How to Redirect Input and Output in Bash

Let us understand redirect input and output in Bash through examples.

ls -l > file.txt

This command lists the files in the current directory in a long format and redirects the output to a file named "file.txt". The contents of "file.txt" will be overwritten if the file already exists.

In [1]:

ls -l > file.txt

let us look at the above file.

In [2]:

cat file.txt | head -1
total 345280

find . -name "*.txt" >> files.txt

This command searches for all files with the ".txt" extension in the current directory and redirects the output to a file named "files.txt". The output will be appended to the file if it already exists.

In [28]:

find . -name "*.txt" >> files.txt

Let us check the content of above file.

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cat files.txt | wc -l
5916

There are 5916 .txt files in the current directory

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cat files.txt | tail -1
./requirements.txt

sort < unsorted.txt > sorted.txt

This command sorts the contents of a file named "unsorted.txt" and redirects the output to a new file named "sorted.txt".

Let us create a dummy example file.

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echo 3 >> unsorted.txt 
echo 1 >> unsorted.txt 
echo 2 >> unsorted.txt

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cat unsorted.txt
3
1
2

Let us sort now.

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sort < unsorted.txt > sorted.txt

Let us check the contents now.

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cat sorted.txt
1
2
3

grep "error" /var/log/syslog | mail -s "Error Log" email

This command searches the file "/var/log/syslog" for the string "error" and pipes the output to the "mail" command, which sends an email with the subject "Error Log" to the address "email".

command 2> error.log

This command runs the command and redirects the error output to a file named "error.log".

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echo 1 2> error.log
1

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cat error.log

There is no error in the above the command that is why error.log is empty

command 1&> combined.log

This command runs the command and redirects both standard output and error output to a file named "combined.log".

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ech 1= j&> combined.log

Note I intentionally added a typo in the above command. Let us check the output.

In [15]:

cat combined.log
Command 'ech' not found, did you mean:

  command 'sch' from deb scheme2c (2012.10.14-1ubuntu1)
  command 'ecl' from deb ecl (16.1.3+ds-4)
  command 'bch' from deb bikeshed (1.78-0ubuntu1)
  command 'ecj' from deb ecj (3.16.0-1)
  command 'ecc' from deb ecere-dev (0.44.15-1build3)
  command 'ecs' from deb ecere-dev (0.44.15-1build3)
  command 'ecp' from deb ecere-dev (0.44.15-1build3)
  command 'dch' from deb devscripts (2.20.2ubuntu2)
  command 'echo' from deb coreutils (8.30-3ubuntu2)
  command 'ecm' from deb gmp-ecm (7.0.4+ds-5)

Try: apt install <deb name>

command > /dev/null

This command runs the command and discards the standard output.

In [7]:

echo 1 > /dev/null

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