32
votes
5answers
9k views

When would you use an additional file descriptor?

I know you can create a file descriptor and redirect output to it. e.g. exec 3<> /tmp/foo # open fd 3. echo a >&3 # write to it exec 3>&- # close fd 3. But you can do the same ...
31
votes
8answers
7k views

Is there a way to modify a file in-place?

I have a fairly large file (35Gb), and I would like to filter this file in situ (i.e. I don't have enough disk space for another file), specifically I want to grep and ignore some patterns — is there ...
12
votes
1answer
1k views

Why is my bash prompt getting bugged when I browse the history?

Often when I start looking at history of commands some of the characters from a command displayed aren't erased, for example: What's happening: prompt$ some_command prompt$ some_commanother_command ...
10
votes
2answers
4k views

real memory usage

if I understand correctly, in the following output produced by free, 3535m is the actual free memory available to applications, only 413m is used, is this correct? need some clarification on the ...
188
votes
2answers
45k views

Difference between nohup, disown and &

What is the difference between $ nohup foo and $ foo & and $ foo & $ disown
51
votes
4answers
11k views

Convince grep to output all lines, not just those with matches

Say I have the following file: $ cat test test line 1 test line 2 line without the search word another line without it test line 3 with two test words test line 4 By default, grep returns each ...
55
votes
4answers
15k views

What's the point in adding a new line to the end of a file?

Some compilers (especially C or C++ ones) give you warnings about: No new line at end of file I thought this would be a C-programmers-only problem, but github displays a message in the commit view: ...
51
votes
4answers
6k views

In what order do piped commands run?

I've never really thought about how the shell actually executes piped commands. I've always been told that the "stdout of one program gets piped into the stdin of another," as a way of thinking about ...
49
votes
4answers
39k views

Reserved space for root on a filesystem - why?

I understand that by default, newly created filesystems will be created with 5% of the space allocated for root. I also know you can change the defined space with: tune2fs -m 1 /dev/sdXY What I'm ...
35
votes
5answers
5k views

Is there a one-liner that allows me to create a directory and move into it at the same time?

I find myself repeating a lot of: mkdir longtitleproject cd longtitleproject Is there a way of doing it in one line without repeating the directory name? I'm on bash here.
23
votes
11answers
11k views

Is there a robust command line tool for processing csv files?

I work with CSV files and sometimes need to quickly check the contents of a row or column from the command line. In many cases cut, head, tail, and friends will do the job; however, cut cannot easily ...
17
votes
4answers
17k views

sending text input to a detached screen

I'm trying to run a minecraft server on my unRAID server. The server will run in the shell, and then sit there waiting for input. To stop it, I need to type 'stop' and press enter, and then it'll ...
52
votes
3answers
13k views

Why is there a big delay after entering a wrong password?

I notice a weird (well, according to me) thing about passwords. For example, if I type an incorrect password during login, there will be a few seconds' delay before the system tells me so. When I try ...
11
votes
4answers
7k views

What should I do before dual-booting Windows 7 and Debian on EFI?

I'm about to purchase an Asus motherboard with EFI firmware and wanted to prepare myself for installing Windows and Debian once the hardware arrives. I was hoping that someone who has gone down this ...
25
votes
2answers
9k views

What mount points exist on a typical Linux system?

I have 2 questions. During Linux installation we specify memory space for 2 mount points - root and swap. Are there any other mount points created without the users notice? Is this statement ...
7
votes
4answers
6k views

How do I connect to a pc through another pc using ssh

I have three Computers. PC1 and PC2 is on a private LAN, where PC1 is known to PC2 as 192.168.0.2 PC2 and PC3 is on a another LAN, where PC2 is known to PC3 as 192.168.123.101 How can I connect to ...
4
votes
2answers
715 views

What information do I need to solve an APT dependency issue?

I've seen several times people with unmet dependencies, which apt-get doesn't tells straightforward what's the problem, like this one: The following packages have unmet dependencies: ...
8
votes
3answers
41k views

Shell Script for logging into a ssh server

I tried writing a shell script which can do automatic login into a ssh server using password which is mentioned in the script. I have written the following code: set timeout 30 /usr/bin/ssh -p 8484 ...
2
votes
1answer
2k views

“No such file or directory” lies on Optware installed binaries

I've been using Optware to install packages on my ARM-based NAS for a while - the usual stuff like Transmission, Samba and others. However, I'd been having problems with Transmission hanging not long ...
261
votes
9answers
109k views

When should I not kill -9 a process?

I am always very hesitant to run kill -9, but I see other admins do it almost routinely. I figure there is probably a sensible middle ground, so: When and why should kill -9 be used? When and why ...
87
votes
19answers
75k views

How can I get my external IP address in bash?

I need to find my external IP address from a bash script. At the moment I use this function: myip () { lwp-request -o text checkip.dyndns.org | awk '{ print $NF }' } But it depends on ...
44
votes
4answers
29k views

What is the difference between Halt and Shutdown commands?

What is the difference between the halt and shutdown commands?
21
votes
7answers
89k views

Unix/Linux undelete/recover deleted files

Is there a command to recover/undelete deleted files by rm? $ rm -rf /path/to/myfile How can I recover myfile? If there is such a tool how can I use it?
40
votes
8answers
27k views

How to test what shell I am using in a terminal?

How to check what shell I am using in a terminal? What is the shell I am using in MacOS?
17
votes
4answers
5k views

The merits of a partitionless filesystem

I ran into something a couple of weeks ago that I'd never seen before: A filesystem (ext3 I believe) installed to a storage device without a partition. In essence /dev/sdb was the entire filesystem. I ...
17
votes
2answers
1k views

gnu find and masking the {} for some shells - which?

The man page for gnu find states: -exec command ; [...] The string `{}' is replaced by the current file name being processed everywhere it occurs in the arguments to the command, ...
9
votes
1answer
9k views

How can I make variables “exported” in a bash script stick around?

I have multiple Amazon EC2 accounts and want to quickly be able to switch variables, such as $EC2_HOME, using a script. I have have a shell script set up like this: #!/bin/sh export ...
7
votes
2answers
10k views

Why is my variable local one 'while read' loop, but not in another seemingly similar loop? [duplicate]

Why do I get different values for $x from the snippets below? #!/bin/bash x=1 echo fred>junk ; while read var ; do x=55 ; done <junk echo x=$x # x=55 .. I'd expect this result x=1 cat ...
6
votes
2answers
3k views

Why is there a discrepancy in disk usage reported by df and du? [duplicate]

I have a Linux(CentOS) server, the OS+packages used around 5GB. Then, I transferred 97GB data from a Windows server to two folders on this Linux server, after calculated the disk usage, I see the ...
4
votes
2answers
6k views

ext4: How to account for the filesystem space?

I've recently formated a 1.5 TB drive with the intention of replacing ntfs with ext4. Then I noticed that the files I saved don't fit on the new partition. df: ext4 (ext3 & ext2 show the same ...
19
votes
2answers
5k views

Does (should) LC_COLLATE affect character ranges?

Collation order through LC_COLLATE defines not only the sort order of individual characters, but also the meaning of character ranges. Or does it? Consider the following snippet: unset LANGUAGE ...
121
votes
15answers
55k views

How to have tail -f show colored output

I'd like to be able to tail the output of a server log file that has messages like: INFO SEVERE etc, and if it's SEVERE, show the line in red; if it's INFO, in green. What kind of alias can I ...
100
votes
1answer
120k views

How to set default file permissions for all folders/files in a directory?

I want to set a folder such that anything created within it (directories, files) inherit default permissions and group. Lets call the group "media". And also, the folders/files created within the ...
43
votes
4answers
4k views

What features are in zsh and missing from bash, or vice versa?

As a Linux user, I've always just used bash because it was the default on every distro I used. People using other Unix systems such as BSD seem to use other shells far more frequently. In the ...
88
votes
7answers
8k views

Why is cd not a program?

I've always wondered why cd isn't a program, but never managed to find the answer. Anyone know why this is the case?
38
votes
5answers
7k views

Timing out in a shell script

I have a shell script that's reading from standard input. In rare circumstances, there will be no one ready to provide input, and the script must time out. In case of timeout, the script must execute ...
45
votes
6answers
11k views

The result of ls * , ls ** and ls ***

I know using the command ls will list all the directories. But what does the ls * command do ? I used it and it just lists the directories. Is the star in front of ls means how deep it can list the ...
44
votes
2answers
23k views

Why can rm remove read-only files?

If I create a file and then change its permissions to 444 (read-only), how come rm can remove it? If I do this: echo test > test.txt chmod 444 test.txt rm test.txt ...rm will ask if I want to ...
25
votes
6answers
26k views

Lightweight outgoing SMTP server

What do you use as a lightweight SMTP for outgoing e-mails only? I am running an app that sends e-mails via SMTP when events occur once in a while, and I don't need a fully featured e-mail server. ...
31
votes
9answers
3k views

What is the /boot partition really for?

I'm reading a relatively old text on Linux partitions and file systems (the LPIC 1 Certification Bible). It says: Some versions of the Linux boot loaders cannot access a kernel that is outside ...
27
votes
4answers
2k views

Quick way to include a directory path when calling mv? [duplicate]

Often I will work out of the root of a rails directory where I find my self navigating the same path twice every time I would like to move a file: mv app/views/layouts/application.html.erb ...
13
votes
1answer
13k views

File descriptors & shell scripting

I am having a very hard time understanding how does one use file descriptors in shell scripts. I know the basics such as exec 5 > /tmp/foo So fd 5 is attached to foo for writing. exec 6 < ...
29
votes
7answers
7k views

Block network access of a process?

Is it possible to block the (outgoing) network access of a single process?
40
votes
7answers
11k views

what does the @ mean in ls -l?

I am using Mac OSX. When I type ls -l I see something like drwxr-xr-x@ 12 xonic staff 408 22 Jun 19:00 . drwxr-xr-x 9 xonic staff 306 22 Jun 19:42 .. -rwxrwxrwx@ 1 xonic staff 6148 25 ...
23
votes
3answers
3k views

What is the difference between these commands for bringing down a Linux server?

Reading "What is the difference between Halt and Shutdown commands?" , I generally have an idea what does the command shutdown does, with or without -h/-r options. The "halt" command performs ...
18
votes
7answers
861 views

Reading and searching long man pages

I finally got fed up when wanting to read about bash's read and it's -s option with man bash. I found the right spot eventually (around line 4500), but it was a frustrating as usual, since both /read ...
14
votes
3answers
9k views

How to find out which (not installed) package a file belongs to?

On Debian systems (and derivatives): $ dpkg --search /bin/ls coreutils: /bin/ls That is, the file /bin/ls belongs to the Debian package named coreutils. But this only works if the package is ...
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 ...
11
votes
3answers
694 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 ...

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