Archive for gener de 2013

tr (translation) modify string in a text file

28 gener 2013
    • tr

(translation). A filter useful to replace all instances of characters in a text file or “squeeze” the whitespace.


cat some_file | tr '3' '5' > new_file

This will run the cat program on some file, the output of this command will be sent to the trcommand, tr will replace all the instances of 3 with 5, like a search and replace. You can also do other things such as:

cat some_file | tr '[A-Z]' '[a-z]' > new_file

This will run cat on some_file and convert any capital letters to lowercase letters (you could use this to change the case of file names too…).




Tee Command Usage Examples

28 gener 2013

Tee command is used to store and view (both at the same time) the output of any other command.

Tee command writes to the STDOUT, and to a file at a time as shown in the examples below.

Example 1: Write output to stdout, and also to a file

The following command displays output only on the screen (stdout).

$ ls

The following command writes the output only to the file and not to the screen.

$ ls > file

The following command (with the help of tee command) writes the output both to the screen (stdout) and to the file.

$ ls | tee file

Example 2: Write the output to two commands

You can also use tee command to store the output of a command to a file and redirect the same output as an input to another command.

The following command will take a backup of the crontab entries, and pass the crontab entries as an input to sed command which will do the substituion. After the substitution, it will be added as a new cron job.

$ crontab -l | tee crontab-backup.txt | sed 's/old/new/' | crontab –

Misc Tee Command Operations

By default tee command overwrites the file. You can instruct tee command to append to the file using the option –a as shown below.

$ ls | tee –a file

You can also write the output to multiple files as shown below.

$ ls | tee file1 file2 file3