Substring extraction (${var:offset:length}) in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the syntax ${var:offset:length} do in bash scripting?

It extracts a substring from the variable var, starting at position offset and taking length characters.

beginner

If var="hello world", what is the result of ${var:6:5}?

The substring starting at position 6 with length 5 is world.

intermediate

What happens if the length in ${var:offset:length} is omitted?

The substring will include all characters from offset to the end of the string.

intermediate

Can the offset be negative in ${var:offset:length}? What does it mean?

Yes, a negative offset counts from the end of the string backwards. For example, ${var: -3} starts 3 characters from the end.

beginner

How would you extract the first 4 characters of a variable var in bash?

Use ${var:0:4} to get the first 4 characters starting at position 0.

What does ${var:3:2} extract from var="abcdefg"?

A "ef"

B "cd"

C "de"

D "bc"

Starting at position 3 (0-based), the substring is "de".

If var="hello", what does ${var:1} return?

A "h"

B "he"

C "o"

D "ello"

Without length, it returns substring from offset 1 to end: "ello".

What happens if offset is negative in ${var:offset:length}?

A Counts from start of string

B Counts from end of string backwards

C Causes an error

D Returns empty string

Negative offset counts backwards from the end of the string.

Which command extracts the first 3 characters of var?

A ${var:0:3}

B ${var:3:0}

C ${var:3:3}

D ${var:1:3}

Start at 0, take 3 characters extracts the first 3 characters.

If var="bash scripting", what does ${var:5} return?

A "scripting"

B "bash scripting"

C "script"

D "bash"

Offset 5 means start at 6th character to end, which is "scripting".

Explain how to extract a substring from a bash variable using the syntax ${var:offset:length}. Include what happens if length is omitted or offset is negative.

Think about how you cut a piece from a string, starting point and size.

Describe a real-life example where substring extraction in bash scripting can be useful.

Imagine you have a filename and want just the extension.