06. Arrays and Strings (Part 2)

Strings

Strings are characters that are grouped together as one data value
Strings have space as delimiters, so if spaces are part of a string, use quotes to preserve the spaces and keep everything within the quotes as one string
When working with strings, it is often necessary to edit or process only a part of a string
Even though it is easy on the command line to use vi to edit a string of characters, it is not possible to do so in a script
In a script, some utilities are often used to edit strings:
- sed, combined with regular expressions
- tr
- awk, when the string has a regular delimiter
The bash shell also has a set of string operations that can be used in a script to manipulate strings

String Length

To get the length (the number of characters) of a string:

${#stringName}

Example:

name=“abc def”

echo $name print abc def

echo ${#name} print 7

Review
- Note that “abc def” above needs to be in quotes because of the space character
- Without quotes, abc will be stored in name, and def will cause an error
- To store both abc and def without using quotes, we need to use an array: name=( abc def )
  - name[0] will be abc and name[1] will be def

Position of a Substring

To find the position of the first character in a substring within a string:

expr index $string $substring

- the value returned is the position of the first character of the substring that can be found in the string
- position in a string starts at 1

Example:

str=“123456789”

expr index “$str” 234 print 2 (not index 1)

expr index “$str” 432 print 2 (2 is the first character found)

Substring Extraction by Index

Just like with a filter, the extraction copies out a substring from the string. It does not change the string itself.
To extract a substring from a string:

${stringName:$startIndex:$length}

- startIndex is the index to start the extraction. It can be a value in a variable
- length is the number of characters extracted. It can be a value in a variable
- if there is no length, then extract to the end of the string
- index of characters starts at 0

Example:

str=“123456789”

echo ${str:4:3} print 567

echo ${str:4} print 56789

Substring Extraction with Shortest Match

Just like with a filter, the extraction copies out a substring from the string. It does not change the string itself.
To extract a substring from a string with shortest match, starting at the beginning of the string:

${stringName#pattern}

To extract a substring from a string with shortest match, starting at the end of the string:

${stringName%pattern}

To be extracted, the substring must match pattern, and pattern can contain wildcards (not regex)
Example:

str=“my.script.example.sh”

echo ${str#*.} print script.example.sh - my. is the shortest substring that matches the *. pattern and is extracted

echo ${str%.*} print my.script.example - .sh is the shortest substring that matches the pattern .* and is extracted

Substring Extraction with Longest Match

Just like with a filter, the extraction copies out a substring from the string. It does not change the string itself.
To extract a substring from a string for with longest match, starting at the beginning of the string:

${stringName##pattern}

To extract a substring from a string for with longest match, starting at the end of the string:

${stringName%%pattern}

To be extracted, the substring must match pattern
pattern can contain wildcards (not regex)
Example:

str=“my.script.example.sh”

echo ${str##*.} print sh - my.script.example. is the longest substring that matches the *. pattern

echo ${str%%.*} print my - .script.example.sh is the longest substring that matches the pattern .*

Substring Substitution

Just like with a filter, the substitution works with a copy of the string. It does not change the string itself.
To substitute a substring at the first match:

${stringName/pattern/replacementStr}

- pattern is the pattern that substring must match. pattern can contain wildcards (not regex)
- replacementStr is the substring that will replace the part of the string that matches the pattern

Example:

str=“myscriptexample.sh”

echo ${str/e/E} # print myscriptExample.sh - note that only first e matches, second e is ignored

echo ${str/./} # print myscriptexamplesh - replaced . with nothing, or delete .

To substitute globally multiple substrings in the string:

${stringName//pattern/replacementStr}

- pattern is the pattern that substring must match. pattern can contain wildcards (not regex)
- replacementStr is the substring that will replace the part of the string that matches the pattern

Example:

str=“this is cis 18c”

echo ${str//is/IS} # print thIS IS cIS 18c - all is match the pattern and are replaced with IS

echo ${str// /-} # print this-is-cis-18c - all spaces match the pattern and are replaced with -

Substring Substitution at Begin or End of String

To substitute when a substring at the beginning the string matches:

${stringName/#pattern/replacementStr}

- pattern is the pattern that substring at the beginning of the string must match. pattern can contain wildcards (not regex)
- replacementStr is the substring that will replace the matched substring

To substitute when a substring at the end the string matches:

${stringName/%pattern/replacementStr}

- pattern is the pattern that substring at the beginning of the string must match. pattern can contain wildcards (not regex)
- replacementStr is the substring that will replace the matched substring

Example:

str=“my.script.example”

echo ${str/#my/your} print your.script.example

echo ${str/%my/your} print my.script.example (no substitute since there is no match)

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