Unix/Linux Commands and Shell Scripting

The >> operator in Unix/Linux shell redirects output and appends it to a file, preserving existing content. The > operator overwrites the file, << is for here-documents (inline input), and <> is not a standard shell redirection operator.

whoami displays the username of the currently logged-in user. pwd shows present working directory path, who lists all logged-in users, and uname displays system information (kernel name, version, etc.).

The 'ls -a' command lists all files in a directory, including hidden files (those starting with a dot), whereas 'ls -l' shows detailed information but excludes hidden files by default.

The touch command creates an empty file without opening it for editing. cat displays file contents, less views files page-by-page, and create is not a standard Unix file creation command.

The kill command sends signals (typically SIGTERM or SIGKILL) to terminate processes using their PID. stop, delete, and sleep are not standard Unix process termination commands.

The pattern ??[rgi]* matches files where the 3rd character is r, g, or i: ?? (any 2 chars), [rgi] (3rd char), * (rest). *[rgi]* would match r/g/i anywhere, ?3[rgi]* incorrectly requires '3', **[rgi]? has invalid glob syntax.

The touch command updates both access and modification times of a file (creates file if it doesn't exist). chmod changes permissions, chown changes ownership, and mtime is not a valid command.

The vim editor uses the hidden configuration file '.vimrc' located in the user's home directory to load custom startup settings and preferences.

In vim, :se nu (short for set number) enables line numbering. nonu disables numbering, n is not a valid vim option, and history is unrelated to line numbers.

The more command displays file contents one page at a time with pagination. cat dumps entire file, less is a more advanced pager, view opens files in vim read-only mode.

ls -a lists all files including hidden files (those starting with .). ls alone only shows non-hidden files, cat displays file contents, type shows command type, dir is an alias for ls.

The 'touch' command in Unix/Linux is used to update the access and modification times of a file to the current system time, or to create an empty file if it does not exist.

Vim stores its configuration in .vimrc in the user's home directory. .bashrc is for bash configuration, .exrc is for vi/ex (older vi clone), .etcrc and .usrc are not standard config files.

:se nu in vim enables line numbers (short for set number). bs controls backspace behavior, history is not a standard vim option, noai disables auto-indent, nonu disables line numbers.

While view opens files in vim read-only mode, cat is the standard command to display file contents without modification. more and less are pagers, head/tail show file portions.

The kill command terminates processes using their PID. stop, abort, and exit are not standard Unix process termination commands.

In shell scripting, -eq (equal), -ne (not equal), and -lt (less than) are all integer comparison operators used within [ ] or (( )) constructs. They compare numeric values, not strings. Other common operators include -gt, -ge, and -le.

In shell scripting, ! (NOT), -a (AND), and -o (OR) are Boolean logic operators used for conditional tests. The -a and -o operators work within test constructs, while ! can also be used independently. Modern bash also supports && and || for complex conditions.

The tail command displays the last lines of a file. tail -15 shows the last 15 lines. Conversely, head displays the beginning of a file. The -n option (default 10) specifies line count, so tail -15 filename is correct.