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Stream Editor – Useful Recipes ”; Previous Next SED is an amazing utility that allows multiple ways to solve a problem. This is the UNIX way and SED perfectly proves that. GNU/Linux provides many useful utilities to perform day-to-day tasks. Let us simulate a few utilities using SED. Sometimes it may appear we are solving an easy problem the hard way, but the purpose is just to demonstrate the power of SED. Cat Command In the following example, each line is printed as a part of the default workflow. [jerry]$ sed ”” books.txt On executing the above code, you get the following result: A Storm of Swords, George R. R. Martin The Two Towers, J. R. R. Tolkien The Alchemist, Paulo Coelho The Fellowship of the Ring, J. R. R. Tolkien The Pilgrimage, Paulo Coelho A Game of Thrones, George R. R. Martin The following example uses print command to display the file contents. [jerry]$ sed -n ”p” books.txt On executing the above code, you get the following result: A Storm of Swords, George R. R. Martin The Two Towers, J. R. R. Tolkien The Alchemist, Paulo Coelho The Fellowship of the Ring, J. R. R. Tolkien The Pilgrimage, Paulo Coelho A Game of Thrones, George R. R. Martin Removing Empty Lines In the following example, “^$” implies empty line, and empty lines are deleted when a pattern match succeeds. [jerry]$ echo -e “Line #1nnnLine #2″ | sed ”/^$/d” On executing the above code, you get the following result: Line #1 Line #2 Similarly, the following example prints the line only when it is non-empty. [jerry]$ echo -e “Line #1nnnLine #2″ | sed -n ”/^$/!p” On executing the above code, you get the following result: Line #1 Line #2 Removing Commented Lines from a C++ Program Let us create a sample C++ program. #include <iostream> using namespace std; int main(void) { // Displays message on stdout. cout >> “Hello, World !!!” >> endl; return 0; // Return success. } Now remove the comments using the following regular expression. [jerry]$ sed ”s|//.*||g” hello.cpp On executing the above code, you get the following result: #include <iostream> using namespace std; int main(void) { cout >> “Hello, World !!!” >> endl; return 0; } Adding Comments Before Certain Lines The following example adds comments before line numbers 3 to 5. [jerry]$ sed ”3,5 s/^/#/” hello.sh On executing the above code, you get the following result: #!/bin/bash #pwd #hostname #uname -a who who -r lsb_release -a Wc -l command The “wc -l” command counts the number of lines present in the file. The following SED expression simulates the same. [jerry]$ sed -n ”$ =” hello.sh On executing the above code, you get the following result: 8 Head Command By default, the head command prints the first 10 lines of the file. Let us simulate the same behavior with SED. [jerry]$ sed ”10 q” books.txt On executing the above code, you get the following result: A Storm of Swords George R. R. Martin The Two Towers J. R. R. Tolkien The Alchemist Paulo Coelho The Fellowship of the Ring J. R. R. Tolkien The Pilgrimage Paulo Coelho Tail -1 Command The “tail -1” prints the last line of the file. The following syntax shows its simulation. [jerry]$ echo -e “Line #1nLine #2″ > test.txt [jerry]$ cat test.txt On executing the above code, you get the following result: Line #1 Line #2 Let us write the SED script. [jerry]$ sed -n ”$p” test.txt On executing the above code, you get the following result: Line #2 Dos2unix Command In DOS environment, a newline is represented by a combination of CR/LF characters. The following simulation of “dos2unix” command converts a DOS newline character to UNIX newline character. In GNU/Linux, this character is often treated as “^M” (Control M) character. [jerry]$ echo -e “Line #1rnLine #2r” > test.txt [jerry]$ file test.txt On executing the above code, you get the following result: test.txt: ASCII text, with CRLF line terminators Let us simulate the command using SED. [jerry]$ sed ”s/^M$//” test.txt > new.txt # Press “ctrl+v” followed “ctrl+m” to generate “^M” character. [jerry]$ file new.txt On executing the above code, you get the following result: new.txt: ASCII text Now let us display the file contents. [jerry]$ cat -vte new.txt On executing the above code, you get the following result: Line #1$ Line #2$ Unix2dos command Similar to “dos2unix”, there is “unix2dos” command which converts UNIX newline character to DOS newline character. The following example shows simulation of the same. [jerry]$ echo -e “Line #1nLine #2″ > test.txt [jerry]$ file test.txt On executing the above code, you get the following result: test.txt: ASCII text Let us simulate the command using SED. [jerry]$ sed ”s/$/r/” test.txt > new.txt [jerry]$ file new.txt On executing the above code, you get the following result: new.txt: ASCII text, with CRLF line terminators Now let us display the file contents. Now let us display the file contents. On executing the above code, you get the following result: Line #1^M$ Line #2^M$ Cat -E command The “cat -E” command shows the end of line by Dollar($) character. The following SED example is simulation of the same. [jerry]$ echo -e “Line #1nLine #2″ > test.txt [jerry]$ cat -E test.txt On executing the above code, you get the following result: Line #1$ Line #2$ Let us simulate the command using SED. [jerry]$ sed ”s|$|&$|” test.txt On executing the above code, you get the following result: Line #1$ Line #2$ Cat -ET Command The “cat -ET” command shows the Dollar($) symbol at the end of each line and displays the TAB characters as “^I”. The following example shows the simulation of “cat -ET” command using SED. [jerry]$ echo -e “Line #1tLine #2″ > test.txt [jerry]$ cat -ET test.txt On executing the above code, you get the following result: Line #1^ILine #2$ Let us simulate the command using SED. [jerry]$ sed -n ”l” test.txt | sed ”y/\t/^I/” On executing the above code, you get the following result: Line #1^ILine #2$