Shells

Variables MESSAGE="HELLO WORLD" echo $MESSAGE MESSAGE="HELLO WORLD $LOGNAME" echo $MESSAGE MESSAGE="HELLO WORLD $USER" echo $MESSAGE

comment #this is a comment #this just prints to screen echo hello mum save this as hello.sh change permissions run as sh hello.sh

utilities and commands a utility is a program, a command is an instruction uname -a > outputfile.txt (will display vital statistics on the system.)

Five different types of command simple command pipeline list command function definition compound command

Grouping Commands (echo start ; cat hello.sh ; echo end) > new.txt adds start and end to hello.sh command1 || (command2 && command3)

Exit Status 0 if previous command successful non zero if previous command not successful chmod u-w testfile cat > testfile echo $? 1

basic calculator bc /3 scale=5 (5 decimal places) sqrt(2)

operations + addition - subtraction * multiplication % integer remainder ^ power

Functions sqrt length scale (need option -l for library) s, sine c, cosine a, inverse tanget e, exponetial l, natural logarithm. echo "1+2" | bc echo 2/3 | bc –l (-l is better!!!!)

The Test Statement two ways test arguments [arguments] [ -e ouput.txt ] && echo file exists if the file exists, echo the message [ -e ouput.txt ] || echo file not exist if the file does not exist, echo the message

If…file exists test -e ouput.txt && echo file exists if the file exists, echo the message test -e ouput.txt || echo file not exist if the file does not exist, echo the message

File operators used by test -d, if exists and is directory -e, if exists -f, if exists and is regualar file -r, if exists and is readable -s, if exists and has non-zero size -w, if exists and is writable -x, if exists and is executable

String operators used by test -n, if string has non-zero length -x, if string has zero length string, if string is not null s1=s2, if strings are equal s1!=s2, if strings are different.

Example – NAME not set $ [ "$NAME" ] || echo NAME not set NAME not set $ [ $NAME ] || echo NAME not set NAME not set

Arithmetic Operators used by test -eq, equal -ne, not equal -gt, greater than -ge, greater than or equal to -lt, less than -le, less than or equal to.

Example -eq and = 0 and 00 are numerically equal $ [ 0 -eq 00 ] && echo equal equal 0 and 00 are different strings. $ [ 0 = 00 ] && echo equal $

White Space is important # [ "0" -eq "00" ] && echo equal equal # [ "0" -eq "00"] && echo equal bash: [: missing `]'

IF statement if [ hell = hell ] then echo equal else echo not equal fi if [ hell1 = hell2 ] then echo equal else echo not equal fi

For Loops for name in values do commands done

For loop example for i in $( ls ) do [ -r $i ] && echo $i is readable done

While Loops while true do echo tick tock (this the sound of a clock) date sleep 1 done

Until Loops until command1 do command2 done