Master Essential Linux Commands: From pwd to umask Explained

pwd – Show Current Directory

The pwd command (print working directory) displays the absolute path of the current directory.

Syntax: pwd [option] Usually used without options.

cd – Change Directory

The cd command (change directory) switches the working directory to the specified path.

Syntax: cd [option] [dir] Without arguments, it jumps to the user's home directory defined by $HOME.

It is a built‑in Bash command; use help cd for help.

tree – Display Directory Tree

tree

lists the contents of a directory in a hierarchical tree format.

Syntax: tree [option] [directory] Common options are shown in the original tables (e.g., -L for depth, -a to include hidden files).

mkdir – Create Directories

mkdir

creates one or more new directories.

Syntax: mkdir [option] [directory] Use -p to create parent directories as needed.

Multiple directories can be created in a single command.

touch – Create Empty Files or Update Timestamps

touch

creates an empty file or updates the access and modification timestamps of existing files.

Syntax: touch [option] [file] Do not confuse with mkdir, which creates directories.

ls – List Directory Contents

ls

lists files and directories with optional details.

Syntax: ls [option] [file] Common options: -l (long format), -a (all files), -h (human‑readable sizes), -i (inode), -t (sort by modification time).

Typical output columns: inode, permissions, link count, owner, group, size, modification time, name.

cp – Copy Files or Directories

cp

copies source files or directories to a destination.

Syntax: cp [option] [source] [dest] Use -r or -R for recursive directory copy.

Preserve attributes with -p.

mv – Move or Rename Files

mv

moves or renames files and directories.

Syntax: mv [option] [source] [dest] Use -i for interactive prompting before overwriting.

rm – Remove Files or Directories

rm

deletes files or directories (dangerous operation).

Syntax: rm [option] [file] Use -r to remove directories recursively.

Prefer rm -i or move to a temporary location before deletion.

rmdir – Remove Empty Directories

rmdir

removes a directory only if it is empty.

Syntax:

rmdir [option] [directory]

ln – Create Links

ln

creates hard links or symbolic (soft) links.

Syntax: ln [option] [source] [target] Hard link: ln source target (both share the same inode).

Symbolic link: ln -s source target (creates a separate file with its own inode).

readlink – Show Symbolic Link Target

readlink

displays the path that a symbolic link points to.

Syntax:

readlink [option] [file]

find – Search Files

find

searches for files in a directory hierarchy and can execute actions on matches.

Syntax: find [-H] [-L] [-P] [pathname] [expression] Common options include -name, -type, -size, -exec, etc.

xargs – Build and Execute Command Lines from Standard Input

xargs

reads items from standard input and constructs command lines to run another command.

Syntax:

xargs [option]

rename – Batch Rename Files

rename

renames files by applying a string substitution.

Syntax: rename from to file Example: rename "finished" "" * removes the word "finished" from all filenames.

Example: rename .jpg .png *.jpg changes extensions.

basename – Strip Directory and Suffix

basename

prints the filename portion of a path, optionally removing a suffix.

Syntax:

basename [name] [suffix]

dirname – Strip Filename

dirname

prints the directory portion of a path.

Syntax:

dirname [name]

chattr – Change File Attributes

chattr

modifies extended attributes of a file (e.g., immutable, append‑only).

Syntax: chattr [option] [mode] [files] Example: chattr +i file makes the file immutable.

Example: chattr +a .bash_history sets append‑only mode.

lsattr – List File Attributes

lsattr

displays the extended attributes of files or directories.

Syntax:

lsattr [options] [files]

file – Determine File Type

file

reports the type of a file based on its content.

Syntax:

file [option] [file]

md5sum – Compute and Verify MD5 Checksums

md5sum

calculates the MD5 hash of files and can verify them against stored values.

Syntax: md5sum [option] [file] Useful for verifying integrity of downloaded or backed‑up files.

chown – Change Owner and Group

chown

changes the user and/or group ownership of files and directories.

Syntax: chown [option] [OWNER] [:[GROUP]] [file] Examples: chown user file, chown :group file, chown user:group file.

chmod – Change Permissions

chmod

modifies the read/write/execute permissions of files and directories.

Syntax: chmod [option] [mode] [file] Modes can be symbolic (e.g., u+x) or numeric (e.g., 755).

Common options: -R for recursive changes.

umask – Set Default Permission Mask

umask

defines the default permission bits that are masked out when new files or directories are created.

Syntax: umask [option] [mode] Default file creation permission is 666; directory creation is 777.

The resulting permission is calculated as default - umask, adjusting odd digits upward to the next even value.

Example: umask 022 yields files with 644 and directories with 755 permissions.