learn-shell.md

Learn Shell in Operating System

Basic bash commands

Command	Description
echo	display string or variables
file	determine file type
ls	list directory contents
ll	list directory contents with long listing format
ls -a	list directory contents without ignoring files start with "."
chmod	change mode for file permission, + for adding, - for removing, = for setting
mkdir	make directories
rm	remove files
rm -r	recursively remove files, often use when removing directories
rm -f	forcely remove files
cp	copy files
cp -r	copy directories
mv	move files
cat	concatenate files and print all contents
more	print one screenful contents
less	similar to more
head -n	print contents with a certain number of lines at the beginning of file
tail -n	print contents with a certain number of lines at the end of file
tail -f	monitoring appending contents
grep	search and print the contents with the keyword in the file
ps	report a snapshot of the current processes
ps aux	report every current processes on the system using BSD syntax
kill	kill process with pid
dirname	show the directory name without full path

• Pipe operator: |

  • The output of the first command is piped to the second command as its input.

• Output redirect operator: > and >>

  • The > overwrites the file if it exists or creates it if it doesn't exist.
  • The >> appends to a file or creates the file if it doesn't exist.

Comparator

Command	Description
-eq	equal
-ne	not equal
-gt	greater than
-ge	greater or equal
-lt	less than
-le	less or equal

Bash Shell Script

#!/bin/bash
# You can also use other interpreters like /usr/bin/python
# If you want the new shell to be executed, you should give it the execution authority
# The command is "chmod +x learn-shell.sh" 

foo=123  # no space between '='

echo $foo  # dollar sign for accessing variables
echo ${foo}bar  # braces for solving ambiguity in expression

if [ 10 -eq 20 ]  # two spaces between the content inside the []
then echo 10 is equal to 20 
elif [ 10 -lt 20 ]  # else if  
then echo 10 is less than 20
else echo 10 is larger than 20
fi  # end if

i=0
while [ $i -lt 10 ]  # while loop
do  # begin command
    echo i = $i
    i=$(($i + 1))  # Use two pairs of parentheses inside dollar sign for expression
    i=`expr $i + 1`  # Using `` and expr is equivalent to using $(())
    # When using 'expr', there must be spaces between operators
    # Execute command inside `` first and replace `command...` as its output
done  # end while

for var in 0 1 2 3 4 5  # for loop
do  
    echo $var  # print 0 to 5
done  # end for

function_name() {  # Use parentheses to declare a function, cannot use parameters inside
    echo I am a function
}

function function_with_params {  # Explicitly declare a function, no parentheses
    echo hello $1, $2  # Use dollar signs for params
} # Function should be pre-defined before calling

function_with_params sun moon  # calling function

lines_in_text () {
    cat $1 | wc -l 
}
num_lines=`lines_in_text $1`  # use the argument of the calling command in terminal 
echo The file $1 has $num_lines lines in it.

hello="hello world"
hello=${hello// /\\ } # change space into backslash space

declare -a array  # declare an array
array+=("$hello" "!") # append elements
for i in ${array[@]} # traverse the elements in array
do
    echo $i
done

mapfile -t files < <(ls pwd) # store the outputs of commands into the array 'files'

# To be continue