Bash string concatenate possible? [SOLVED]

How to concatenate strings with underscore, newline, whitespace or any other character in bash? How to append substrings in a string in bash or shell script? How to append starings in a variable using for loop with whitespace? Bash join strings with separator.

These are some of the common questions which we will address in this tutorial. concatenate means nothing but linking or putting things together in some sort of chain or series. You may choose to put two or multiple strings together by any separation character.

Basic concatenation of two strings with no separator

This is quite a straight forward use case wherein we have two variables with some string and you want to concatenate them

For example, I have two variables:

#!/bin/bash

VAR1='Hello ' # There is an extra space after Hello
VAR2='World'

# We don't have any separation character
# Concat both strings from the variables
echo ${VAR1}${VAR2}

As you have noticed VAR1 already contains an extra space so we are just printing both variables together, the output from this script would be:

]# ./eg_1.sh
Hello World

Now we can also join VAR1 and VAR2 into a third variable VAR3 and the output would be the same i.e. "Hello World"

#!/bin/bash

VAR1='Hello ' # There is an extra space after Hello
VAR2='World'

# We don't have any separation character
# Concat both strings from the variables
VAR3="${VAR1}${VAR2}"

echo $VAR3

Join strings with special character as separator

Now the above was a very basic example, let' take it to the next level. In this example we will concatenate strings with underscore, you may choose any other separator such as comma, whitespace, hyphen etc.

Concatenate in a loop (append strings to a variable)

Now assuming you have a requirement where in you have to test network connectivity of multiple hosts and then print a summary with list of success and failed hosts. This would require you to keep storing the list of success and failed hosts, this can be done using various methods but we will use += operator to store the list of strings in a variable (or convert to array based on your requirement)

ALSO READ

Bash check File Exists with Best Practices [5 Methods]

In this example script I have defined a list of 4 hosts out of which 2 are reachable (yeah I know this already) while two are un-reachable but we will use a script to check this. Based on the ping output we will append the string in different variable

#!/bin/bash

# Define empty variables
SUCCESS=""
FAILED=""

# Declare an array as we have to iterate over individual value
declare -a HOSTS='172.20.10.2 172.20.10.3 172.20.10.4 172.20.10.5 172.20.10.6'

# Run a for loop over individual host
for server in ${HOSTS[@]}; do
    # ping with 2 packages and check for connectivity
    ping -c 2 -q $server >/dev/null 2>&1
    if [[ $? -eq 0 ]]; then
       # If reachable then append output to SUCCESS variable
       SUCCESS+="$server "
    else   
       # if not reachable then append to FAILED variable
       FAILED+="$server " 
    fi
done

# print summary
echo "Reachable: $SUCCESS"
echo "Not Reachable: $FAILED"

Output from this script:

~]# ./eg_1.sh
Reachable: 172.20.10.3 172.20.10.4 
Not Reachable: 172.20.10.2 172.20.10.5 172.20.10.6

Here if you notice this if condition is the main part which stores the server list. I have intentionally added an empty whitespace after $server so that there is a gap between list of servers when added to SUCCESS or FAILED

if [[ $? -eq 0 ]]; then
    # If reachable then append output to SUCCESS variable
    SUCCESS+="$server "
else   
    # if not reachable then append to FAILED variable
    FAILED+="$server " 
fi

You may choose to strip the last whitespace character at the end of execution of that creates a problem in your output:

# Strip last whitespace char
SUCCESS=`echo $SUCCESS | sed 's/ $//g'`
FAILED=`echo $FAILED | sed 's/ $//g'`

Concatenate strings using new line character

You can use the same operator += to append strings with new line character, although printing the output would require certain extra handling. In this example I have created an array with some values, I will iterate over these values and then join them together with a new line character at the end

#!/bin/bash

# Declare an array as we have to iterate over individual value
declare -a HOSTS='ab cd ef gh ij'

# Run a for loop over the list of individual value
for server in ${HOSTS[@]}; do
    OUT+="${server}\n"
done

# print output
echo -en "OUTPUT: $OUT"

Conclusion

In this tutorial we learned about bash string concatenation using different joining character such as whitespace, newline, special characters etc. You can use += operator in all sorts of scenarios to combine strings. But one thing to remember is that by default in a loop += will append the string in the end of variable, if you have a different requirement than that would need special handling.