36
votes
4answers
4k views

unix, difference between path starting with '/' and '//'

In unix/linux, any number of consecutive forwardslashes in a path is generally equivalent to a single forwardslash. eg. $ cd /home/shum $ pwd /home/shum $ cd /home//shum $ pwd /home/shum $ cd ...
14
votes
2answers
43k views

How can I assign the output of a command to a shell variable?

I want to assign the result of an expression to a variable and concatenate it with a string, then echo it. Here's what I've got: #!/bin/bash cd ~/Desktop; thefile= ls -t -U | grep -m 1 "Screen Shot"; ...
11
votes
3answers
745 views

Order of redirections

I don't quite understand how the computer reads this command. cat file1 file2 1> file.txt 2>&1 If I understand, 2>&1 simply redirect Standard Error to Standard Output. By that ...
9
votes
3answers
2k views

Trapping errors in command substitution using “-o errtrace” (ie set -E)

According to this ref manual: -E (also -o errtrace) If set, any trap on ERR is inherited by shell functions, command substitutions, and commands executed in a subshell environment. The ERR ...
14
votes
2answers
1k views

Manipulating X key and pointer grabs on the command line

What command line tools exist to list and manage X server grabs? (That's grab as in key and pointer grabs, i.e. restricting the use of a particular key or mouse button to a particular application, or ...
37
votes
3answers
7k views

What does size of a directory mean in output of 'ls -l' command?

What does size of a directory mean in output of ls -l command?
7
votes
3answers
1k views

Linux Bulk/Remote Administration

Beside our internal IT infrastructure, we've got around 500 Linux machines hosting our services for the on-line world. They are grouped in a bunch of clusters like Database A-n, Product A-n, NFS, ...
4
votes
1answer
500 views

Difference between ' and " on command line (bash)?

I used to use '' and "" interchangeably on the command line, but I recently noticed that '$HOME/some/dir' is not expanded, while "$HOME/some/dir" is. I searched around a little bit and found that "" ...
4
votes
1answer
3k views

Rename All Files with a Certain Name

I'm trying to find certain files with the name "stringx" and replace the name (but not the extension) with "stringy". So basically for stringx.txt and stingx.cs, I'd want stringy.txt and stringy.cs. ...
208
votes
6answers
84k views

What does env x='() { :;}; command' bash do and why is it insecure?

There is apparently a vulnerability (CVE-2014-6271) in bash: Bash specially crafted environment variables code injection attack I am trying to figure out what is happening, but I'm not entirely sure ...
176
votes
3answers
95k views

How to unfreeze after accidentally pressing Ctrl-S in a terminal?

It's a situation that has happened quite often to me: after I press (with a different intention) Ctrl-S in a terminal, the interaction (input or output) with it is frozen. It's probably a kind of ...
103
votes
6answers
227k views

How to forward X over SSH from Ubuntu machine?

I have a machine running Ubuntu which I SSH to from my Fedora 14 machine. I want to forward X from the Ubuntu machine back to Fedora so I can run graphical programs remotely. Both machines are on a ...
70
votes
27answers
17k views

Quick directory navigation in the terminal

I would like to frequently switch between directories that are in totally unrelated paths, for example /Project/Warnest/docs/ and ~/Dropbox/Projects/ds/test/. But I don't want to type cd ...
54
votes
5answers
44k views

What is a Superblock, Inode, Dentry and a File?

From the article Anatomy of Linux File Systems by Tim Jones, I read that Linux views all the file systems from the perspective of a common set of objects and these objects are superblock, inode, ...
80
votes
2answers
52k views

Getting 256 colors to work in tmux

I have 256 colors working just fine in konsole,. I thought I'd give tmux a try because, unlike screen, it seems to support vi mode. However I find that the colors of my prompt show up and this is ...
65
votes
12answers
28k views

How to grep a specific line _and_ the first line of a file?

Assuming a simple grep such as: $ psa aux | grep someApp 1000 11634 51.2 0.1 32824 9112 pts/1 SN+ 13:24 7:49 someApp This provides much information, but as the first line of the ps ...
56
votes
6answers
54k views

How do you move all files (including hidden) from one directory to another?

How do I move all files in a directory (including the hidden ones) to another directory? For example, if I have a folder "Foo" with the files ".hidden" and "notHidden" inside, how do I move both ...
23
votes
1answer
35k views

How to loop over the lines of a file?

Say I have this file: hello world hello world This program #!/bin/bash for i in $(cat $1); do echo "tester: $i" done outputs tester: hello tester: world tester: hello tester: world I'd ...
60
votes
16answers
93k views

SSH tunneling error: “channel 1: open failed: administratively prohibited: open failed”

When I open this ssh tunnel: ssh -nXNT -p 22 localhost -L 0.0.0.0:8984:remote:8983 I get this error when trying to access the HTTP server running on localhost:8984: channel 1: open failed: ...
35
votes
4answers
5k views

What is the difference between a builtin command and one that is not?

Is there any intrinsic difference between a builtin command and another command which can nominally do the same thing? eg. Do builtins get "special" treatement? ... is there less overhead running ...
20
votes
5answers
3k views

Understanding UNIX based permissions and their attributes

I've never really got how chmod worked up until today. I followed a tutorial that explained a big deal to me. For example, I've read that you've got three different permission groups: owner (u) ...
18
votes
4answers
7k views

Four tasks in parallel… how do I do that?

I have a bunch of PNG images on a directory. I have an application called pngout that I run to compress these images. This application is called by a script I did. The problem is that this script does ...
19
votes
1answer
17k views

What is meant by mounting a device in Linux?

I've heard the term "mounting" when referring to devices in Linux. What is its actual meaning? How it handling now unlike older versions? I haven't done that manually via the command-line. Can you ...
11
votes
2answers
60k views

Can I resize the root partition without uninstalling and reinstalling Linux (or losing data)?

I have started using a machine that has both Debian and Windows 7 installed. However, after installing some programs I started getting a message that there is not enough space. I knew that the system ...
30
votes
10answers
16k views

How to understand what's taking up space?

I'm looking for a linux alternative to WinDirStat. I would like to know what is taking up space on my hard drives. A program that works on console and doesn't require a UI is preferred .
14
votes
4answers
5k views

Evolution of Operating systems from Unix

Can you explain the evolution hierarchy of operating systems (Linux and Windows) from Unix?
29
votes
5answers
7k views

What process created this X11 window?

Given an X11 window ID, is there a way to find the ID of the process that created it? Of course this isn't always possible, for example if the window came over a TCP connection. For that case I'd ...
21
votes
2answers
16k views

How to colorize output of ls ?

I'm running CentOS in Linux text mode. When I run the command ls /usr/, the output is too hard to read (dark blue on black). How can I change the text coloring?
19
votes
3answers
8k views

Determining Specific File Responsible for High I/O

This is a simple problem but the first time I've ever had to actually fix it: finding which specific files/inodes are the targets of the most I/O. I'd like to be able to get a general system overview, ...
41
votes
2answers
44k views

How do I do a ls and then sort the results by date created?

In what order are the dated ordered by? Certainly not alphanumeric order. ls -lt sorts by modification time. But I need creation time.
30
votes
2answers
13k views

What do the brackets around processes mean?

mark@mark-VirtualBox:~/auto/lib$ ps -Awwo pid,comm,args PID COMMAND COMMAND 1 init /sbin/init 2 kthreadd [kthreadd] 3 ksoftirqd/0 [ksoftirqd/0] 5 ...
16
votes
8answers
39k views

Changing extension to multiple files

I would like to change file extension from *.txt file to *.text file, tried using the basename command, but I am having trouble on changing more than 1 file, I am getting a basename: too many ...
17
votes
6answers
3k views

Any way to sync directory structure when the files are already on both sides?

I have two drives with the same files, but the directory structure is totally different. Is there any way to 'move' all the files on the destination side so that they match the structure of the ...
15
votes
2answers
19k views

How to run grep with multiple AND patterns?

I would like to get the multi pattern match with implicit AND between patterns, i.e. equivalent to running several greps in a sequence: grep pattern1 | grep pattern2 | ... So how to convert it to ...
9
votes
2answers
3k views

How to restrict internet access for a particular user on the lan using iptables in Linux

Let's say there are two users on the LAN, A and B. How do I restrict user A from internet access using iptables rules and saving the rules so that after reboot, they are still effective. And suppose I ...
7
votes
3answers
1k views

Construct a command by putting a string into a tty

I managed to do this echo -n "command" > /dev/tty1 The letters appear, and the cursor moves, but they are "ghosts" - if you hit enter, nothing happens (they are not in stdin). Edit: In the ...
23
votes
2answers
4k views

Where do the files go if you mount a drive to a folder that already contains files?

I just accidentally mounted a new drive to a folder that already contained files. I don't care about them and have them somewhere else, but that folder appears empty now. I'm curious what happened ...
9
votes
3answers
19k views

Extracting a regex matched with 'sed' without printing the surrounding characters

To all the 'sed' doctors out there: I have a seemingly trivial 'sed' question to which I have not been able to find a solution. How can you get 'sed' to exctract a regular expression it has matched ...
7
votes
2answers
6k views

How can I rename a lot of files using a regex?

I want to rename a lot of files on Mac OS X (10.7.2).. I don't have the perl package for the rename command. My files have names like "T452-102456-0.png" and I want to delete the "-0" part. I know I ...
5
votes
2answers
1k views

Portability of file descriptor links

I've always wondered this but never took the time to find out, so I'll do so now - how portable is the usage shown here of either /proc/$$/fd/$N or /dev/fd/$N? I understand POSIX guarantees /dev/null, ...
16
votes
3answers
4k views

What defines the maximum size for a command single argument?

I was under the impression that the maximum length of a single argument was not the problem here so much as the total size of the overall argument array plus the size of the environment, which is ...
6
votes
6answers
2k views

How to clean up file extensions?

I have a directories with .MP3 files which I'd like to change the extensions to .mp3. What's the easiest way to do this? I'm think something along the lines of: find /RootPath -type f -iname "*.mp3" ...
4
votes
2answers
598 views

Precedence of user and group owner in file permissions

I just ran into something unexpected (for me) regarding file permissions on Linux (Arch Linux). Basically I have: userX in groupX fileX userX:groupX ---rwx---- What puzzles me: I cannot perform ...
7
votes
3answers
8k views

Automount USB drives with no GUI requirement (halevt replacement)

For minimal installations of Debian, I have been using halevt to enable automounting of USB drives. Halevt is reliable and requires no configuration aside from installing the package. Now, halevt has ...
307
votes
7answers
27k views

How can I find the implementations of Linux kernel system calls?

I am trying to understand how a function, say mkdir, works by looking at the kernel source. This is an attempt to understand the kernel internals and navigate between various functions. I know mkdir ...
161
votes
4answers
29k views

/usr/bin vs /usr/local/bin on Linux

Why are there so many places to put a binary in Linux? There are atleast these five: /bin/ /sbin/ /usr/bin/ /usr/local/bin/ /usr/local/sbin/ And on my office box, I do not have write permissions ...
83
votes
2answers
15k views

Why do we use su - and not just su?

I don't understand why su - is preferred over su to login as root.
55
votes
6answers
2k views

Resources for portable shell programming

What resources exist for portable shell programming? The ultimate answer is to test on all targeted platforms, but that's rarely practical. The POSIX / Single UNIX specification is a start, but it ...
54
votes
8answers
31k views

How can I move files and view the progress (e.g. with a progress bar)?

When moving large directories using mv, is there a way to view the progress (%)? The cp command on gentoo had a -g switch that showed the progress.
54
votes
7answers
23k views

How to check if a shell is login/interactive/batch

I think I understand the differences between an interactive, a login and a batch shell. See the following links for more help: What is the difference between a 'Login' and an 'Interactive' bash ...

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