3. String operations

Length of the string: ${#parameter}
```
foo="This string is long."
```
```
echo "'$foo' is ${#foo} characters long."
```
However, ${#@} and ${#*} give the number of the positional parameters.
Extract a substring:
${parameter:offset}
${parameter:offset:length}
```
echo ${foo:5}
```
```
echo ${foo:5:6}
```
```
echo ${foo: -5}
```
```
echo ${foo: -5:2}
```
Notice that a space is needed before the - in order to avoid confusion with a default value.
Remove text from the beginning and from the end:
${parameter#pattern}
${parameter##pattern}
${parameter%pattern}
${parameter%%pattern}
```
foo=file.txt.zip
```
```
echo ${foo#*.}
```
```
echo ${foo##*.}
```
```
echo ${foo%.*}
```
```
echo ${foo%%.*}
```
Replace:
${parameter/pattern/string}
${parameter//pattern/string}
${parameter/#pattern/string}
${parameter/%pattern/string}
```
foo=XYZ.XYZ.XYZ
```
```
echo ${foo/XYZ/ABC}
```
```
echo ${foo//XYZ/ABC}
```
```
echo ${foo/%XYZ/ABC}
```
```
echo ${foo/#XYZ/ABC}
```
If the replacement is omitted, then the matched pattern will be deleted.
```
echo ${foo/XYZ}
```
```
echo ${foo//XYZ}
```
```
echo ${foo/%XYZ}
```
```
echo ${foo/#XYZ}
```

Let's modify the previous longest-word example to use ${#j} instead of $(echo -n "$j" | wc -c) for getting the length of a word:

diff -u longest-word3.sh longest-word2.sh

diff -u longest-word3.sh longest-word2.sh

--- longest-word2.sh 2023-06-28 01:48:25.000000000 +0000
+++ longest-word3.sh 2023-09-30 16:13:27.942258976 +0000
@@ -7,7 +7,7 @@
         max_word=
         max_len=0
         for j in $(strings "$i"); do
-            len="$(echo -n "$j" | wc -c)"
+            len="${#j}"
             if (( len > max_len )); then
                 max_len="$len"
                 max_word="$j"

vim longest-word3.sh

longest-word3.sh

#!/bin/bash

# longest-word2: find longest string in a file

for i; do
    if [[ -r "$i" ]]; then
        max_word=
        max_len=0
        for j in $(strings "$i"); do
            len="${#j}"
            if (( len > max_len )); then
                max_len="$len"
                max_word="$j"
            fi
        done
        echo "$i: '$max_word' ($max_len characters)"
    fi
done

ls -l /usr/bin > dirlist-usr-bin.txt

./longest-word3.sh dirlist-usr-bin.txt

It is not only simpler, but also more efficient:

time ./longest-word3.sh dirlist-usr-bin.txt

time ./longest-word2.sh dirlist-usr-bin.txt

Case conversion:

foo=ABCD

echo ${foo,}

echo ${foo,,}

foo=abcd

echo ${foo^}

echo ${foo^^}

We can also declare a variable to keep only uppercase or lowercase content:

declare -u foo

foo=aBcD

echo $foo

declare -l foo

foo=aBcD

echo $foo

unset foo

foo=aBcD

echo $foo

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