Shell Scripting Part4 – Input, Output, and Redirection
In the previous post, we talked about the parameters and options in detail. Today, we will talk about something essential in shell scripting, which are Input, Output, and Redirection.
You can display the output from your shell scripts in two ways:
- Display output on the screen.
- Send output to a file.
Standard file descriptors
Everything is a file in Linux, and that includes input and output.
Each process can have nine file descriptors opened at the same time. The file descriptors 0, 1, 2 are kept for the bash shell usage.
You can use the above file descriptors to control input and output.
You need to fully understand these three because they are like the backbones of your shell scripting. So we are going to describe every one of them in detail.
STDIN stands for standard input, which is the keyboard by default.
You can replace the STDIN, which is the keyboard and replace it with a file by using the input redirect symbol (<), it sends the data as keyboard typing. No magic!!
When you type the cat command without anything, it accepts input from STDIN. Any line you type, the cat command prints that line to the screen.
The STDOUT stands for the standard output, which is the screen by default.
You can redirect output to a file using the >> symbol.
If we have a file contains data, you can append data to it using this symbol like this:
pwd >> myfile
The output generated by pwd is appended to myfile without deleting the existed content.
The following command tries to redirect the output to a file using > symbol.
ls –l xfile > myfile
I have no file called xfile on my PC, and that generates an error that is sent to STDERR.
The shell sends errors to the screen by default.
If you need to redirect the errors to a log file instead of sending it to the screen, you can redirect errors using the redirection symbol.
We can redirect the errors by placing the file descriptor which is 2 before the redirection symbol like this:
ls -l xfile 2>myfile
As you can see, the error now is in the file and nothing on the screen.
Redirecting errors and normal output
To redirect errors and the normal output, you have to precede each with the proper file descriptor like this:
ls –l myfile xfile anotherfile 2> errorcontent 1> correctcontent
The ls command result is sent to the correctcontent file using the 1> symbol. And error messages were sent to the errorcontent file using the 2> symbol.
You can redirect normal output and errors to the same file using &> symbol like this:
ls –l myfile xfile anotherfile &> content
here, we redirect the errors and normal output to a file named content.
There are two ways for output redirection:
- Temporarily redirection.
- Permanently redirection.
For temporary redirections, you can use the >&2 symbol like this:
#!/bin/bash echo "Error message" >&2 echo "Normal message"
So if we run it, we will see both lines printed normally because, as we know, errors go to the screen by default.
You can redirect errors to a file like this:
./myscript 2> myfile
Shell scripting is Awesome! Normal output is sent to the screen, while the echo message which has >&2 symbol sends errors to the file.
If you have much data that need to be redirected, you can have a permanent redirection using the exec command like this:
#!/bin/bash exec 1>outfile echo "Permanent redirection" echo "from a shell to a file." echo "without redirecting every line"
If we look at the file called outfile, we will see the output of the echo lines.
We redirect the STDOUT at the beginning, what about in the middle of a script like this:
#!/bin/bash exec 2>myerror echo "Script Begining ..." echo "Redirecting Output" exec 1>myfile echo "Output goes to the myfile" echo "Output goes to myerror file" >&2
The exec command redirects all errors to the file myerror, and we send the standard output to the screen.
We use the statement exec 1>myfile to redirect output to the myfile file, and finally, errors go to the myerror file using >&2 symbol.
You can redirect input to a file instead of STDIN using exec command like this:
exec 0< myfile
This command tells the shell to take the input from a file called myfile instead of STDIN and here is an example:
#!/bin/bash exec 0<testfile total=1 while read line; do echo "#$total: $line" total=$(($total + 1)) done
Shell scripting is easy.
You know how to use the read command to get user input. If you redirect the STDIN to a file, the read command will try to read from STDIN, which points to the file.
Some Linux system administrators use this technique to read the log files for processing, and we will discuss more ways to read the log on the upcoming posts in a professional way.
Creating custom redirection
You know that there are nine file descriptors, you use only 3 of them for input, output, and error.
The remaining six file descriptors are available for use for input and output redirection.
We can use the exec command to assign a file descriptor for output like this:
#!/bin/bash exec 3>myfile echo "This line appears on the screen" echo "This line stored on myfile" >&3 echo "This line appears on the screen"
Creating input file descriptors
To redirect input file descriptors do the following:
1- Save the STDIN to another file descriptor.
2- Redirecting it to a file.
3- Revert STDIN to its original location.
Look at the following code to understand these steps:
#!/bin/bash exec 7<&0 exec 0<myfile total=1 while read line; do echo "#$total: $line" total=$(($total + 1)) done exec 0<&7 read -p "Finished? " res case $res in y) echo "Goodbye" ;; n) echo "Sorry, this is the end." ;; esac
We saved the STDIN to file descriptor 7, and redirect the STDIN to a file.
The STDIN reverted to its original location after iterating over file lines.
The last read command just to make sure that STDIN is reverted, and you can use the keyboard normally.
Close file descriptors
When the script exits, the file descriptors will close automatically. If you want to close the file descriptor yourself, redirect the file descriptor to this symbol &- it will be closed.
#!/bin/bash exec 3> myfile echo "Testing ..." >&3 exec 3>&- echo "Nothing works" >&3
As you can see, it gives an error of bad file descriptor because it is no longer exist.
The lsof command is used to list all the opened files on the system and background processes.
On many Linux systems like Fedora, the lsof command is located under /usr/sbin.
This is some of the important options for lsof command:
-p: for process ID.
-d: for the file descriptor.
You can get the process PID using the $$ variable.
We can use the -a to combine the results of -p option and -d option.
Now, testing the command from a script:
#!/bin/bash exec 4> myfile1 exec 5> myfile2 exec 6< myfile3 lsof -a -p $$ -d 0,1,2,4,5,6
The shell script creates the file descriptors 4 and 5 for writing and 6 for reading.
Suppressing command output
Sometimes you don’t want to see any output. We redirect the output to the black hole, which is /dev/null.
For example, we can suppress errors like this:
ls -al badfile anotherfile 2> /dev/null
You can truncate a file without deleting it completely using the same command.
cat /dev/null > myfile
Now you understand the input, and output, how to redirect them, how to create your file descriptor, and redirect to it.
Mokhtar is the founder of LikeGeeks.com. He works as a Linux system administrator since 2010. He is responsible for maintaining, securing, and troubleshooting Linux servers for multiple clients around the world. He loves writing shell and Python scripts to automate his work.
2 thoughts on “Shell Scripting Part4 – Input, Output, and Redirection”
I didn’t understand the example of creating file descriptors.
Why did we “exec 0<myfile"? Not "exec 7 myfile”? Shouldn’t it be “cat myfile > /dev/null”?
The demonstrated example shows how to save the STDIN to a file descriptor and redirect STDIN to a file temporarily for testing purpose.
Then we revert STDIN back.