Credit to Jason Fong (a few edits by Catherine Lai)
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 (go back up one level)
cd ..
remove the directory
rmdir new_dir
create blank file
touch new_file
write contents to the file: Note > will overwrite whatever was in that file before, >> does not overwrite contents of file but rather appends it to the end
echo hello world > new_file
echo hello world2 >> new_file
print contents of file to terminal
cat new_file
browse contents of file
less new_file
while browsing using less you can use the following keyboard shortcuts:
- q to quit
- j,k to navigate up and down
- u,d to navigate pages up and down
search for the string "hello" in file
grep "hello" new_file
redirect output of a command to another new file
grep "hello" 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 on the command line to examine what the flags '-a' and '-l' do
add a bash command to the file
echo "echo 'hello world'" > test.sh
The > redirects the output of the command to a 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
The >> tell the shell to concatenate the output of the command to the file test.sh but doesn't overwrite it
add executable permission to it
chmod a+x test.sh
now we can execute this file using dot notation
./test.sh
- add 'sha-bang' (i.e.,
#!) 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
Now have a look at what the values we've assigned to these variables
echo var1 is $var1
echo var2 is $var2
echo var3 is $var3
echo var4 is $var4
echo var5 is $var5
Some examples of for loops over files
touch 1.txt 2.txt 3.txt #make a few dummy files for demonstration
In a bash script, you can write a for loop over multiple lines like so:
for filename in ./*; do #loop over files in current directory
echo $filename #* is a wildcard that matches anything
done
You can also write it all in one line if you want to run it on the command line (note the use of semicolons ;):
for filename in ./* ; do echo $filename ; done
Some more example for loops:
loop over files with given extension
for filename in *.txt; do
echo $filename
done
loop over files in the parent directory, .. is an alias for the parent directory)
for filename in ../*; do
echo $filename
done
how to loop over numbers: a C++ style for loop which iterates over variable i from 0 to 4 (stop at 5)
for ((i=0; i<5; i++)); do
echo $i
done
Another example: use the seq command
for j in $(seq 5 10); do
echo $j
done
how to call a script from within a for loop (e.g. in another script)
for ((i=0; i<5; i++)); do
./test.sh
done
how to call a script with arguments:
First let's create a script called test2.sh that writes out "printing number" and then the first and second command line arguments (i.e., whitespace separated strings that are written immediately after the command)
$1 refers to the first command line argument, $2 refers to the second.
touch test2.sh
chmod a+x test2.sh # make it executable
echo "echo 'printing number' \$1 \$2" > test2.sh # $1 refers to first command line argument
Now put the script into a for loop.
#note: we use \$ to escape the dollar sign
for ((i=0; i<5; i++)); do
./test2.sh $i wow
done
It should print out:
printing number 0 wow
printing number 1 wow
printing number 2 wow
printing number 3 wow
printing number 4 wow
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
Another if example
if [ $boolvar = should_echo_no ]; then #make sure to add the spaces around the square brackets
echo 'yes'
else
echo 'no'
fi