What is referred to above as buckets is what we call variables in most programming languages. Furthermore, building onto the basic concept of a variable, a concept we call typing is introduced.

Typing is a name for expected storage and assignment behavior of a variable. So, how does this look when we return to buckets?

In the little bucket world, we have created for our programs, buckets have names. That’s it.

Now suppose that our programs don’t have the luxury of having an infinite number of Jetson bags at their disposal to use as buckets. Before a bucket is named and filled with its contents, the program must decide the shape and constitution of every bucket it touches. I know it seems a little overkill but it is a necessary evil.  All buckets are bound by their type.

String Manipulation in Bash

How does this look when we return to bash?

Functions, arrays, and strings are stored in variables. Bash uses what we call attributes to flag changes in behaviors on assignment. Despite there being a system to modify the behavior of variable assignment, when it all comes down to it, values are stored in variables as strings.

In bash, a program lives to put strings into variables and name them for later use. These strings are available to manipulate throughout the lifetime of the program. The program lives to manipulate strings.

Here we will cover bash string manipulation in breath and as much depth as possible to be accessible to readers of all makes and shapes. Read on.

What is string manipulation in bash

String manipulation is an operation on a string changing its contents. In bash, string manipulation comes in two forms: pure bash string manipulation, and string manipulation via external commands. Here we will touch both types of string manipulation.

Suppose that we have a variable in bash holding a string we wish to manipulate named string. In the case that more than one string exists, we name the strings string, string2, … Also, we may opt to name a string something more meaningful than string to promote understanding of the string content and intended use.

Concatenate Strings — Listing strings in a string

In bash, the easy way to concatenate strings is to list strings in order. The resulting string is a new string containing all strings listed.

Example: String concatenation by listing strings in string

Output

Listing strings in an array

In bash, another way to concatenate strings is to list strings in an array and use parameter expansion to expand array into the concatenated string. However, unlike the example above, removing white space separating array elements may require extra work.

Example: String concatenation by listing strings in an array

Output

Shorten A String — Shift a string to the left

One way to shorten a string is to shift its contents to the left. String contents shifted to the left disappear, resulting in a shorter string.

Example: Shift left loop

Output

Shift a string to the right, get string substring

Another way to shorten a string in bash is to get the substring of a string. The resulting operation can be used to implement a shift operation to the right similar to the method above.

Example: Shift right loop

Output

Example: Shift loop pyramid

For fun, let’s combine the two shift loop examples above to draw a step in our terminal.

Example: Shift loop step

Output

Capitalize the whole string in Bash

In bash4 or later you can capitalize the printable characters using parameter expansion as follows.

Suppose that we throw the first 10 words of the output from the Shift loop step example into a variable called pyramid. Note that unmodified, the expected behavior of echoing its content is as follows.

Command

Output

Notice that as you would expect, there are no caps. Let’s blow it up. That is, we are going to make all of its characters caps.

Command

Output

That was easy!  How about if we want to only convert the first letter in a string to caps like a sentence?  Yes, we can! All we need to do is try a little less hard, one character less to be exact.

Capitalize only the first letter in a string

Maybe capitalizing the whole string is not the bash string manipulation technique you are looking for. What if you only need to capitalize the first letter like a sentence?

Commands

Output

Now suppose that we are more interested in converting strings into lowercase. Luckily, there is a pure bash way to do this; that is use parameter expansion.

Convert the whole string to lowercase in Bash

Convert a string to all lowercase in bash by using the double-comma ( “,,”) parameter expansion operator.

Commands

Output

Convert only the first letter in a string to lowercase

Convert the first character in a string lowercase in bash by using the single-comma ( “,”) parameter expansion operator.

Commands

Output

Assign an empty string a value and return its value

Often you want to set a fallback for an empty string and have its value persist throughout a bash script such as the case when optionally accepting variables from the environment. This can be accomplished using parameter expansion.

Commands

Output

Notes: str is assumed to be unassigned

Replace a pattern occurring in a string once  in Bash

We have a string sitting in a variable and want to replace the first occurrence of a substring. Here’s how.

Basic usage

Commands

Output

Replace all occurrences of a pattern in a string in Bash

We have a string in a variable and want to replace all occurrences of a substring. Here’s how.

Basic usage

Commands

Output

How to manipulate strings in bash using external commands

To manipulate strings in bash using an external command, we need to use a feature that the bash manual calls command substitution. In short, whatever is inside $( ) or ` ` is treated as a command and substituted in place. Frankly, I prefer the first way; however, you may use either. The easy way to use command substitution is to assign the result of command substitution to a variable as follows.

Commands

In the case of string manipulation using an external command in bash, we would need to pipe the echo of a string to the command, unless passing the string to the command as a parameter is accepted. Here is what the new result should look like.

Commands

Now, let’s try doing something real. However, reduce a string containing words to the last word in the string? For this example, let’s use the external command gawk.

Notes on the following commands. Let’s make everything lowercase and get rid of periods. The quote is by Linus Torvalds. It is a really popular quote.

Commands

Output

Bottom line on string manipulation in bash

Here we covered how to manipulate strings the pure bash way as well as using external commands. For pure bash string manipulation techniques, a feature called parameter expansion was used. On the other hand, for the case of external commands, command substitution was used.  Admittingly, in writing this piece, I improved my ability to manipulate strings in bash. Hopefully, you did as well.

Note that the topic of this discussion was not treated in entirety. However, exercises are left for those that would like to tinker a little more. For other string manipulations not contained in this article, you may contact me directly or contact the editor.

That’s enough string manipulation, for now … Thanks,