bash string - ghdrako/doc_snipets GitHub Wiki

• https://www.thegeekstuff.com/2010/07/bash-string-manipulation/
• https://earthly.dev/blog/bash-string/
• https://tldp.org/LDP/abs/html/string-manipulation.html

Assigning content to a variable and printing its content

VariableName='value'
echo $VariableName
# or
VariableName="value"
echo ${VariableName}
# or
VariableName=value
echo "$VariableName"

String length

String Length: Determine the length of a string using ${variable}.

length=${message}

${#string} # Identify String Length inside Bash Shell Script

variableName=value
echo ${#variablename}


words="here are some words"
echo "'$words' is ${#words} characters long

Concatenate strings inside Bash Shell using variables

var=${var1}${var2}${var3}
# or
var=$var1$var2$var3
# or
var="$var1""$var2""$var3"

# The following will insert "**" between the strings
var=${var1}**${var2}**${var3}
# or
var=$var1**$var2**$var3
# or
var="$var1"**"$var2"**"$var3"

# The following concatenate the strings using space:
var=${var1} ${var2} ${var3}
# or
var="$var1" "$var2" "$var3"
# or
echo ${var1} ${var2} ${var3}

var=$var1 $var2 $var3  # not work !!!

Concatenate string using +=

concat=""
concat+="1"
concat+="2"
echo "$concat"
12

Concatenate strings inside Bash Shell using an array

o create an array:
arr=("value1" value2 $value3)

To print an array:
echo ${arr[@]}

To print length of an array:
echo ${#arr[@]}

Using indices (index starts from 0):
echo ${arr[index]}

Note: echo ${arr} is the same as echo ${arr[0]}

Extract a substring from a string

Substring Extraction: Extract part of a string using

${variable:start:length}.
message="Hello, World!"
echo ${message:0:5} 
Outputs: Hello

${string:position} # Extract substring from $string at $position
${string:position:length} # Extract $length of characters substring from $string starting from $position
${string:position}  --> returns a substring starting from $position till end
${string:position:length} --> returns a substring of $length characters starting from $position.

Example

$ NAME=Baeldung
$ echo ${NAME:6}
ng
$ echo ${NAME:0:4}
Bael

$ word="bash"
$ echo "${word:0:1}"
b

$ word="bash"
$ echo "${word:0:1}"
b

$ word="bash"
$ echo "Head: ${word:0:1}"
$ echo "Rest: ${word:1}"
OutputHead: b
Rest: ash

Note: $length and $position must be always greater than or equal to zero. If the $position is less than 0, it will print the complete string. If the $length is less than 0, it will raise an error and will not execute.

Bash Last Character

word="bash" 
$ echo "${word:(-1)}"
$ echo "${word:(-2)}"
$ echo "${word:(-3)}"

Outputh
sh
ash

Remove the last characters from string

word="bash" 
echo "${word:0:${#word}-1}"
echo "${word:0:${#word}-2}"
echo "${word:0:${#word}-3}"

bas
ba
b

Remove the last characters from string use the pattern expansion

$ word="bash" 
$ echo "${word%?}
$ echo "${word%??}
$ echo "${word%???}

bas
ba
b

If the regex doesn’t match, it doesn’t remove anything.

$ word="one"
$ echo "${word%????}" # remove first four characters
one 

Substring matching:

In Bash, the shortest and longest possible match of a substring can be found and deleted from either front or back.

Syntax:

To delete the shortest substring match from front of $string:
${string#substring}

To delete the shortest substring match from back of $string:
${string%substring}

To delete the longest substring match from front of $string:
${string##substring}

To delete the shortest substring match from back of $string of $string:
${string%%substring}

Pattern Matching

if [ "file.jpg" = *.jpg ](/ghdrako/doc_snipets/wiki/-"file.jpg"-=-*.jpg-); then echo "is jpg"; fi

Pattern Matching - extended globbing

• *(pattern) – matches any number of occurrence of pattern
• ?(pattern) – matches zero or one occurrence of pattern
• +(pattern) – matches one or more occurrence of pattern
• !(pattern) – negates the pattern, matches anything that doesn’t match the pattern

$ shopt -s extglob
$ if [ "file.jpg" = *.jp?(e)g ](/ghdrako/doc_snipets/wiki/-"file.jpg"-=-*.jp?(e)g-); then echo "is jpg"; fi

Pattern Matching - regular expression

• https://github.com/ghdrako/doc_snipets/wiki/bash-regexp

$ if [ "file.jpg" =~ .*\.jpe?g ](/ghdrako/doc_snipets/wiki/-"file.jpg"-=~-.*\.jpe?g-); then echo "is jpg"; fi
is jpg

We can use Extended Regular Expressions here, like when calling grep with the -E flag. If we use the capture groups, they’ll be stored in the BASH_REMATCH array variable and can be accessed later.

filename="bash.string.txt"
# Shortest Substring Match
echo ${filename#*.}  # output -> string.txt
echo ${filename%.*}  # output -> bash.string
# Longest Substring Match
echo ${filename##*.} # txt
echo ${filename%%.*} # bash

${string/pattern/replacement}  # Replace only first match
filename="bash.string.txt"
echo "After Replacement:" ${filename/str*./operations.} # output -> bash.operations.txt


${string//pattern/replacement} # Replace all the matches
filename="Path of the bash is /bin/bash"
echo "After Replacement:" ${filename//bash/sh}  # Path of the sh is /bin/sh

${string/#pattern/replacement} #  syntax replaces with the replacement string, only when the pattern matches beginning of the $string.
${string/%pattern/replacement} #  syntax replaces with the replacement string, only when the pattern matches at the end of the given $string.

filename="/root/admin/monitoring/process.sh"

echo "Replaced at the beginning:" ${filename/#\/root/\/tmp}  # /tmp/admin/monitoring/process.sh
echo "Replaced at the end": ${filename/%.*/.ksh}             # /root/admin/monitoring/process.ksh

Bash String Replace

$ phrase="create to create"
$ echo "${phrase/create/make}"  # replace only first 
make to create
echo "${phrase//create/make}"   # replaca all 
make to make

$ number="Phone Number: 234-234-2343"
$ echo "${number//[0-9]/X}
Phone number: XXX-XXX-XXXX

String condition

if [ "one" == "one" ](/ghdrako/doc_snipets/wiki/-"one"-==-"one"-); then
    echo "Strings are equal."
fi

if [ "one" != "two" ](/ghdrako/doc_snipets/wiki/-"one"-!=-"two"-); then
    echo "Strings are not equal."
fi

if [ "aaa" < "bbb" ](/ghdrako/doc_snipets/wiki/-"aaa"-<-"bbb"-); then
    echo "aaa is smaller."
fi

Output:
Strings are equal.
Strings are not equal.
aaa is smaller.

Compare strings to globs

file="todo.gz"
if [ "$file" = *.gz ](/ghdrako/doc_snipets/wiki/-"$file"-=-*.gz-); then     # (Note that the glob is not quoted.)
    echo "Found gzip file: $file"
fi
if [ "$file" = todo.* ](/ghdrako/doc_snipets/wiki/-"$file"-=-todo.*-); then
    echo "Found file named todo: $file"
fi

Output
Found gzip file: todo.gz
Found file named todo: todo.gz

Bash String Regex Match

name="aardvark"
if [ "$name" =~ ^aa ](/ghdrako/doc_snipets/wiki/-"$name"-=~-^aa-); then   # checking if a string starts with aa
    echo "Starts with aa: $name"
fi

Output
Starts with aa: aardvark

name="aardvark"
if [ "$name" =~ dvark ](/ghdrako/doc_snipets/wiki/-"$name"-=~-dvark-); then  #  string contains a certain substring
    echo "Contains dvark: $name"
fi

Output
Contains dvark: aardvark

Split Strings using regexp group

if [ "1](/ghdrako/doc_snipets/wiki/tom|1982"-=~-(.*)\|(.*)\|(.*)-); 
then 
  echo "ID = ${BASH_REMATCH[1]}" ; 
  echo "Name = ${BASH_REMATCH[2]}" ; 
  echo "Year = ${BASH_REMATCH[3]}" ; 
else 
  echo "Not proper format"; 
fi

Output
ID = 1
Name = tom
Year = 1982

Split string using $IFS Internal Field Separator

By default, bash treats spaces as the delimiter between separate elements.

list="foo bar baz"
for word in $list; do # <-- $list is not in double quotes
  echo "Word: $word"
done

Output
Word: foo
Word: bar
Word: baz

If you wrap space-delimited items in brackets, you get an array.

list="foo bar baz"
array=($list)
echo "Zeroth: ${array[0]}"  
echo "First: ${array[1]}"  
echo "Whole Array: ${array[*]}" 

Output
Zeroth: foo
First: bar
Whole Array: foo bar baz

text="1|tom|1982"

IFS='|' 
array=($text)
unset IFS;

echo "ID = ${array[1]}" ; 
echo "Name = ${array[2]}" ; 
echo "Year = ${array[3]}" ; 

Output
ID = 1
Name = tom
Year = 1982

trimming the whitespace aroud string

echo "   lol  " | xargs
lol