All-in-one mini-tutorial

[Simon]

Written by Jason Fong

#find out current location in disk (stands for "print working directory")
pwd

#list contents of current directory
ls 

#list contents of current directory with more information, and in list format
ls -l 

#make a directory
mkdir new_dir

#change into a directory
cd new_dir

#leave the directory
cd ..

#remove the directory 
rmdir new_dir

#create blank file
touch new_file

#write contents to the file 
echo hello world >> new_file # >> does not overwrite contents of file, appends
echo hello world2 >> new_file

#print contents of file to terminal
cat new_file

#browse contents of file
# q to quit
# j,k to navigate up and down 
# u,d to navigate pages up and down
less new_file

#search for string in file
grep 2 new_file

#redirect output of a command to another new file
grep 2 new_file > newer_file 

#overwrite contents of file
echo bye world > new_file # > does overwrite contents of the file

#create a bash file using a text editor
touch test.sh #touch creates a file if one doesn't exist

#check read (r), write (w) and execute (x) permissions of the test.sh file 
ls -al #type 'man ls' to examine what the flags 'a' and 'l' do

#add a bash command to the file
echo "echo 'hello world'" > test.sh # > redirects output to file

#examine the contents of this file 
less test.sh

#execute the file
bash test.sh

#add another line to the file
echo "echo 'bye bye world'" >> test.sh # >> concatenates and doesn't overwrite

#add executable permission to it
chmod a+x test.sh 

#now we can execute this file using dot notation
./test.sh

#add 'sha-bang' to the file so that it's interpreted as a bash script
#use a text editor to open the file and add #!/bin/bash to the very top 
#or use "vim test.sh" or "nano test.sh" to edit it 

#how to create a variable
var1='hello world'
var2=helloworld
var3=999
var4=$var3 #can assign a variable to the contents of another variable
var5=${var4}appendedtext #can use curly brackets to clarify variable names

#how to use a variable
echo var1 is $var1
echo var2 is $var2
echo var3 is $var3
echo var4 is $var4
echo var5 is $var5

#for loops over files
touch 1.txt 2.txt 3.txt #make a few dummy files for demonstration
for filename in *; do #loop over files in current directory
    echo $filename #* is a wildcard that matches anything
done
for filename in *.txt; do #loop over files with given extension
    echo $filename
done
for filename in ../*; do #loop over files in the parent directory 
    echo $filename #('..' is an alias for the parent directory)
done

#how to loop over numbers
for ((i=0; i<5; i++)); do #c++ style for loop
    echo $i
done
for j in $(seq 5 10); do #use seq command
    echo $j
done

#how to call a script from within another script
for ((i=0; i<5; i++)); do
    ./test.sh
done

#how to call a script with arguments
touch test2.sh
chmod a+x test2.sh
echo "echo 'printing number' \$1 \$2" > test2.sh # $1 refers to first command line argument
#note: we use \$ to escape the dollar sign
for ((i=0; i<5; i++)); do
    ./test2.sh $i wow
done

#how to iterate over the lines contained within a text file
touch test3.txt
chmod a+x test3.txt
echo 'line1' >> test3.txt
echo 'line2' >> test3.txt
echo 'line3' >> test3.txt
echo 'line4' >> test3.txt
echo 'line5' >> test3.txt
for line in `cat test3.txt`; do #note: we are using `! not ' or "...
    echo $line
done

#be careful using single and double quotes
#they have different behaviours when expanding variables
echo 'var4 contains $var4'
echo "var4 contains $var4"

#if statement - check if two strings are equal
boolvar=lol101
if [ $boolvar = lol101 ]; then #make sure to add the spaces around the square brackets
    echo 'yes'
else
    echo 'no'
fi

if [ $boolvar = should_echo_no ]; then #make sure to add the spaces around the square brackets
    echo 'yes'
else
    echo 'no'
fi

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