Understand and use essential tools
Core command-line skills
You must be fluent with paths before anything else. Know that > truncates while >> appends, that 2> captures stderr, that &> or > file 2>&1 captures both, and that | connects stdout only. tee writes to a file and the terminal at once. Pipes combined with grep -E, cut, sort -u, wc -l, and sed -i cover most text-processing questions.
Searching and archives
find filters on -name, -type, -user, -perm, -size, and -mtime, and runs commands with -exec ... \; (one call per file) or -exec ... + (batched). tar -cf, -xf, and -tf combined with z (gzip), j (bzip2), or J (xz) cover archive questions; the archive name comes before the file list.
Links, ownership, permissions
A hard link shares the inode of the original, so editing either changes both; hard links cannot span filesystems or point at directories. A symbolic link stores a path and may cross filesystems or dangle. Octal chmod (644, 755, 2770 for setgid) and symbolic chmod (g+w, o-r) are both fair game, as is umask subtracting from 666/777.
Trap: the grader inspects the state of the filesystem, not your intent - a symlink where a hard link was requested fails. Remember find -perm 644 means exactly 644, while -perm -644 means at least those bits.
Rule of thumb: one inode, one file. If ls -i prints the same number for two names, they are hard links.
Worked example: find /var/log -type f -size +10M -exec gzip {} \;