1. CIRC Summer School 2017 Baowei Liu 7.25.2017
Bash Scripting
CIRC Summer School 2017
Baowei Liu

2. source, . VS execute 1. builtin command in Linux.
source filename [argument]
1. builtin command in Linux.
2. read and execute commands from filename in the current shell environment
3. usually used for setting environment variables
. Filename [argument]
synonymous to source in Bash
Make it executable and execute script
start a new process when running and close when completed

3. Conditional Expression and if
if conditional expression
then ….
else fi

4. Conditional Expression and Operators
[[ conditional expression ]] : new, supported by Bash
[ … ] : traditional/standard, portable
(( … )) : arithmetic operations.
(( $a > $b )) ==, !=, <, <=, >, >= (white space are not critical)
[[ ”$a” –gt “$b” ]] –gt –le, =, !=, >, <, (white space are critical
[ “$a” \> “$b” ] –gt, –le \> \<

5. Integer Comparison Suggestion:
Always use (( )) : for integer comparison
> < in [[ ]] is for string comparison!!
Example: compareIntegers.sh

6. Strings Comparison Compare strings: [[ “$a” = “$b” ]]
White spaces around [[ ]], [] and operators are necessary!!
Double-quoting variables!
Examples: compareStrings.sh; compareStrings2.sh

8. Compound Operators
&&, ||
if [[ … ]] && [[ … ]]
then …. fi

9. Loop Constructs –while loop
Conditional Expression
while [[ conditional expression ]] do ….
done
Arithmetic Expansion
while (( arithmetic expression )) …

10. Loop Constructs –until loop
Conditional Expression
until [[ conditional expression ]] do ….
done
Arithmetic Expansion
until (( arithmetic expression )) …

11. Flow Control Constructs: Case
case expression in pattern1) statement;; pattern2) *) …. esac ) ;; and *

12. Strings and Manipulation
Create a string: string variable
Display a string: $stringName
Length of a string: ${#stringName}
Substring: a Bash string just holds one element
Compare strings
Concatenate of string: = or +=
Substring extraction
Substring replacement

13. Substring Extraction ${string:position:length} ${string:position}
Example:substring_extraction.sh

14. Substring Replacement
${string/substring/replacement}
${string//substring/replacement}: replace all matches
${string/#substring/replacement}: replace one match from the front
${string/%substring/replacement}: replace one match from the end
String match/replacement can be parameterized

15. Substring Removal
${string#substring}: Remove the shortest match from the front
${string##substring}: Remove the longest match from the front
${string%substring}: remove the shortest match from the back
${string%%substring}: remove the longest match from the back
Example: rm_substr.sh

16. Common Use Get extension: ${filename##*.}
Get filename without extension ${filename%.*}
Get extension: ${filename##*.}
Get directory name: ${pathname%/*}
Get filename: ${pathname##*/}
Example: FileName/fileName.sh

17. Filename Expansion / Globbing
Expanding filenames containing special characters
Wild cards * ?
Square brackets [set]: “-”
Special characters: ^ (exclude/ other than)
Escaping backslash: protect a subsequent special character

18. Project
Write a Bash script using if/case to count how many files there are in files/ for each of these types: txt, html, xml, data
Key tech: editor, chmod, command substitution, for loop, arithmetic expansion, echo, case or if, string compare, substring removal