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How to print Stdout of a command in reverse order on a Linux Shell

Recently encountered a challenge to display the standard output (stdout) of a Linux command in the reverse order.

For eg; ls -lrt would list the files and directories based on the created/updated date but would like to get the output in the reverse direction.

Solution: Linux shell is abundant of features. It has a built-in command namely tac part of the coreutils written by Jay Lepreau and David MacKenzie. It concatenates and prints files in reverse order.

ie., stdout of the command is given as standard Input to tac and it copies standard input to standard output reversing the records each line by separately.

Step 1: List files in the default order.

[root@ssl httpd]$ ls -rlt
total 12
lrwxrwxrwx 1 root root 29 Sep 8 15:58 modules -> ../../usr/lib64/httpd/modules
lrwxrwxrwx 1 root root 19 Sep 8 15:58 logs -> ../../var/log/httpd
drwxr-xr-x 2 root root 4096 Sep 8 15:58 conf
lrwxrwxrwx 1 root root 10 Sep 8 15:58 run -> /run/httpd
drwxr-xr-x 2 root root 4096 Nov 28 16:41 conf.modules.d
drwxr-xr-x 2 root root 4096 Nov 28 16:41 conf.d

Step 2: List files in the reverse order of their default order.

[root@ssl httpd]$ ls -lrt | tac
drwxr-xr-x 2 root root 4096 Nov 28 16:41 conf.d
drwxr-xr-x 2 root root 4096 Nov 28 16:41 conf.modules.d
lrwxrwxrwx 1 root root 10 Sep 8 15:58 run -> /run/httpd
drwxr-xr-x 2 root root 4096 Sep 8 15:58 conf
lrwxrwxrwx 1 root root 19 Sep 8 15:58 logs -> ../../var/log/httpd
lrwxrwxrwx 1 root root 29 Sep 8 15:58 modules -> ../../usr/lib64/httpd/modules

This built-in command tac can be used on any Linux command or on your own scripts as well.

Updated on December 28, 2018

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