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# Strings
# Introduction
## Trim leading and trailing white-space from string
A collection of pure `bash` alternatives to external processes and programs. The `bash` scripting language is more powerful than people realise and you can accomplish most tasks without the need or dependency of external programs.
This is an alternative to `sed`, `awk`, `perl` and other tools. The
function below works by finding all leading and trailing white-space and
removing it from the start and end of the string. The `:` built-in is used in place of a temporary variable.
Calling an external process in `bash` is expensive and excessive use will cause a noticeable slowdown. By sticking to built-in methods (*where possible*) your scripts and programs will be faster, require less dependencies and you'll gain a better understanding of the language itself.
**Example Function:**
```sh
trim_string() {
# Usage: trim_string " example string "
: "${1#"${1%%[![:space:]]*}"}"
: "${_%"${_##*[![:space:]]}"}"
printf '%s\n' "$_"
}
```
**Example Usage:**
```shell
$ trim_string " Hello, World "
Hello, World
$ name=" John Black "
$ trim_string "$name"
John Black
```
## Trim all white-space from string and truncate spaces
This is an alternative to `sed`, `awk`, `perl` and other tools. The
function below works by abusing word splitting to create a new string
without leading/trailing white-space and with truncated spaces.
**Example Function:**
```sh
# shellcheck disable=SC2086,SC2048
trim_all() {
# Usage: trim_all " example string "
set -f
set -- $*
printf '%s\n' "$*"
set +f
}
```
**Example Usage:**
```shell
$ trim_all " Hello, World "
Hello, World
$ name=" John Black is my name. "
$ trim_all "$name"
John Black is my name.
```
## Use regex on a string
We can use the result of `bash`'s regex matching to replace `sed` for a
large number of use-cases.
**CAVEAT**: This is one of the few platform dependant `bash` features.
`bash` will use whatever regex engine is installed on the user's system.
Stick to POSIX regex features if aiming for compatibility.
**CAVEAT**: This example only prints the first matching group. When using
multiple capture groups some modification is needed.
**Example Function:**
```sh
regex() {
# Usage: regex "string" "regex"
[[ $1 =~ $2 ]] && printf '%s\n' "${BASH_REMATCH[1]}"
}
```
**Example Usage:**
```shell
$ # Trim leading white-space.
$ regex ' hello' '^\s*(.*)'
hello
$ # Validate a hex color.
$ regex "#FFFFFF" '^(#?([a-fA-F0-9]{6}|[a-fA-F0-9]{3}))$'
#FFFFFF
$ # Validate a hex color (invalid).
$ regex "red" '^(#?([a-fA-F0-9]{6}|[a-fA-F0-9]{3}))$'
# no output (invalid)
```
**Example Usage in script:**
```shell
is_hex_color() {
if [[ "$1" =~ ^(#?([a-fA-F0-9]{6}|[a-fA-F0-9]{3}))$ ]]; then
printf '%s\n' "${BASH_REMATCH[1]}"
else
printf '%s\n' "error: $1 is an invalid color."
return 1
fi
}
read -r color
is_hex_color "$color" || color="#FFFFFF"
# Do stuff.
```
## Split a string on a delimiter
This is an alternative to `cut`, `awk` and other tools.
**Example Function:**
```sh
split() {
# Usage: split "string" "delimiter"
IFS=$'\n' read -d "" -ra arr <<< "${1//$2/$'\n'}"
printf '%s\n' "${arr[@]}"
}
```
**Example Usage:**
```shell
$ split "apples,oranges,pears,grapes" ","
apples
oranges
pears
grapes
$ split "1, 2, 3, 4, 5" ", "
1
2
3
4
5
# Multi char delimiters work too!
$ split "hello---world---my---name---is---john" "---"
hello
world
my
name
is
john
```
## Change a string to lowercase
**CAVEAT:** Requires `bash` 4+
**Example Function:**
```sh
lower() {
# Usage: lower "string"
printf '%s\n' "${1,,}"
}
```
**Example Usage:**
```shell
$ lower "HELLO"
hello
$ lower "HeLlO"
hello
$ lower "hello"
hello
```
## Change a string to uppercase
**CAVEAT:** Requires `bash` 4+
**Example Function:**
```sh
upper() {
# Usage: upper "string"
printf '%s\n' "${1^^}"
}
```
**Example Usage:**
```shell
$ upper "hello"
HELLO
$ upper "HeLlO"
HELLO
$ upper "HELLO"
HELLO
```
## Trim quotes from a string
**Example Function:**
```sh
trim_quotes() {
# Usage: trim_quotes "string"
: "${1//\'}"
printf '%s\n' "${_//\"}"
}
```
**Example Usage:**
```shell
$ var="'Hello', \"World\""
$ trim_quotes "$var"
Hello, World
```
## Strip all instances of pattern from string
**Example Function:**
```sh
strip_all() {
# Usage: strip_all "string" "pattern"
printf '%s\n' "${1//$2}"
}
```
**Example Usage:**
```shell
$ strip_all "The Quick Brown Fox" "[aeiou]"
Th Qck Brwn Fx
$ strip_all "The Quick Brown Fox" "[[:space:]]"
TheQuickBrownFox
$ strip_all "The Quick Brown Fox" "Quick "
The Brown Fox
```
## Strip first occurrence of pattern from string
**Example Function:**
```sh
strip() {
# Usage: strip "string" "pattern"
printf '%s\n' "${1/$2}"
}
```
**Example Usage:**
```shell
$ strip "The Quick Brown Fox" "[aeiou]"
Th Quick Brown Fox
$ strip "The Quick Brown Fox" "[[:space:]]"
TheQuick Brown Fox
```
## Strip pattern from start of string
**Example Function:**
```sh
lstrip() {
# Usage: lstrip "string" "pattern"
printf '%s\n' "${1##$2}"
}
```
**Example Usage:**
```shell
$ lstrip "The Quick Brown Fox" "The "
Quick Brown Fox
```
## Strip pattern from end of string
**Example Function:**
```sh
rstrip() {
# Usage: rstrip "string" "pattern"
printf '%s\n' "${1%%$2}"
}
```
**Example Usage:**
```shell
$ rstrip "The Quick Brown Fox" " Fox"
The Quick Brown
```
## Check if string contains a sub-string
**Using a test:**
```shell
if [[ "$var" == *sub_string* ]]; then
printf '%s\n' "sub_string is in var."
fi
# Inverse (substring not in string).
if [[ "$var" != *sub_string* ]]; then
printf '%s\n' "sub_string is not in var."
fi
# This works for arrays too!
if [[ "${arr[*]}" == *sub_string* ]]; then
printf '%s\n' "sub_string is in array."
fi
```
**Using a case statement:**
```shell
case "$var" in
*sub_string*)
# Do stuff
;;
*sub_string2*)
# Do more stuff
;;
*)
# Else
;;
esac
```
## Check if string starts with sub-string
```shell
if [[ "$var" == sub_string* ]]; then
printf '%s\n' "var starts with sub_string."
fi
# Inverse (var doesn't start with sub_string).
if [[ "$var" != sub_string* ]]; then
printf '%s\n' "var does not start with sub_string."
fi
```
## Check if string ends with sub-string
```shell
if [[ "$var" == *sub_string ]]; then
printf '%s\n' "var ends with sub_string."
fi
# Inverse (var doesn't start with sub_string).
if [[ "$var" != *sub_string ]]; then
printf '%s\n' "var does not end with sub_string."
fi
```
The contents of this book provide a reference for solving the problems encountered when writing programs and scripts in `bash`. The examples are in function format showcasing how to incorporate these solutions into your code.
<!-- CHAPTER END -->