BASH Programming

How to Use Environment Variables in Sed Command

An environment variable is a dynamically named value. Environmental variables are usually exported to a terminal using the command shown below.

export $SOMEVARIABLE=value

Files are available in all terminals after source; for this purpose, environmental variables are stored inside bash files.

Suppose the user has to use or change environmental variables with the help of the sed command. In that case, Users cannot use the command as mentioned above for this. Different functions and methods have to be used for the sed command. In this article, we will see some such methods, which are as follows.

In the Linux command-line, sed is a potent processing tool. Using compact sed one-liners, users often do text replacement which is quite convenient. When users replace sed with shell variables, it also has some disadvantages.

How to Use Environment Variables in Sed Command

Let’s have an example, here we have a file named test.txt.

cat test.txt
CURRENT_TIME = # fill the
current date and time
JAVA_HOME = # fill the

We will write a shell script to populate the JAVA_HOME path and current time in the above given current system. This process is easy, but there are some problems in it which are possible. Here we will write a script using GNU sed.

As we have mentioned above, here, we will substitute the JAVA_HOME path and current time. For this, we will first fill the current time in the right places here. We can use the date command to get the current time.

sed -i -r 's/^(CURRENT_TIME =).*/\1 $MY_DATE/' test.txt

The script written above is not too difficult to understand. By first substituting the command in a variable MY_DATE, get the time and current date and save it.

After getting the data using sed substitution, the user fills it in the file. We have used the -i option of the GNU sed command to perform in-place editing.

Now we will check and execute our script.

$ ./
$ cat test.txt
JAVA_HOME = # fill the JAVA_HOME path

We see in the output that the line with “CURRENT_TIME =” has been replaced. However, the literal “$MY_DATE” is filled in instead of the time and current date.

The reason why shell variables are not expanded within single quotes is that we used single quotes under the sed command.

Double quotation marks are used in the sed command to allow quick fix shell variable expansion.

$ cat
sed -i -r "s/^(CURRENT_TIME =).*/\1 $MY_DATE/" test.txt

Now we will test the script again.

$ ./
$ cat test.txt
CURRENT_TIME = Wed Jan 27 10:02:05 PM CET 2021
JAVA_HOME = # fill the JAVA_HOME path

After filling the time and date in the right places, the JAVA_HOME path is filled.

We see which delimiter we should use by adding another sed command to our script.

$ cat
sed -i -r "s/^(CURRENT_TIME =).*/\1 $MY_DATE/" test.txt
sed -i -r "s/^(JAVA_HOME =).*/\1 $JAVA_HOME/" test.txt

Checking the above script.

$ ./
sed: -e expression #1, char 24: unknown option to `s'

We see that the new sed command that has been added is not working. If we test it for the second time, we see that only its variable is different, but working is the same as the sed command. To Solve This, We Should Take the Following Measures:

The Delimiter Does Not Exist in the Variable

To know this well, users must first understand what the environment variable $JAVA_HOME contains.

$ echo $JAVA_HOME

We can see those shell variables are expanded within double-quotes. So our second sed command comes after variable expansion.

sed -i -r "s/^(JAVA_HOME =).*/\1 /usr/lib/jvm/default/" test.txt

The slashes (/) in the variable’s value interfere with the ‘s’ command (s/pattern/replacement/), which is why the above sed command does not work. In this way, we can select other characters as delimiters of the ‘s’ command.

Users can slightly modify the second sed command to solve this by using ‘#’ as the delimiter of the s command.

sed -i -r "s#^(JAVA_HOME =).*#\1 $JAVA_HOME#" test.txt

Now we test the above script.

$ ./
$ cat test.txt
CURRENT_TIME = Wed Jan 27 10:36:57 PM CET 2021
JAVA_HOME = /usr/lib/jvm/default

Solution 2 works in most cases. Also, we see that ‘#’ in filenames is a valid character on most *nix file systems. If we execute our script to JAVA_HOME on a system set to /opt/#jvm#, the user’s script fails again. We Will Do the Following Work for Our Script to Work in all Cases

  1. If the user takes ‘#’ as the delimiter for better readability, they must select a delimiter for the sed command.
  2. We have to escape all the delimiter characters which are in the contents of the variable.
  3. Lastly, collect the remaining material in the sed command.

Users can use bash substitution to escape the delimiter. For example, the user can escape all ‘#’ characters in variable $VAR.

$ VAR="foo#bar#blah"
$ echo "${VAR//#/\\#}"

Now we will apply our script here.

$ cat
sed -i -r "s/^(CURRENT_TIME =).*/\1 $MY_DATE/" test.txt
sed -i -r "s#^(JAVA_HOME =).*#\1 ${JAVA_HOME//#/\\#}#" test.txt

We will test by executing our script with the spurious JAVA_HOME variable to see if it works as expected.

$ JAVA_HOME=/opt/#/:/@/-/_/$/jvm ./
$ cat test.txt
CURRENT_TIME = Thu Jan 28 11:23:07 AM CET 2021
JAVA_HOME = /opt/#/:/@/-/_/$/jvm

We conclude that our script works even though we have many special characters in our JAVA_HOME variable.


In this article, we saw how to use environmental variables with the sed command. They also make many mistakes that they cause, which we have also mentioned in this article and their redressal. We hope that from this article you will get the complete knowledge that you need.

About the author

Prateek Jangid

A passionate Linux user for personal and professional reasons, always exploring what is new in the world of Linux and sharing with my readers.