Unix/Linux Shell Scripting and Commands

The pwd command (print working directory) displays the full path of the current working directory. cwd is not a valid UNIX command, and wd doesn't exist as a standard command.

The * matches any number of characters (including zero), while ? matches exactly one character. These are the two primary wildcard characters used for filename pattern matching in UNIX shells.

The du (disk usage) command summarizes disk usage for directories and files. df shows filesystem free space, quota displays disk quotas, and ulimit controls resource limits.

The bg command resumes a stopped job in the background, while & placed after a command starts it directly in the background. fg brings a background job to foreground.

The uname command displays system information including the UNIX flavor/variant. SHELL is an environment variable for the current shell, not system type.

The SHELL environment variable contains the path to your default shell (e.g., /bin/bash). uname shows system type, not shell. Use echo $0 to see the current shell.

The grep command searches files for patterns matching regular expressions. find locates files by name/attributes, not content patterns. pattern and check are not valid commands.

The ~ symbol is a shortcut for your home directory, so cd ~ navigates there. cd . stays in current directory, cd .. goes to parent (and has capitalization error), cd - goes to previous directory.

The cp -p command preserves file attributes including modification/access times, mode, and ownership. cp -r and cp -R recursively copy directories but reset timestamps.

The grep -v command inverts the match, showing lines that don't contain the pattern. grep -i makes the search case-insensitive, not inverted.

chmod 754 sets: owner (7) = rwx (all access), group (5) = r-x (read+execute), others (4) = r-- (read only). The numbers sum permission bits: r=4, w=2, x=1.

$? stores the exit status of the most recently executed command. A value of 0 indicates success, while non-zero values indicate various error conditions. $$ gives the current shell's PID, $! gives the last background job's PID, and $# gives the argument count.

$# contains the number of positional parameters passed to the script or function. $$ is the PID, $? is exit status, and $! is the last background job's PID.

$$ expands to the process ID (PID) of the current shell. This is useful for creating unique temporary filenames or tracking the shell process. $? is exit status, $! is last background job PID, $# is argument count.

$! contains the process ID of the most recently executed background command. $$ is current shell PID, $? is exit status, $# is argument count.

${parameter:-word} is parameter expansion that returns 'word' if parameter is unset or null, but doesn't assign it. This differs from ${parameter:=word} which would assign the value. Option A correctly describes this default value behavior.

All three methods set a variable to null/empty: name= (no value), name='' (empty single quotes), name="" (empty double quotes). In shell scripting, these all result in an empty string value, effectively NULL.

+(pattern) in extglob matches one or more occurrences of the pattern. So +(X) matches X, XX, XXX, etc. *(X) matches zero or more, ?(X) matches zero or one, !(X) matches anything except the pattern.

The -z test operator returns true if a string has zero length (is empty/null). -n tests for non-zero length, -s tests if a file exists and is non-empty.