Shell programming 1.ppt
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This document provides an overview of shell programming and shell scripts. It discusses that shell scripts are text files containing Unix commands that will be executed in order. The #! line specifies the shell to use. Shell scripts are useful for automating tasks and avoiding repetition. Examples demonstrate basic shell script elements like variables, command output assignment, control flow statements like for loops and conditionals.
Shell programming 1.ppt
- 1. SHELL PROGRAMMING @2020 copyright KalKey training
- 2. SHELL SCRIPTS (1) Basically, a shell script is a text file with Unix commands in it. Shell scripts usually begin with a #! and a shell name – For example: #!/bin/sh – If they do not, the user's current shell will be used Any Unix command can go in a shell script – Commands are executed in order or in the flow determined by control statements. Different shells have different control structures – The #! line is very important – We will write shell scripts with the Bourne shell (sh) @2020 copyright KalKey training
- 3. SHELL SCRIPTS (2) Why write shell scripts? – To avoid repetition: If you do a sequence of steps with standard Unix commands over and over, why not do it all with just one command? – To automate difficult tasks: Many commands have subtle and difficult options that you don’t want to figure out or remember every time. @2020 copyright KalKey training
- 4. A SIMPLE EXAMPLE (1) tr abcdefghijklmnopqrstuvwxyz thequickbrownfxjmpsvalzydg < file1 > file2 – “encrypts” file1 into file2 Record this command into shell script files: – myencrypt #!/bin/sh tr abcdefghijklmnopqrstuvwxyz thequickbrownfxjmpsvalzydg – mydecrypt #!/bin/sh tr thequickbrownfxjmpsvalzydg abcdefghijklmnopqrstuvwxyz @2020 copyright KalKey training
- 5. BOURNE SHELL VARIABLES Remember: Bourne shell variables are different from variables in csh and tcsh! – Examples in sh: PATH=$PATH:$HOME/bin HA=$1 PHRASE="House on the hill" export PHRASE Note: no space around = Make PHRASE an environment variable @2020 copyright KalKey training
- 6. ASSIGNING COMMAND OUTPUT TO A VARIABLE Using backquotes, we can assign the output of a command to a variable: #!/bin/sh files=`ls` echo $files Very useful in numerical computation: #!/bin/sh value=`expr 12345 + 54321` echo $value @2020 copyright KalKey training
- 7. USING EXPR FOR CALCULATIONS Variables as arguments: % count=5 % count=`expr $count + 1` % echo $count 6 – Variables are replaced with their values by the shell! expr supports the following operators: – arithmetic operators: +,-,*,/,% – comparison operators: <, <=, ==, !=, >=, > – boolean/logical operators: &, | – parentheses: (, ) – precedence is the same as C, Java @2020 copyright KalKey training
- 8. CONTROL STATEMENTS Without control statements, execution within a shell scripts flows from one statement to the next in succession. Control statements control the flow of execution in a programming language The three most common types of control statements: – conditionals: if/then/else, case, ... – loop statements: while, for, until, do, ... – branch statements: subroutine calls (good), goto (bad) @2020 copyright KalKey training
- 9. FOR LOOPS for loops allow the repetition of a command for a specific set of values Syntax: for var in value1 value2 ... do command_set done – command_set is executed with each value of var (value1, value2, ...) in sequence @2020 copyright KalKey training
- 10. FOR LOOP EXAMPLE (1) #!/bin/sh # timestable – print out a multiplication table for i in 1 2 3 do for j in 1 2 3 do value=`expr $i * $j` echo -n "$value " done echo done @2020 copyright KalKey training
- 11. FOR LOOP EXAMPLE (2) #!/bin/sh # file-poke – tell us stuff about files files=`ls` for i in $files do echo -n "$i " grep $i $i done – Find filenames in files in current directory @2020 copyright KalKey training
- 12. FOR LOOP EXAMPLE (3) #!/bin/sh # file-poke – tell us stuff about files for i in *; do echo -n "$i " grep $i $i done – Same as previous slide, only a little more condensed. @2020 copyright KalKey training
- 13. CONDITIONALS Conditionals are used to “test” something. – In Java or C, they test whether a Boolean variable is true or false. – In a Bourne shell script, the only thing you can test is whether or not a command is “successful” Every well behaved command returns back a return code. – 0 if it was successful – Non-zero if it was unsuccessful (actually 1..255) – We will see later that this is different from true/false conditions in C. @2020 copyright KalKey training
- 14. THE IF STATEMENT Simple form: if decision_command_1 then command_set_1 fi Example: if grep unix myfile >/dev/null then echo "It's there" fi grep returns 0 if it finds something returns non-zero otherwise redirect to /dev/null so that "intermediate" results do not get printed @2020 copyright KalKey training
- 15. IF AND ELSE if grep "UNIX" myfile >/dev/null then echo UNIX occurs in myfile else echo No! echo UNIX does not occur in myfile fi @2020 copyright KalKey training
- 16. IF AND ELIF if grep "UNIX" myfile >/dev/null then echo "UNIX occurs in file" elif grep "DOS" myfile >/dev/null then echo "Unix does not occur, but DOS does" else echo "Nobody is there" fi @2020 copyright KalKey training