Essential Commands, Flags & Workflows

Question 1

Open the manual page for a command.

Accepted Answer

Use man COMMAND, for example man ls. It opens the installed manual entry when one is available.

Source: man-pages: man(1).

Question 2

Print the current working directory.

Accepted Answer

Use pwd. It prints the absolute path of the directory the shell is currently in.

Source: GNU Coreutils: pwd.

Question 3

Create an empty file or update a file's timestamps.

Accepted Answer

Use touch FILE. It creates an empty file if the path does not exist; otherwise it updates the selected timestamps.

Source: GNU Coreutils: touch.

Question 4

Print a small text file to standard output.

Accepted Answer

Use cat FILE. It copies the file's contents to standard output; use a pager such as less for long files.

Source: GNU Coreutils: cat.

Question 5

Send one command's standard output into another command.

Accepted Answer

Use a pipe: COMMAND1 | COMMAND2. The shell connects the first command's standard output to the second command's standard input.

Source: GNU Bash: pipelines.

Question 6

Where do you see a file's permission bits in ls -l output?

Accepted Answer

At the start of each entry, after the file-type character. A value such as -rw-r--r-- shows the type first, then user, group, and other permissions.

Source: GNU Coreutils: long listing format.

Question 7

Take a quick snapshot of processes associated with the current terminal.

Accepted Answer

Use ps. With its default selection, it gives a one-time process listing rather than a live display.

Source: procps-ng: ps.

Question 8

Create an uncompressed tar archive from files or directories.

Accepted Answer

Use tar -cf ARCHIVE.tar PATH.... -c creates an archive and -f names the archive file.

Source: GNU tar: create.

Question 9

Open an interactive shell on a remote machine over SSH.

Accepted Answer

Use ssh USER@HOST. SSH connects to the remote host and can start an authenticated login session.

Source: OpenSSH: ssh.

Question 10

Display the current environment variables.

Accepted Answer

Use env. With no command or assignments, it prints the current environment.

Source: GNU Coreutils: env.

Question 11

Ask a GNU command for its short built-in usage help.

Accepted Answer

Try COMMAND --help. GNU command-line programs conventionally use --help for a brief usage summary; not every non-GNU command follows this convention.

Source: GNU coding standards: --help.

Question 12

List the current directory's visible entries.

Accepted Answer

Use ls. By default, names beginning with . are omitted.

Source: GNU Coreutils: ls.

Question 13

Create a new directory.

Accepted Answer

Use mkdir DIRECTORY. It creates the named directory when its parent already exists.

Source: GNU Coreutils: mkdir.

Question 14

Read a long text file one screen at a time.

Accepted Answer

Use less FILE. It is a pager: move through the file interactively and press q to quit.

Source: less: manual page.

Question 15

Write standard output to a file, replacing its previous contents.

Accepted Answer

Use COMMAND > FILE. The shell opens the file for output and truncates an existing regular file before the command writes to it.

Source: GNU Bash: redirecting output.

Question 16

What do user, group, and other mean in a file mode?

Accepted Answer

They are three permission classes: the owning user, the owning group, and everyone else. Each class can have read, write, and execute bits.

Source: GNU Coreutils: mode structure.

Question 17

Show a broad, user-oriented snapshot of processes on a Linux system.

Accepted Answer

Use ps aux. The BSD-style a and x options broaden the selection, while u selects a user-oriented format.

Source: procps-ng: ps.

Question 18

Extract an uncompressed tar archive into the current directory.

Accepted Answer

Use tar -xf ARCHIVE.tar. -x extracts and -f names the archive file. Review unfamiliar archive contents before extracting them.

Source: GNU tar: extract.

Question 19

Copy a file to or from a remote machine over SSH.

Accepted Answer

Use scp SOURCE USER@HOST:DESTINATION for an upload, or reverse the remote and local paths for a download.

Source: OpenSSH: scp.

Question 20

Print one environment variable by name.

Accepted Answer

Use printenv NAME, for example printenv PATH. It prints that variable's value and returns a nonzero status when the name is absent.

Source: GNU Coreutils: printenv.

Question 21

Check whether Bash will treat a name as an alias, function, builtin, or executable file.

Accepted Answer

Use type NAME. Bash reports how it would interpret the name.

Source: GNU Bash: type.

Question 22

Include hidden entries in a directory listing.

Accepted Answer

Use ls -a. It includes names beginning with ., including . and ...

Source: GNU Coreutils: general output formatting.

Question 23

Copy a file to another path.

Accepted Answer

Use cp SOURCE DESTINATION. If the destination names an existing file, cp can replace it, so add an interactive check when overwrites are uncertain.

Source: GNU Coreutils: cp.

Question 24

Show the first 10 lines of a text file.

Accepted Answer

Use head FILE. Its default output is the first 10 lines; use -n NUMBER to choose another count.

Source: GNU Coreutils: head.

Question 25

Append standard output to a file without replacing its existing contents.

Accepted Answer

Use COMMAND >> FILE. The shell opens the file in append mode, creating it if needed.

Source: GNU Bash: appending redirected output.

Question 26

What do read, write, and execute mean on a regular file?

Accepted Answer

Read allows reading the contents, write allows modifying the contents, and execute allows the file to be run as a program when its format is executable.

Source: GNU Coreutils: mode structure.

Question 27

Find process IDs by process name.

Accepted Answer

Use pgrep PATTERN. It searches currently running processes and prints matching process IDs.

Source: procps-ng: pgrep.

Question 28

Compress one file with gzip while keeping the original file.

Accepted Answer

Use gzip -k FILE. GNU gzip writes FILE.gz and -k keeps the input file.

Source: GNU gzip manual.

Question 29

Fetch only a URL's response headers with curl.

Accepted Answer

Use curl -I URL. It requests headers only; the server may respond differently than it would to a normal download.

Source: curl: --head.

Question 30

Make a shell variable available to commands started from this Bash session.

Accepted Answer

Use export NAME=value. Bash marks the name for inclusion in the environment of subsequently executed commands.

Source: GNU Bash: export.

Question 31

Ask Bash which command it would run for a name.

Accepted Answer

Use command -v NAME. It reports the command, builtin, function, alias, or executable Bash would use, and returns a nonzero status when none is found.

Source: GNU Bash: command.

Question 32

Show a long directory listing with modes, owners, sizes, and timestamps.

Accepted Answer

Use ls -l. The long format adds file type and mode, link count, owner, group, size, timestamp, and name.

Source: GNU Coreutils: long listing format.

Question 33

Move or rename a file.

Accepted Answer

Use mv SOURCE DESTINATION. Within one filesystem this commonly renames the path; it can also move it to another directory.

Source: GNU Coreutils: mv.

Question 34

Show the last 10 lines of a text file.

Accepted Answer

Use tail FILE. Its default output is the last 10 lines; use -n NUMBER to choose another count.

Source: GNU Coreutils: tail.

Question 35

Give a command standard input from a file.

Accepted Answer

Use COMMAND < FILE. The shell opens the file for reading on the command's standard input.

Source: GNU Bash: redirecting input.

Question 36

What do read, write, and execute mean on a directory?

Accepted Answer

Read permits listing names, write permits changing directory entries, and execute permits traversal and access to entries by name. Creating or removing entries normally needs both write and execute permission.

Source: GNU Coreutils: mode structure.

Question 37

Watch a live, updating view of processes and resource use.

Accepted Answer

Use top. It refreshes a process and system summary interactively; press q to quit.

Source: procps-ng: top.

Question 38

Show one path's total disk usage in a human-readable summary.

Accepted Answer

Use du -sh PATH. -s reports one total and -h formats sizes for people.

Source: GNU Coreutils: du.

Question 39

Download a URL using its remote file name.

Accepted Answer

Use curl -O URL. curl saves the response under the file-name part of the URL; check the target directory before downloading.

Source: curl: --remote-name.

Question 40

Search Bash command history interactively.

Accepted Answer

Press Ctrl-R and type part of the command. Bash Readline performs a reverse incremental search through history.

Source: GNU Bash: reverse search.

Question 41

Search manual-page names and summaries by keyword.

Accepted Answer

Use man -k KEYWORD. It searches the manual-page database's short descriptions, like apropos KEYWORD.

Source: man-pages: man.

Question 42

Show a long listing with human-readable file sizes.

Accepted Answer

Use ls -lh. -l selects long format and -h uses human-readable size units.

Source: GNU Coreutils: block size.

Question 43

Ask before cp overwrites an existing destination file.

Accepted Answer

Use cp -i SOURCE DESTINATION. -i prompts before replacing an existing destination.

Source: GNU Coreutils: cp.

Question 44

Keep watching new lines appended to a log file.

Accepted Answer

Use tail -f FILE. -f keeps the command running and prints data as the file grows; stop it with Ctrl-C.

Source: GNU Coreutils: tail.

Question 45

Write only a command's error stream to a file.

Accepted Answer

Use COMMAND 2> ERRORS.log. File descriptor 2 is standard error, so standard output stays unchanged.

Source: GNU Bash: redirections.

Question 46

Give the file owner execute permission without changing the other mode bits.

Accepted Answer

Use chmod u+x FILE. In symbolic mode, u means owner, + adds a bit, and x means execute.

Source: GNU Coreutils: symbolic modes.

Question 47

Ask a process to terminate gracefully by PID.

Accepted Answer

Use kill PID. By default, kill sends SIGTERM, which gives the process a chance to handle the signal and clean up.

Source: Linux man-pages: kill.

Question 48

Show free and used space for mounted filesystems in readable units.

Accepted Answer

Use df -h. It reports filesystem space and formats sizes in powers of 1024 for easier reading.

Source: GNU Coreutils: df.

Question 49

Check whether a host responds to ICMP echo requests.

Accepted Answer

Use ping HOST. It sends ICMP echo requests and reports replies and packet statistics; a blocked reply does not prove the host is down.

Source: iputils: ping.

Question 50

Create a short Bash name for a longer command.

Accepted Answer

Use alias NAME='COMMAND', for example alias ll='ls -lah'. The alias lasts for the current shell unless you add it to shell startup configuration.

Source: GNU Bash: aliases.

Question 51

Interrupt the foreground command from the terminal.

Accepted Answer

Press Ctrl-C. The terminal normally sends SIGINT to the foreground process group.

Source: GNU Bash: signals.

Question 52

Change to another directory.

Accepted Answer

Use cd PATH. cd is a shell builtin because it changes the shell's own working directory.

Source: GNU Bash: cd.

Question 53

Ask before mv overwrites an existing destination file.

Accepted Answer

Use mv -i SOURCE DESTINATION. -i prompts before replacing an existing destination.

Source: GNU Coreutils: mv.

Question 54

Count the lines in a text file.

Accepted Answer

Use wc -l FILE. -l prints the newline count.

Source: GNU Coreutils: wc.

Question 55

Send standard output and standard error to the same file.

Accepted Answer

Use COMMAND > OUTPUT.log 2>&1. The order matters: standard error is redirected to the destination that standard output has at that point.

Source: GNU Bash: duplicating file descriptors.

Question 56

How do octal digits encode read, write, and execute permissions?

Accepted Answer

Use 4 for read, 2 for write, and 1 for execute, then add the values for each of user, group, and other. For example, 6 means read plus write.

Source: GNU Coreutils: numeric modes.

Question 57

Force a process to stop only after graceful termination has failed.

Accepted Answer

Use kill -KILL PID as a last resort. SIGKILL cannot be caught or ignored, so the process cannot run its normal cleanup.

Source: Linux man-pages: signal.

Question 58

List block devices in a tree without changing them.

Accepted Answer

Use lsblk. It reads device and filesystem information and prints a tree by default.

Source: util-linux: lsblk.

Question 59

Clear the display, or recover a terminal whose state is corrupted.

Accepted Answer

Use clear for a normal screen clear. Use reset when the terminal's modes are garbled and need reinitialization.

Source: ncurses: clear and reset.

Question 60

Move to the parent directory.

Accepted Answer

Use cd ... In a path, .. names the parent directory.

Source: POSIX: pathname resolution.

Question 61

Remove one file without recursive deletion.

Accepted Answer

Use rm FILE. It removes the named directory entry; this is not a trash operation, so verify the path first.

Source: GNU Coreutils: rm.

Question 62

Print lines in a file that contain a pattern.

Accepted Answer

Use grep 'PATTERN' FILE. grep prints matching lines; quote the pattern so the shell does not reinterpret special characters.

Source: GNU Grep manual.

Question 63

Discard a command's standard output.

Accepted Answer

Use COMMAND > /dev/null. /dev/null discards bytes written to it; standard error is still visible unless redirected separately.

Source: Linux man-pages: null.

Question 64

Change a file's owning user.

Accepted Answer

Use chown USER FILE. Changing ownership is permission-sensitive and commonly requires elevated privileges.

Source: GNU Coreutils: chown.

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