Cao Yi

Extracting Substrings in Bash Shell Scripts

Index

Obtain the original string: ${var_name}.

$ a='abc123'
$ echo $a
abc123

Modify a section of ${var_name}:

Remove all digit characters and replace them with an empty string

$ echo ${a//[0-9]/}
abc

Remove all non-digit characters and replace them with an empty string

$ echo ${a//[!0-9]/}
123

Replace digits with ‘dd’

$ echo ${a//[0-9]/dd}
abcdddddd

Reference: https://stackoverflow.com/questions/13210880/replace-one-substring-for-another-string-in-shell-script

Another example. Extract digits at the end of a string. Assisted by ChatGPT.

Question: Can you please explain the following commands and outputs?

$ b='a2b23c234'
$ echo $b
a2b23c234
$ echo ${b##*[!0-9]}
234

Answer:

The first command assigns the variable b the value a2b23c234

The second command, echo $b, displays the value of the variable b: a2b23c234

The third command, echo ${b##*[!0-9]}, displays the numeric characters at the end of the variable b, which is 234

Explanation of the third command:

• ${b##*[!0-9]} is a parameter expansion that removes the longest substring matching the pattern *[!0-9] from the beginning of the variable b.
• The pattern *[!0-9] matches any sequence of characters that does not end with a numeric digit.
• As a result, the parameter expansion removes all non-numeric characters from the beginning of the string, leaving only the numeric characters at the end.

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This article has been rewritten using GPT-4. Source.

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