Linux systems offer numerous methods to manipulate text lines, enabling you to extract or replace words, choose delimiters, and trim whitespace from the edges. These capabilities prove invaluable in tasks like scripting for data processing, cleaning files, or capturing specific strings for further commands. This article discusses various commands facilitating these operations, simplifying tasks that otherwise appear complex.

It’s crucial to determine whether you need to extract complete words or character sequences from specific positions. Your choice of commands will vary based on your specific requirements.

For instance, extracting a word based on its position, say the third word, can be efficiently accomplished using the awk command. To extract the third word from a line of text, you might use an awk command as follows:

The notation $3 indicates the third word, taking advantage of the default space delimiter to identify word boundaries.

To utilize the cut command effectively, you need to specify the delimiter using the -d option. Consider a scenario where -f3 indicates the third word:

It is possible to select multiple fields when employing the cut command, as demonstrated in the following instructions:

To apply an alternative delimiter, such as a colon, you might execute a command like so:

Using awk, you have the ability to work with more than one delimiter. In this example, both a colon and a space are used as delimiters. The initial two lines are for displaying the file content, followed by two lines that explain the command and its output.

To extract a selected sequence of characters from a string, the awk command can be utilized. Here, $0 stands for the entire string, 10 indicates the starting character position, and 5 the number of characters to be extracted.

Similarly, to achieve character extraction using the cut command, this example shows how to retrieve characters from the 13^th to the 22^nd positions in the string.

In another instance using the cut command, characters from the 7^th to 12^th positions are extracted from each line in a file, with the head command limiting the output to just the first 4 lines.

The grep command can be used to search for multiple specific words within a file. In the given example, the -o option is employed to display only the matching words, rather than the full lines on which they appear.

Without the -o option, complete lines are displayed.

Multiple-word phrases can also be selected as demonstrated in this example:

The expr command is also useful for selecting parts of a phrase by defining its starting point and length.

The sed command offers a highly efficient method for substituting words within a string.

You can also use this kind of command to replace multiple words as in this example:

To trim spaces at the start and end of strings, employ the xargs command.

The xargs command also eliminates empty lines and tabs. In the following scenario, a file with two lines filled only with tabs and spaces and another beginning with four spaces followed by a quote, is trimmed to solely include the quote.

When utilizing bash parameter expansion, you can determine the start and end points for the snippet you wish to extract. For instance, by defining a variable with a value and then using syntax like what follows below, you can select a segment of it.

Note that the example above makes it clear that this technique starts position numbering at 0. So, in the next example, the 7 represents the eighth character in the string and the -2 means to drop the last 2 characters. As a result, the substring in the first example below has a single character and the second has all but the last two.

In this next example, we first create a variable using “set –” and then use echo to display the eighth and ninth characters. In other words, it starts with the eighth character (7) and then displays two characters.

NOTE: You could display the string created with the set command by simply using the command “echo $1”. This is what is referenced by the “1” in the example above.

Linux provides many commands to help you manipulate text. The awk, cut, grep, expr, sed and xargs commands along with bash parameter expansion provide you with many useful options.

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