A regular expression is a string that can be used to describe several sequences of characters. Regular expressions are used by several different Unix commands, including ed, sed, awk, grep, and, to a more limited extent, vi.
This tutorial would teach you how to use regular expression along with sed.
Here sed stands for stream editor is a stream oriented editor which was created exclusively for executing scripts. Thus all the input you feed into it passes through and goes to STDOUT and it does not change the input file.
Invoking sed:
Before we start, let us take make sure you have a local copy of /etc/passwd text file to work with sed.
As mentioned previously, sed can be invoked by sending data through a pipe to it as follows:
$ cat /etc/passwd | sed
Usage: sed [OPTION]... {script-other-script} [input-file]...
-n, --quiet, --silent
suppress automatic printing of pattern space
-e script, --expression=script
...............................
The cat command dumps the contents of /etc/passwd to sed through the pipe into sed's pattern space. The pattern space is the internal work buffer that sed uses to do its work.
The sed General Syntax:
Following is the general syntax for sed
/pattern/action
Here, pattern is a regular expression, and action is one of the commands given in the following table. If pattern is omitted, action is performed for every line as we have seen above.
The slash characters ( /) that surround the pattern are required because they are used as delimiters.
Range Description
p Prints the line
d Deletes the line
s/pattern1/pattern2/ Substitutes the first occurrence of pattern1 with pattern2.
Deleting All Lines with sed:
Invoke sed again, but this time tell sed to use the editing command delete line, denoted by the single letter d:
$ cat /etc/passwd | sed 'd'
$
Instead of invoking sed by sending a file to it through a pipe, you can instruct sed to read the data from a file, as in the following example.
The following command does exactly the same thing as the previous Try It Out, without the cat command:
$ sed -e 'd' /etc/passwd
$
The sed Addresses:
Sed also understands something called addresses. Addresses are either particular locations in a file or a range where a particular editing command should be applied. When sed encounters no addresses, it performs its operations on every line in the file.
The following command adds a basic address to the sed command you've been using:
$ cat /etc/passwd | sed '1d' |more
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
backup:x:34:34:backup:/var/backups:/bin/sh
$
Notice that the number 1 is added before the delete edit command. This tells sed to perform the editing command on the first line of the file. In this example, sed will delete the first line of /etc/password and print the rest of the file.
This tutorial would teach you how to use regular expression along with sed.
Here sed stands for stream editor is a stream oriented editor which was created exclusively for executing scripts. Thus all the input you feed into it passes through and goes to STDOUT and it does not change the input file.
Invoking sed:
Before we start, let us take make sure you have a local copy of /etc/passwd text file to work with sed.
As mentioned previously, sed can be invoked by sending data through a pipe to it as follows:
$ cat /etc/passwd | sed
Usage: sed [OPTION]... {script-other-script} [input-file]...
-n, --quiet, --silent
suppress automatic printing of pattern space
-e script, --expression=script
...............................
The cat command dumps the contents of /etc/passwd to sed through the pipe into sed's pattern space. The pattern space is the internal work buffer that sed uses to do its work.
The sed General Syntax:
Following is the general syntax for sed
/pattern/action
Here, pattern is a regular expression, and action is one of the commands given in the following table. If pattern is omitted, action is performed for every line as we have seen above.
The slash characters ( /) that surround the pattern are required because they are used as delimiters.
Range Description
p Prints the line
d Deletes the line
s/pattern1/pattern2/ Substitutes the first occurrence of pattern1 with pattern2.
Deleting All Lines with sed:
Invoke sed again, but this time tell sed to use the editing command delete line, denoted by the single letter d:
$ cat /etc/passwd | sed 'd'
$
Instead of invoking sed by sending a file to it through a pipe, you can instruct sed to read the data from a file, as in the following example.
The following command does exactly the same thing as the previous Try It Out, without the cat command:
$ sed -e 'd' /etc/passwd
$
The sed Addresses:
Sed also understands something called addresses. Addresses are either particular locations in a file or a range where a particular editing command should be applied. When sed encounters no addresses, it performs its operations on every line in the file.
The following command adds a basic address to the sed command you've been using:
$ cat /etc/passwd | sed '1d' |more
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
backup:x:34:34:backup:/var/backups:/bin/sh
$
Notice that the number 1 is added before the delete edit command. This tells sed to perform the editing command on the first line of the file. In this example, sed will delete the first line of /etc/password and print the rest of the file.
