More Linux Shell Scripts

Transcription

1 nit 5 ore Linux hell cripts

2 opics for this nit ore shell commands Control statements Constraints xpressions elect command Case command

3 unning hell cripts shell script as a standalone is an executable program ou must use chmod to change the permissions and make the script executable teps to run a script called myscript 1. $ chmod +x myscript 2. un the script: 3. $./myscript ach line of the script is processed in order

4 Login cripts ou don t want to enter aliases, set environment variables, set up command line editing, etc. each time you log in ll of these things can be done in a script that is run each time the shell is started

5 5 L xercise #1 Create a bash logon shell script called hello_user.sh that when executed will print: Hello (your_logon_name!) our home directory is /home/your_home_directory oday's date is whatever Hint: ou can get the date by using the date command

6 6 L xercise #1: olution #!/bin/bash echo "Hello $L!" echo "our home is: $H echo "oday's date is date hen at the command line type $ chmod +x hello_user.sh $./hello_user.sh

7 Control statements Without control statements, execution within a shell scripts flows from one statement to the next in succession Control statements change the flow of execution he three most common types of control statements: conditionals: if/then/else, case,... loop statements: while, for, until, do,... branch statements: subroutine calls (good programming practice), etc

8 xpressions xpressions can be: tring comparison umeric comparison File operators Logical operators 8

9 xpressions expr supports the following operators: arithmetic operators: +,-,*,/,% comparison operators: <, <=, ==,!=, >=, > boolean/logical operators: &, parentheses: (, ) precedence is the same as C, Java

10 10 xample 1 Check if a certain file exists #!/bin/bash if [ -f /etc/fstab ]; then cp /etc/fstab. echo "Done." else echo "his file does not exist." exit 1 fi

11 for loops for loops allow the repetition of a command for a specific set of values. yntax: for var in value1 value2... do command_set done command_set is executed with each value of var (value1, value2,...) in sequence

12 xample 2 Listing all files in a directory. #! /bin/bash for i in * do echo $i done : * is a wild card that stands for all files in the current directory, and for will go through each value in *, which is all the files and $i has the filename.

13 he case tatement use the case statement for a decision that is based on multiple choices case word in pattern1) command-list1 ;; pattern2) command-list2 ;; pattern) command-list ;; esac

14 xample 3 Lets count the number of files that are executable in current directory #!/bin/bash # initialize variable count=0 for filename in `ls`; do if [ x $filename ] then fi done count=`expr $count + 1` echo "otal of $count files executable"

15 he while loop While loops repeat statements as long as the next command is successful.

16 elect command Constructs simple menu from word list llows user to enter a number instead of a word ser enters sequence number corresponding to the word yntax: select WD in L do done CV-CD

17 #!/bin/bash elect example echo "script to make files private" echo "elect file to protect " select FL in * do echo "ou picked $FL ($L)" done chmod go-rwx "$FL" echo "it is now private"

18 Lab 5 nstall the gcc compiler Compile and run a c program edo last week s homework ry to make the script at the bottom of page 918 work Do exercise #8 on age 970 Do exercise #12 on age 971