Bash scripting basics

Transcription

1 Bash scripting basics prepared by Anatoliy Antonov for ESSReS community September

2 Outline Definitions Foundations Flow control References and exercises 2

3 Definitions 3

4 Definitions Script - [small] program for command interpreter Bash is a shell - command-line interpreter, traditional interface for UNIX-like systems Interpreter - environment which reads and executes commands one by one. note: syntax (as well as other) errors are discovered only when execution reaches the respective line 4

5 Bash script Usually - text file with extension.sh Simplest script: list of shell commands Advanced script: generalized program serving several related tasks 5

6 Script anatomy Start file with: #!/bin/bash Place commands on separate lines Use # to add one-line comments echo Hello, world! # prints Hello, World! End with (optional): exit 6

7 Hello World example #!/bin/bash echo Hello World! exit Create file hello.sh Type three lines above, save file Make file executable (type in shell) chmod u+x hello.sh Invoke./hello.sh 7

8 Foundations 8

9 Script and commands Main part of script are shell commands Execution of command may produce a result (integer/string value or list of values) Execution of command always has an exit status (0 - successful completion, error code otherwise) 9

10 Variables Bash has number of built-in/special variables, e.g. $? retrieve exit status of last command, $1 ($2,..., ${10},...) access command-line parameters Put $ in front of variable name to substitute it with respective value (variable substitution) Define own variables by simple assignment (IMPORTANT: no spaces around = ) MYSTR= Hello # prints: Hello echo $MYSTR i=0 let i=$i+1 # prints: i=1 echo i=$i 10

11 Quoting Quote single strings consisting of several words or including special characters Double quotes allow variable substitution, single quotes do not MYSTR= Hello, World # try without quotes :) echo $MYSTR! # prints: Hello, World! echo $MYSTR! # prints: $MYSTR! To print special characters like $, escape them echo \ \$\ \$\ \$\ # prints: $ $ $ 11

12 Executing shell commands Direct execution precise command cat file.txt command from variable $CMD file.txt parameterized command cat $FILENAME Command substitution (to use result) $FILENAME=`cat name.txt` # backquotes Combine as you wish :) 12

13 Arithmetics Bash has only integers Operators: +,-,*,/ plus, minus, multiplication, integer division ** exponentiation % modulo (remainder of integer division) To perform an operation, use construct: let z=5**3 # assign 5*5*5=125 to z let x=$y/2 # assign value of y divided by 2 to x 13

14 Flow control 14

15 IF operator if COMMAND then #command(s) fi Commands between then and fi are executed only when exit status of COMMAND is 0 (success) COMMAND may be test (see next pages) or any external command 15

16 Test Test checks condition and returns appropriate exit status: 0 for true, 1 for false Syntax: condition in square brackets [ -e log/log000 ] [ $x -ne 3 ] [ $str = stop ] 16

17 Test: files -e file exists -f file is regular file (not a directory or device file) -s file is not zero size -d file is directory -r (-w, -x) file has read (write, execute) permission for user running the script etc. if [! -e $FILENAME ] #! negates the condition then echo file not found: $FILENAME fi 17

18 Test: integers -eq is equal to -ne is not equal to -gt is greater than -ge is greater than or equal to -lt is less than -le is less than or equal to if [ "$a" -ne "$b" ] then echo $a is not equal to $b fi 18

19 Test: strings = is equal to!= is not equal to < is less than (ASCII order) > is greater than (ASCII order) -z string is null (has zero length) -n string is not null Always quote the tested strings! if [ $a = $b ] # spaces are necessary! then echo strings are equal fi 19

20 Extending IF if COMMAND then #command(s) else #other command(s) fi if COMMAND then #command(s) elif COMMAND2 then #other command(s) else #yet another command(s) fi fi 20

21 Parametrized Hello #!/bin/bash if [ -z "$1" ] then #condition is true, parameter is empty echo "Hello!" else #parameter is non-empty, use it! echo "Hello, $1!" fi #end of conditional operator exit bash>./hello.sh Hello! bash>./hello.sh Jack Hello, Jack! 21

22 FOR loop for variable in list do #command(s) done Repeats commands between do and done for each element of list, assigning element to variable Useful when number of iterations (repetitions) is known 22

23 Lists Space-separated elements (words or numbers) for planet in Mercury Venus Earth for word in $var1 $var2 $var3 String with elements for planet in Mercury Venus Earth Mars # spaces inside vars? List of files for file in * # enumerates all files in current dir Command substitution for var in `cat list.txt` 23

24 WHILE loop while COMMAND do done #command(s) Repeats commands between do and done while exit status of COMMAND is 0 Usually COMMAND is test Useful when number of iterations is not known 24

25 References and exercises 25

26 And there is much more! Extensive guide is available at from The Linux Documentation Project Google <your task> in bash :) 26

27 Exercises 1. Count number of words in a text file 2. Create file filelist.txt containing current date and list of files in current directory. If filelist.txt exists, write output to filelist2.txt, and so on. Note: echo bla-bla-bla >> file.txt may help 3. Output Fibonacci numbers until Nth, N is given as command-line parameter Fibonacci numbers: {Fn-2 + Fn-1} Note: space-separated, not on separate lines 4. List and count all programs in /bin, /sbin, /usr/bin, /usr/sbin which start with ch (e.g. chmod, chown, chgrp) 27