sed command - 's' for Subtitution.

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The most useful `sed` utility is s i think. ie subtitution.

Here is one example.

1)
+$ echo "dim" | sed s/dim/light/
light
+$

if the syntax is wrong;

+$ echo "dim" | sed s/dim/light
sed: command garbled: s/dim/light

2)

+$ cat file1
boy

+$ sed s/boy/girl/ file1 > file2

+$ cat file2
girl

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