CODEWITHSUNDEEP
linuxshell
McFiddlyWiddly
McFiddlyWiddly

Reputation: 581

How does 'ls' command work when it has multiple arguments?

How does the 'ls' command work in Linux/Unix?

So that's some reference.

But I was wondering how a command such as 

ls -1 | grep 'myfile'

would be executed by the shell, i.e. when is exec called, when is fork called, when id dup called(if at all).

Also, how is this entire command parsed?

Upvotes: 1

Views: 4357

Answers (2)

cardamom
cardamom

Reputation: 7421

I'm answering here specifically the textual question of the title,

How does 'ls' command work when it has multiple arguments?

...not addressing the question which came underneath. Was needing to check if a list of files was present in a directory and this question's phrasing was the closest to what I needed.

If you need to do this, either separate them with a space or wrap them in curly brackets with commas and no space as follows:

ls something.txt something_else.txt

or

ls {something.txt,something_else.txt}

Upvotes: 0

shanmuga
shanmuga

Reputation: 4499

What does fork do

Fork is the primary (and historically, only) method of process creation on Unix-like operating systems.

What does exec do

In computing, exec is a functionality of an operating system that runs an executable file in the context of an already existing process

What does this mean
When you run a command (that is not built in like exit, cd), shell creates a child process using fork. This child process then uses exec executes the binary file for that command (e.g: /bin/ls)

What happens when during input/output redirecction
Every process is supplied with three streams standard input (STDIN), standard output (STDOUT) and standard error (STDERR). By default these streams are mapped to parent process's respective streams. Thus commands like wc or nano which reads from STDIN can be supplied with data from parent shell process's STDIN, and their output is captured by parent shell process and displayed.

However, when using redirection like

ls /tmp /abcd 1>out.log and 2>err.log

stdout is now mapped to a file output stream of out.log, similarly stderr is mapped to err.log. And the output is written to corresponding files.

PIPE chaining

ls -1 | grep 'myfile'

In shell PIPE | is used to chain the STDOUT of first command to STDIN of second command.

This means output of ls -1 (list of files and directories) is given as input to grep myfile which searches for lines containing "myfile" and prints to its STDOUT. The combined effect is to search filename containing char sequence "myfile"

Upvotes: 1

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