Linux Shell Script

1. Linux Shell Script

2. Shell script • The first line is used to specify shell program • #!/bin/sh • Variables variable=“text” variable=0 variable=`program arguments` • To use the declared variable s=“hello” echo $s echo ${s}_123 • System environment variables $ export

3. Passing argument • Special variables • Number of arguments $# • 1st, 2nd, .. arguments $1, $2, ... • Each argument $@ = .$1. .$2.... • All arguments $* = .$1 $2 .... • Exit status $?

4. echo - display text or variable to stdout • echo “hello” • read - read from stdin read s echo “you type: $s” • expr- evaluate the expression read i j=`expr $i + 10` echo $j

5. Shell Arithmetic • Use to perform arithmetic operations. • Syntax:expr op1 math-operator op2Examples: $ expr 1 + 3$ expr 2 - 1$ expr 10 \/ 2$ expr 20 \% 3$ expr 10 \* 3$ echo `expr 6 + 3`

6. if .- conditional statement if condition then ... fi if [ “$s” = “abc” ]; then ... fi • Most conditions can be written by using test or [...] • Test if file size > 0 -s filename • Test if file exists -f filename • Test file permission -r -w -x filename • String equality string1 = string2 • String inequality string1 != string2

7. Test if string length = 0 -z string • Test if string length > 0 -n filename • Equality i1 -eq i2 • Inequality i1 -neq i2 • Greater than i1 -gt i2 • Greater than or equal to i1 -ge i2 • Less than i1 -lt i2 • Less than or equal to i1 -le i2 • Negate !

8. Case case variable in value1) ...;; value2) ...;; *)...;; esac case $f in “english”) echo .hello.;; “thai”) echo .sawasdee.;; esac

9. While Loop while condition do ... done i=0 while [ $i -lt 10]; do echo “Hello” i=`expr $i + 1` done

10. For Loop for variable [ in range ] do ... done for f in *.txt; do cat $f done

11. echo command • \n new line • \a alert (bell) • \b backspace • \c suppress trailing new line • \r carriage return • \t horizontal tab