Interview Q&A / Linux & Shell

1. How do you count how often each line appears, sorted from most to least?

   Sort the input, count duplicates with uniq -c, then sort numerically in reverse: sort file | uniq -c | sort -nr. uniq only compares adjacent lines, so the first sort is required. Add head to keep the top results.

   BeginnerQuick answer1 min read
2. How do you print specific columns of a command's output with awk?

   awk splits each line into fields on whitespace and gives them as $1, $2 and so on, with $NF as the last and $0 as the whole line. awk '{print $1, $3}' prints columns 1 and 3. Use -F to set another separator, such as -F: for /etc/passwd.

   BeginnerQuick answer1 min read
3. How do you replace text in a file using sed?

   sed 's/old/new/' replaces the first match on each line; add g to replace every match on a line. Add -i to edit the file in place, ideally with a backup suffix like -i.bak, otherwise sed only prints the changed text.

   BeginnerQuick answer1 min read
4. What do the grep options -i, -v, -c, -n and -w do?

   -i ignores case, -v inverts the match to show non-matching lines, -c prints only a count of matching lines, -n adds line numbers and -w matches whole words only. They combine, so grep -ivn error file works.

   BeginnerQuick answer1 min read
5. What does the wc command count, and how do you use its options?

   wc counts lines, words and bytes in a file or input. wc -l counts lines, -w words, -c bytes and -m characters. It is most often used at the end of a pipeline, such as grep error log | wc -l, to count matches.

   BeginnerQuick answer1 min read
6. When do you need grep -E, and what is the difference from basic grep patterns?

   Basic grep treats +, ?, | and () as ordinary characters unless escaped. grep -E (extended regex) gives them their regex meaning, so grep -E 'cat|dog' matches either word. Use -F when you want a plain fixed string with no regex at all.

   IntermediateQuick answer1 min read
7. When would you use cut instead of awk?

   cut extracts fixed fields by a single-character delimiter or character positions and is simple and fast. awk handles runs of spaces, conditions and calculations. Use cut for neat CSV-like data and awk for messy whitespace-aligned output.

   IntermediateQuick answer1 min read