Linux Commands

Vocabulary flashcards covering key Linux/Unix command concepts, options, and file-system operations mentioned in Page 1 notes.

ls

List the contents of a directory.

ls -a

Show all files, including hidden ones (dotfiles).

ls -R

Recursively list a directory and all its subdirectories.

ls -l

Long listing: shows permissions, owner, size, and date. Also the alias l or ll.

ls -S

Sort entries by size, largest first.

ls -Sr

Sort by size with reverse order.

Single quotes

Preserve literal value; no shell expansion inside.

Double quotes

Preserve most characters but allow variable expansion and command substitution.

Command substitution

Run a command inside another command and substitute its output (e.g., date or $(date)).

Shell variable

A variable defined in the shell; local unless exported.

export

Promote a shell variable to an environment variable for child processes.

PATH

Colon-separated directories to search for executables; view with echo $PATH.

Backslash escape

Escape a metacharacter so the shell treats it literally.

unset

Remove a shell variable or function from the current session.

alias

Create a shortcut for a command or sequence of commands; view with alias.

asterisk (*) in

matches 0 or more characters in filenames.

whereis

Find the locations of a command, its binary, source, and man page.

locate

Find files by name using a prebuilt index; update with updatedb.

updatedb

Update the locate database used for fast searching.

man

Manual pages for commands; access command documentation.

man -f

Show a brief description of a command from its manual page.

man -k

Search manual pages by keyword (apropos).

Globbing with brackets [ ]

Match exactly one character from the set inside the brackets.

root directory

Top of the filesystem; represented by /.

pwd

Print the current working directory.

cd

Change the current directory; examples: cd Documents, cd .., cd ../../Downloads.

Absolute path

An full path starting from root, e.g., /home/user.

Relative path

A path relative to the current directory.

ln

An additional directory entry for the same inode; same file content.

Hard Link

ln -s

A shortcut pointing to another file.

Soft link

cp

Copy files or directories; common options: -i (confirm before overwrite), -r (recursive), -v (verbose).

mv

Move or rename files; options: -i, -v, -n (no clobber).

rm

Remove files or directories; options: -r (recursive), -f (force), -i (prompt).

touch

Create an empty file or update file timestamps.

cat

Display or concatenate file contents.

echo

Print text to standard output; can redirect to a file.

mkdir

Create a directory; -p creates parent directories as needed.

head

Display the first N lines of a file (default 10).

tail

Display the last N lines of a file; -n specifies how many lines.

tr

Translate or delete characters; e.g., uppercase conversion (tr 'a-z' 'A-Z').

sort

Sort lines of text in a file or from input.

wc

Count lines, words, and bytes; use -l, -w, and -c.

cut

Extract fields from lines; -d sets the delimiter, -f selects fields.

grep

Search for a pattern in files using basic regular expressions.

egrep

grep with extended regular expressions (supports ?, +, |).

groupadd

Create a new group; -g selects a specific GID.

useradd

Create a new user account; -D shows default values when needed.

passwd

Set or update a user's password.

who

Show users currently logged in.

w

Show who is logged in and what they are doing.

usermod

Modify a user account; -aG adds to supplementary groups; -g sets primary group.

userdel

Delete a user account.

su

Switch user to another user within the current session.

exit

Exit the current shell/session.

chmod

Change file permissions; use symbolic (u/g/o/a +/-/=) or octal (e.g., 754).

chmod 754

Permissions: user rwx, group r-x, others r--.

cp -v

Copy files and directories with verbose output, showing what is being copied.

cp -i

Copies files or directories interactively, prompting before overwriting. The user must confirm before replacing an existing file.

cp -r

Copies files and directories recursively, including all subdirectories.

mv -v

Moves files and directories with verbose output, displaying the actions being taken during the move.

mv -i

Moves files or directories interactively, prompting before overwriting any existing files.

mv -n

Moves files or directories without overwriting existing files, retaining the original files.