In this section, you will learn about handling files for reading and writing. In Chapter 8, Automating Decision Making in Scripts, you will learn about checking various attributes of files along with decision making constructs, such as if, case, and similar.
The exec command is very interesting. Whenever we run any command in shell, the new subshell or process gets created, and the command runs in this newly created process. When we run any command as argument to the exec command, exec will replace the current shell with the command to be executed. It does not create or spawn a new process to run the command.
In the Bash shell environment, every process has three files opened by default. These are standard input, display, and error. The file descriptors associated with them are 0, 1, and 2 respectively. In the Bash shell, we can assign the file descriptor to any input or output file. These are called file descriptors.
The syntax for declaring output.txt as output is as follows:
exec fd > output.txt
This command will declare the number fd as an output file descriptor.
The syntax for closing the file is as follows:
exec fd<&-
To close fd, which is 5, enter the following:
exec 5<&-
We will try to understand these concepts by writing scripts.
Let's understand the opening, closing, and writing of a file.
Write a Shell script file_01.sh, shown as follows:
#!/bin/bash # We will open file for writing purpose # We are assigning descriptor number 3 for file sample_out.txt exec 3> sample_out.txt # We are sending output of command "echo" to sample_out.txt file echo "This is a test message for sample_out.txt file" >&3 # Run command date & store output in file sample_out.txt date >&3 # Closing file with file descriptor 3 exec 3<&-
Save the file, give the permission to execute, and run the script as follows:
$ chmod u+x file_01.sh $ ./file_01.sh $ cat sample_out.txt
Output:
This is a test message for sample_out.txt file Tue Sep 29 23:19:22 IST 2015
Let's write script to read from a file:
Write the script file_02.sh, shown as follows:
#!/bin/bash # We will open file sample_input.txt for reading purpose. # We are assigning descriptor 3 to the file. exec 3< sample_input.txt cat <&3 # Closing file exec 3<&-
Save the file, give the permission to execute, and run the script as follows:
$ chmod u+x file_02.sh
We will create the sample_input.txt file as follows:
$ echo "Hello to All" > sample_input.txt
Run the script and check the result:
$ ./file_02.sh
Output:
Hello to All
In the earlier examples, we opened the file either for reading or writing. Now, we will see how to open the file for reading and writing purposes:
exec fd<> fileName
If the file descriptor number is not specified, then 0 will be used in its place. The file will be created if it does not exist. This procedure is useful to update files.
Let's understand following script.
Write the Shell script file_03.sh as follows:
#!/bin/bash file_name="sample_out.txt" # We are assing fd number 3 to file. # We will be doing read and write operations on file exec 3<> $file_name # Writing to file echo """ Do not dwell in the past, do not dream of the future, concentrate the mind on the present moment. - Buddha """ >&3 # closing file with fd number 3 exec 3>&-
We can use command read to get data from a file to store it in variables. The procedure for using the read command to get text from a file is as follows:
read -u fd variable1 variable2 ... variableN
Now, we will see how to read from one file and write to another. Let's write script file_04.sh as follows:
#!/bin/bash # We are assigning descriptor 3 to in_file.txt exec 3< in_file.txt # We are assigning descriptor 4 to out_file.txt exec 4> out_file.txt # We are reading first line of input.txt read -u 3 line echo $line echo "Writing content of in_file.txt to out_file.txt" echo "Line 1 - $line " >&4 # Closing both the files exec 3<&- exec 4<&-
Save the file, give the permission to execute, and run the script as follows:
$ chmod u+x file_04.sh $ echo "Sun is at the center of Solar System." > in_file.txt $ cat in_file.txt
Output:
Sun is at the center of Solar System. $ ./file_04.sh
Output:
Sun is at the center of Solar System. Writing content of in_file.txt to out_file.txt $ cat out_file.txt
Output:
Line 1 - Sun is at the center of Solar System.
In this example, we read the complete line in the variable line and we used the same variable to write it to another file.
Let's write one more script file_05.sh to get the hostname and addresses:
#!/bin/sh cp /etc/hosts hosts2 grep -v '^#' hosts2 > hosts3 exec 3< hosts3 # opening hosts3 as input file exec 4> hostsfinal # opening hostsfinal as output file read <& 3 address1 name_1 extra_info read <& 3 address2 name_2 extra_info echo $name_1 $address1 >& 4 echo $name_2 $address2 >& 4 exec 3<&- # Closing hosts3 exec 4<&- # Closing hostsfinal
In this script, we used the variables address1, name_1, extra_info, address2, and name_2 to store useful information.
We will write the script to display the actual file descriptors associated with the file.
Let's write the script file_06.sh, shown as follows:
#!/bin/bash # we are assigning file descriptor 3 to input file test.txt exec 3< test.txt # we are assigning file descriptor 4 to output.txt exec 4> output.txt # we are using read command to read line from file read -u 3 line echo "Process id of current process is $$" my_pid=$$ echo "Currently following files are opened by $0 script :" ls -l /proc/$my_pid/fd # We are closing both files test.txt and output.txt exec 3<&- exec 4>&-
You will learn how to use the while loop and the read command to read a file line by line. You will learn more about the while loop in the upcoming chapters.
Let's write the script file_07.sh, shown as follows:
#!/bin/bash echo "Enter the name of file for reading" read file_name exec<$file_name while read var_line do echo $var_line done
For executing the preceding script, we will need to create a file with some text in it. Then, we will pass this file name for reading purposes.
The following is the syntax for storing the output of a command in a file:
Command >& fd ./script >& fd
The following is the illustrative example script file_08.sh:
#!/bin/bash exec 4> output.txt cat /proc/cpuinfo >&4 exec 3<&-
Save the file, give the permission to execute, and run the script as follows:
$ chmod u+x file_08.sh $ ./file_08.sh
Output:
In this example, we have executed the command cat /proc/cpuinfo and we have stored the output in file output.txt.
The following is the summary of the exec command for using various file handling-related operations:
|
Command |
What it does |
|---|---|
|
exec command |
This command will replace shell and execute it. Therefore, it will not return to its original shell, which started it. |
|
exec > data.txt |
This opens |
|
exec < data.txt |
This opens |
|
exec 3< data.txt |
This opens |
|
sort <&3 |
This will sort |
|
exec 4>data.txt |
This opens |
|
ll >&4 |
The output of |
|
exec 6<&5 |
This makes fd 6 a copy of fd 5 |
|
exec 4<&– |
This closes fd 4. |