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Shell practice: Introduction to the sed stream editor
Quick Edit
Sed (Stream EDitor) [1] automates repetitive operations on a text file and is especially effective when used in a shell script and with regular expressions (regex). In this article, I send program output to the screen. If you want to participate and practice, simply use the text files provided [2].
Sed Commands
The program calls up and accepts commands from virtually anywhere. You can pass in commands directly or read them in from a file. The data can be piped, redirected, or input from a text file. The output can be sent to the screen (usually stdout), through a pipe to the next command, or redirected to a destination file. (See the "Sed Call Options" box.) To resolve shell variables, you sometimes need to substitute the " for the ' character.
Sed Call Options
Sed simply reads the text file and returns the results through stdout:
sed [COMMAND] [TEXTFILE]
Same with input redirection:
sed [COMMAND] < [TEXTFILE]
Inclusion of sed in one or more pipes:
[PROGRAM1] | sed [COMMAND] | ......
Commands stored in a separate file and read in:
... sed -f [SCRIPT] .....
Output of sed redirected to a text file, but omitting error messages:
... sed [COMMAND] > [TARGETFILE]
The same, but including error messages in the target file:
... sed [COMMAND] > [TARGETFILE] 2>&1
Syntax
The basic syntax structure is shown in Figure 1.
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