tr (translate characters) in Linux CLI - Step-by-Step Execution

Concept Flow - tr (translate characters)

Input text stream

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tr command reads characters

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Check each character

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If char in set1?

No→Output char unchanged

Yes↓

Replace with corresponding char in set2

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Output translated char

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Repeat for all chars

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End of input stream

The tr command reads input characters one by one, replaces characters found in the first set with corresponding characters from the second set, and outputs the result.

Execution Sample

Linux CLI

echo "hello" | tr "hel" "HEl"

This command replaces 'h' with 'H', 'e' with 'E', and 'l' with 'l' (unchanged) in the word 'hello'.

Execution Table

Step	Input Char	Is in set1?	Action	Output Char
1	h	Yes	Replace with 'H'	H
2	e	Yes	Replace with 'E'	E
3	l	Yes	Replace with 'l'	l
4	l	Yes	Replace with 'l'	l
5	o	No	Output unchanged	o
6	End of input	-	-	-

💡 Reached end of input stream, translation complete.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
Current Char	-	h	e	l	l	o	End
Output Stream	""	"H"	"HE"	"HEl"	"HEll"	"HEllo"	"HEllo"

Key Moments - 2 Insights

Why does 'l' remain lowercase in the output even though it is in the first set?

Because the corresponding character in the second set for 'l' is also 'l', so it replaces 'l' with 'l' unchanged, as shown in steps 3 and 4 in the execution_table.

What happens to characters not in the first set?

They are output unchanged, as seen with 'o' in step 5 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output character at step 2?

A E

B e

C H

D l

💡 Hint

Check the 'Output Char' column at step 2 in the execution_table.

At which step does the input character 'o' get processed?

A Step 3

B Step 5

C Step 4

D Step 6

💡 Hint

Look for 'o' in the 'Input Char' column in the execution_table.

If the second set was "HEL" instead of "HEl", what would be the output character at step 3?

A l

B E

C L

D H

💡 Hint

Compare the second set characters and see what replaces 'l' at step 3.

Concept Snapshot

tr command syntax: tr SET1 SET2
Reads input characters one by one.
If character is in SET1, replace with corresponding char in SET2.
If not, output character unchanged.
Useful for simple character substitutions in streams.

Full Transcript

The tr command reads each character from the input stream. It checks if the character is in the first set (SET1). If yes, it replaces it with the corresponding character from the second set (SET2). If not, it outputs the character unchanged. This process repeats until the end of the input stream. For example, echo "hello" | tr "hel" "HEl" replaces 'h' with 'H', 'e' with 'E', and 'l' with 'l' (unchanged), resulting in 'HEllo'.