[SOLVED] a question on shell internal command

sryzdn · 02-03-2014, 02:47 AM

Hi,

Why do I receive this when I want to know the type of "ls". Isn't it an internal shell command?

Quote:

$ type ls
ls is aliased to `ls --color=auto'

colucix · 02-03-2014, 03:09 AM

ls is not a shell built-in but a program provided by coreutils. The message you receive states that you have an alias that causes ls to display a colored output and the alias is usually defined in a script under /etc/profile.d. To see what ls really is, try

Code:

type -a ls

normanlinux · 02-03-2014, 03:10 AM

ls is found, normally, at /usr/bin/ls, and is part of the coreutils package
It defaults to *not* displaying in colour but has options to control this, the most useful being that used in your alias.

This is common on Linux systems as, while we like (and are used to) the colour coding on our terminals the escape sequences used would screw up the following processes if used in a pipe or redirected to a file.

sryzdn · 02-03-2014, 03:30 AM

Quote:

Originally Posted by normanlinux

This is common on Linux systems as, while we like (and are used to) the colour coding on our terminals the escape sequences used would screw up the following processes if used in a pipe or redirected to a file.

Thank you, but I don't understand why colour coding on terminals may screw up the processes?!
When we execute "ls", depending on what we have on the list their colours are different. But what does it have to do with escapes?

normanlinux · 02-03-2014, 03:50 AM

In order to display the colours, the ls program surrounds the file names with terminal sequences that set and then reset the display colour. These normally start with the escape character 0x1b, so they are called escape sequences. If these were passed to a following process - or included in a file for later processing - the process would not be able clearly t see the file names.