pwd     #print working directory

ls     #list content

ls /dir/subdir     #starts with / -> absolute path

ls dir/subdir     #doesn't start with / -> relative path

ls -R    #see also content in the subdirectory

ls -F     #see also a * after runnable programs, / after directories

cd     #change directory

..     #parent directory

cd ..     #go to parent directory

ls ..     #list parent directory content

~     #home directory

cd ~     #go to home directory

ls ~     #list home directory content

cp     #copy files

cp original.txt duplicate.txt     #copy original.txt and name it duplicate.txt

NB: use quotes if there are spaces in files or directories names

cp original.txt duplicate.txt backup     #copy the 2 files in the backup directory

#mv: move or rename files or directories
#move the files from backup to the parent directory
mv backup/original.txt backup/duplicate.txt ..    

mv original.txt old.txt     #rename original.txt in old.txt

mv backup backuptwo     #rename the backup directory

rm     #remove files

rm old.txt duplicate.txt     #remove the 2 files

rmdir     #remove a directory only if it's empty

rmdir backuptwo

mkdir     #make a new directory

mkdir backup

cat     #print file content

cat test.txt

less     #display file content one page at time

less test.txt     #now we can use spaceboard for go to next page or q to quit

less test.txt new.txt

:n to go to the next file, :p for go to the previous one, :q to quit

head     #display first 10 lines

head -n 3 test.txt     #only first 3 lines

NB: with tab you have auto completion, double tab in case of ambiguity

man     #find out what a command do

man less

cut      #select column from a file

#select columns from 2 to 5 and 8 using comma as separator
cut -f 2-5,8 -d , file.csv     #-f = fields, -d = delimiter

grep     #select a line in file according to what contain

grep hello file.csv     #select lines with hello

grep patterns
-c: print a count of matching lines rather than the lines themselves
-h: do not print the names of files when searching multiple files
-i: ignore case (e.g., treat "Regression" and "regression" as matches)
-l: print the names of files that contain matches, not the matches
-n: print line numbers for matching lines
-v: invert the match, i.e., only show lines that don't match

grep -n -v hello test.csv     #select lines with lines number without hello

grep -c hello file.csv two.csv     #how many lines with hello in the two files 

sed     #replace

sed 's/hello/hi/g' test.txt     #replace hello with hi in test.txt

\> redirects output to a file

head -n 3 test.txt > test2.txt

| create pipe

#take first 9 rows, then take the last two of the result (8-9)
head -n 9 test.txt > tail -n2

#1. select first column of the comma delimited file.csv in dir
#2. remove lines with "Date" (maybe the header)
#3. take the first ten lines
cut -d , -f 1 dir/file.csv | grep -v Date | head -n 10

wc count of the character (-c), words (-w), lines (-l) in a file

grep hello file.csv | wc -l     #num of records with hello

wildcards
* -> matches 0 or more characters
? -> matches a single character
[...] -> matches any one of the characters in the brackets
{...} -> matches any of the comma-separated patterns in the brackets

cut -d , -f 1 dir/*.csv     #first field of all csv files in dir

cut -d , -f 1 dir/c*.csv     #first field of all csv files in dir starting with c

sort put data in order (alphabetically without patterns)

sort patterns
-r -> reverse alphabetical order
-r -> sort numerically
-b -> ignore leading blanks
-f -> case insensitive

#take column 2 of the file, only the lines with hello, reverse order
cut -d , -f 2 dir/file.csv | grep -v hello| sort -r

uniq remove adjacent duplicate lines

# take second column of the file without duplicates
cut -d , -f2 file.csv | sort -r | uniq

# same but with count of how often each occours
cut -d , -f2 file.csv | sort -r | uniq -c

# same the result in new.csv
cut -d , -f2 file.csv | sort -r | uniq -c > new.csv

ctrl+c for stop a running program

some environment variables
HOME -> User's home directory
PWD -> Present working directory
SHELL -> Which shell program is being used
USER -> User's ID

echo     #prints its argument

echo hello     #print hello

echo $USER      #print the value of the variable

testvar=dir/file.csv    #assign dir/file.csv to the variable testvar

head -n 1 testvar

expr for numeric calculation but without decimal

expr 1+3

bc is a calculator program, you can use it in a pipe

echo "5 + 7.5" | bc

echo "scale = 3; 10 / 3" | bc     #scale for how many decimals

for loops
for [variable] in [list] ; do [body] ; done

# print second line of each csv in directory dir
for file in dir/*.csv; do head -n 2 $file | tail -n 1; done

nano file.txt     #edit file.txt with nano text editor

history     #see your command history

history | tail -n 3 > steps.sh    #save your last 3 steps to file

head -n 1 dir/*.csv > header.sh     #save command in sh file

bash headers.sh     #tell shell to run commands in the file

$@     #all of the command-line parameters given to the script

Es. if unique-lines.sh contains sort $@ | uniq, executing:

bash unique-lines.sh dir/file.csv

will run this command:

sort dir/file.csv | uniq

if you execute this:

bash unique-lines.sh dir/file.csv dir/file2.csv

it processes both the files.
Use $1, $2, and so on to refer to specific command-line parameters:

head -n $2 $1 | tail -n 1 | cut -d , -f $3 > get-field.sh
#take a filename, the row to select, the column to select, and print
bash get-field.sh dir/file.csv 4 2

you can write for loops in shell scripts without semicolon

# Print the first and last data records of each file.
for filename in $@
do
    head -n 2 $filename | tail -n 1
    tail -n 1 $filename
done

in shell scripts use # for comments

use \ to go to new line