Sh - String Variable

About

string in bash.

Initialization

Function Arg

When calling a function, quote the variable otherwise bash will not see the string as atomic but as an array

output_args()
{
  for arg
  do
    echo "$arg"
  done
}
var="hello Nico"

echo "Without Quotes"
echo "**************"
output_args $var
echo 
echo "With Quotes"
echo "**************"
output_args "$var"

Without Quotes
**************
hello
Nico

With Quotes
**************
hello Nico

Initialization from File

Sh with Bash - Back-tick

#!/bin/sh
echo 'hello' > hello.txt
test=`cat hello.txt `
echo $test

hello

Bash

#!/bin/bash
value=$(<file.txt)
echo "$value"

Quote

Double Quote - $“string”

double quotes ¹⁾ (“) preserves the literal value of all characters within the quotes.

The default for a string variable

var="My 
   string"
echo "$var" # The space and eol are preserved
#vs 
echo $var # Double space and eol are deleted

A double-quoted string preceded by a dollar sign $”string“ will cause the string to be translated according to the current locale.

If the current locale is C or POSIX, the dollar sign is ignored.

If the string is translated and replaced, the replacement is double-quoted.

Single Quote - $'string'

From the bash man page, words of the form $'string' are treated specially.

The word expands to string, with backslash-escaped characters replaced as specified by the ANSI C standard.

The expanded result is single-quoted, as if the dollar sign had not been present.

EOL Bash

The below code will produce:

var=$'Hello\nWorld'
echo $var

Hello
World

EOL Posix compliant

BAR=$(printf "hello\nworld\n")

Escape

The escape character is a backslash \

Function

Concatenation

Write the variable one after another:

Concatenation without space:

var1=Hello
var2=Nico
echo $var1$var2

HelloNico

var1=Hello
var2=Nico
echo $var1 $var2

Hello Nico

concatenation with space

var3="${var1} ${var2}"
echo $var3

Hello Nico

Concat with the += assignment operator. value is expanded and appended to the variable’s value.

MY_VARIABLE=A
MY_VARIABLE+=B
echo $MY_VARIABLE

AB

Concat backslash character

variable="First"$'\n'
variable=$variable"Second"$'\n'
echo "$variable"

First
Second

Split

Cut: Split on a line level

echo -e 'One,Two,Three,Four\nOne,Two,Three,Four' | cut -d ',' -f 1-2,4 --output-delimiter=' '

One Two Four
One Two Four

Substring

With substring_expansion, you can get a part of the string.

Example:

UNKNOWN=unknown
echo ${UNKNOWN:2:3}

kno

The last 3 digits

echo ${UNKNOWN: -3} # the space is important, get the

or with cut

echo "nicolas" | cut -c1-4

nico

Last three character

echo -n "nicolas" | tail -c 3

From to (from 2 to -2) - rev reverse the string

echo -n "nicolas" | cut -c2- | rev | cut -c2- | rev

icola

Remove a part

With Parameter Expansion Removal operation, you can remove a part of the string.

MY_VARIABLE=123
echo ${MY_VARIABLE#1}

Length

With length parameter expansion, you can get the length of a string variable.

MY_VARIABLE=123
echo ${#MY_VARIABLE}

Uppercase / Lowercase / InitCap / CamelCase

Before bash 4:

# From upper to lower
echo "HELLO WORLD" | tr '[:upper:]' '[:lower:]'
# With a variable
X=$(echo "${X}" | tr '[:upper:]' '[:lower:]')

With Bash 4:

hello="HELLO WORLD"
echo  ${hello,,} # Lowercase
echo  ${hello,} # Lowercase First letter
echo  ${hello^^} # Uppercase
echo  ${hello^} # Uppercase First letter

Replace

Bash parameters expansion

Replacement with parameter expansion ?

Replace first match of substring with replacement.

${string/substring/replacement}

# Replacing all / in a path
foo=`pwd`
echo ${foo////_}

Replace all matches of substring with replacement.

${string//substring/replacement}

If substring matches front end of string, substitute replacement for substring.

${string/#substring/replacement}

If substring matches back end of string, substitute $replacement for substring.

 
${string/%substring/replacement}

The Escape character is the \ backslash

word="hello/nico"
# Replacing the / character with a space
echo ${word//\// }

hello nico

Others

Extract

With built-in regexp operator, See How to use Regular expression (regex) in Bash?

regexp=$'[^ ]*\s(.*)' # Mandatory to put the regexp in a variable
if [[ "Hello Nico" =~ $regexp ]]; then echo "Match : First group is (${BASH_REMATCH[1]})"; else echo "No match"; fi;

Match : First group is (Nico)

to Array

to array with read and IFS

IFS=', ' read -r -a array <<< "$string"

example

IFS=',' read -r -a myArray <<< "hello,nico"
echo ${#myArray[*]} # length of 2
echo ${myArray[*]}

hello nico

Comparison

Name	Double Bracket operator	Single Bracket Operator	Example
Greater than	>	\> (*)	[[ a > b ]] \|\| echo "a does not come after b"
Lower than	<	\< (*)	[[ az < za ]] && echo "az comes before za"
Equality	= (or ==)	=	[[ a = a ]] && echo "a equals a"
Not Equal	!=	!=	[[ a != b ]] && echo "a is not equal to b"
Pattern matching	= (or ==)	(not available)	[[ $name = a* ]] \|\| echo "name does not start with an 'a': $name"
RegularExpression matching	=~	(not available)	[[ $(date) =~ ^Fri\ ...\ 13 ]] && echo "It's Friday the 13th!"

Comparison Operator

Comparison operator in bash conditional expression for strings.

= : is equal to. With the whitespace framing the = equals char.

if [ "$a" = "$b" ] 
#  is not the same than (Why???)
if [ "$a"="$b" ]

== is equal to.

if [ "$a" == "$b" ] # Synonym of ''=''

[[ $a == z* ]]   # True if $a starts with a "z" (pattern matching).
[[ $a == "z*" ]] # True if $a is equal to z* (literal matching).

[ $a == z* ]     # File globbing and word splitting take place.
[ "$a" == "z*" ] # True if $a is equal to z* (literal matching).

< is less than, in ASCII alphabetical order

if [[ "$a" < "$b" ]]
if [ "$a" \< "$b" ] # The "<" char needs to be escaped in a [ construct

!= is not equal to. Pattern matching within a [[ ... ]] construct.

if [ "$a" != "$b" ]

-z string has zero length
-n string is not null

Example:

Quote strings within test brackets as the variable may be not initialized

if [ -n "$string1" ]  
then
  echo "String \"string1\" is not null."
else  
  echo "String \"string1\" is null."
fi

You can also use the named comparison operators (demo is for integer but they works also for string)

Double Bracket operator	Single Bracket Operator	Example
-lt	-lt	[[ 8 -lt 9 ]] && echo "8 is less than 9"
-ge	-ge	[[ 3 -ge 3 ]] && echo "3 is greater than or equal to 3"
-gt	-gt	[[ 5 -gt 10 ]] \|\| echo "5 is greater than than 10"
-le	-le	[[ 3 -le 8 ]] && echo "3 is less than or equal to 8"
-eq	-eq	[[ 5 -eq 05 ]] && echo "5 equals 05"
-ne	-ne	[[ 6 -ne 20 ]] && echo "6 is not equal to 20"

Contains

See equality operator above with the [[ ... ]] construct.

Example:

if [[ ! "${DIRECTORY_TO_CLEAN}" =~ ^(/opt/infa/|/home/).* ]] ; then
    echo_err "Directory To Clean parameter must be a directory inside /opt/infa or /home/"
    close_script 1
fi

Documentation / Reference

bash man page
http://stackoverflow.com/questions/3005963/how-can-i-have-a-newline-in-a-string-in-sh

¹⁾

Double-Quotes ref