Create a file: touch <filename>
Or: echo "you've got to hide your love away" > filename
Compare two files: diff <file1> <file2>
Compare two dirs: diff -ur <dir1> <dir2>
Report or remove repeated lines: uniq <filename>
Sort lines by reverse numeric order: ps aux | sort -k2 -r -n
Print second and third columns from file using comma as a field separator: cut -d ',' -f 2,3 <file.csv>
Replace comma with semicolon as field separator: tr ',' ';' < <file.csv>
Replace all consecutive occurrences of space with a single space: cat file | tr -s ' '
Print file with line numbers: nl -ba <filename>
Count lines in a file: wc -l <filename>
Identify the type of a file:
file <file>.csv
<file>.csv: CSV text
Display text both in character and octal format: od -bc <filename.txt>
Tip: Useful for debugging texts for unwanted chars or to visualize data encoded in a non-human readable format (e.g. a binary file).
Change file name (-n for dry-run): rename -n "s/cat/dog/g" cat.txt
Apply uniform spacing between words and sentencese: fmt -u <filename>
Merge file side by side: paste <file1> <file2>
Paginate a file for printing: pr <filename>
Join two files which have a common join field:
cat foodtypes.txt
>1 Protein
>2 Carbohydrate
>3 Fat
cat foods.txt
>1 Cheese
>2 Potato
>3 Butter
join foodtypes.txt foods.txt
>1 Protein Cheese
>2 Carbohydrate Potato
>3 Fat Butter
To join files using different fields, the -1 and -2 flags (or -j if it's the same char or position) options can be passed to join:
cat wine.txt
>Red Beaunes France
>White Reisling Germany
>Red Riocha Spain
cat reviews.txt
>Beaunes Great!
>Reisling Terrible!
>Riocha Meh
join -1 2 -2 1 wine.txt reviews.txt
>Beaunes Red France Great!
>Reisling White Germany Terrible!
>Riocha Red Spain Meh
Join expects that files will be sorted before joining, so you have to sort them if the lines are not in the right order:
join -1 2 -2 1 <(sort -k 2 wine.txt) <(sort reviews.txt)
Split a file into N files based on size of input: split -n <x> <filename>
Use expand or unexpand to convert from tabs to the (equivalent number of) spaces and viceversa.
To tabs N characters apart, instead of the default of 8: expand --tabs=<n> <filename>