The shell is unix's command-line interface. You can type commands in a shell interactively, or write scripts to automate tasks. Use this tag for questions applying to /bin/sh and most compatible shells (ash, bash, ksh, zsh, …).

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23
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12answers
9k views

Object-oriented shell for *nix

Preface: I love bash and have no intention of starting any sort of argument or holy-war, and hopefully this is not an extremely naive question. This question is somewhat related to this post on ...
9
votes
3answers
11k views

What is IFS in context of for looping?

I'm learning bash scripting right now. I was reading this thread: How to loop over the lines of a file? What is IFS? With for and IFS:
20
votes
3answers
44k views

How do I set an environment variable on the command line and have it appear in commands?

If I run export TEST=foo echo $TEST It outputs foo. If I run TEST=foo echo $TEST It does not. How can I get this functionality without using export or a script?
19
votes
6answers
2k views

Do we have more history for cd?

cd - can move to the last visited directory. Can we visit more history other than the last one?
17
votes
6answers
19k views

Grabbing the extension in a file name

How do I get the file extension from bash? Here's what I tried: filename=`basename $filepath` fileext=${filename##*.} By doing that I can get extension of bz2 from the path /dir/subdir/file.bz2, ...
12
votes
5answers
21k views

Is there a way to set the size of the history list in bash to more than 5000 lines?

No matter how much I set the HISTSIZE environment variable to be larger than 5000, when printing the history list with the history builtin, it prints only the last 5000 commands. I need that because I ...
25
votes
6answers
2k views

Why can't I redirect a path name output from one command to “cd”?

I am trying to get cd to accept a directory name redirected to it from another command. Neither of these methods work: $ echo $HOME | cd $ echo $HOME | xargs cd This does work: $ cd $(echo $HOME) ...
24
votes
12answers
6k views

Remove duplicate $PATH entries with awk command

I am trying to write a bash shell function that will allow me to remove duplicate copies of directories from my path environment. I was told that it is possible to achieve this with a one line ...
18
votes
4answers
1k views

Path independent shebangs

I have a script that I want to be able to run in two machines. These two machines get copies of the script from the same git repository. The script needs to run with the right interpreter (e.g. zsh). ...
18
votes
1answer
3k views

Aliases vs functions vs scripts

This site says, "Shell functions are faster [than aliases]. Aliases are looked up after functions and thus resolving is slower. While aliases are easier to understand, shell functions are preferred ...
15
votes
3answers
9k views

How to check if pipe is empty

I have piped line in bash script and want to check if pipe has data, before feeding program Searching I found about test -t 0 but it doesn't work here. Always returns false. So how to be sure that ...
12
votes
3answers
14k views

How to escape quotes in shell?

I'm having trouble with escaping characters in bash. I'd like to escape single and double quotes while running a command under a different user. For the purposes of this question let's say I want to ...
7
votes
5answers
20k views

Disable user shell for security reasons

We have several user accounts that we create for automated tasks that require fine-grained permissions, such as file transfer across systems, monitoring, etc. How do we lock down these user accounts ...
15
votes
7answers
4k views

How to pipe output from one process to another but only execute if the first has output?

How can I rewrite this command to only email if there is output from the mailq | grep? mailq | egrep 'rejected|refused' -A 5 -B 5 | mail -s 'dd' email@email Is this even possible on one line?
11
votes
8answers
400 views

How to access the history on the fly in unix?

for example, if I do a mkdir thisismyfolder912 I remember there is some easier way to switch to thisismyfolder912 than having to do a cd thisismyfolder912 What is that way and how does it work? ...
10
votes
5answers
3k views

Why is the terminal case-sensitive?

When I do - CD .. instead of cd .. it gives me error saying - CD: command not found Why is the terminal case sensitive when it comes to linux commands? I mean you should be able to execute the ...
10
votes
4answers
3k views

Can I make `cut` change a file in place?

The man page doesn't give me much hope, but I'm hoping it's an undocumented (and/or GNU-specific) feature.
11
votes
1answer
658 views

How to “correctly” start an application from a shell

I find it hard to phrase the question precisely but I will give my best. I use dwm as my default window manager and dmenu as my application launcher. I hardly use GUI applications aside from my ...
7
votes
2answers
3k views

what is meant by connecting STDOUT and STDIN?

This is a newbie question. I'm reading a book, it says: Every process has at least three communication channels available to it: “standard input” (STDIN), “standard output” (STDOUT), and ...
2
votes
1answer
2k views

Cut + Copy + Paste in Linux Puppy Wary

Trying to cut + copy + paste from the standard urxvt shell in Puppy Linux Wary 5.3. If I follow this post on http://crunchbang.org/forums/viewtopic.php?id=15085 and I press: in urxvt: ctrl + ...
10
votes
2answers
773 views

Strange difference between pwd and /bin/pwd

I added a symlink to the current directory with ln -s . aa. If I execute cd aa, and after that I executed pwd, the response is /home/sim/aa. But if I execute /bin/pwd it prints /home/sim (the current ...
7
votes
1answer
2k views

Best way to remove file extension from a string?

So, I'm using a script I've made to convert videos to the webm format. A certain program calls the script, sending %f which is the full, absolute file name of the video, like this: converter.sh %f ...
7
votes
3answers
451 views

“mkdir foo; svn mv * foo”

I often want to do some variant of this idiom: $ mkdir 2010 $ svn mv * 2010 Of course, I get an error because the glob matches 2010 as well: svn: Cannot copy path '2010' into its own child ...
3
votes
2answers
2k views

When sh is a symlink to bash or dash, bash limits itself to POSIX-compliance, so it should be 100% compatible with sh?

From Difference between bash and sh: ABck to the question: If you have /bin/sh as a link to bash, then bash will not behave the same when called as /bin/sh as it does when called as /bin/bash. ...
1
vote
2answers
2k views

Busy box Read file line by line

There is no read applet comming with busy box. Is there any way to read a txt file line by line using busybox? What I have now is while read line do echo $line done < "$InputFile"
7
votes
4answers
618 views

How can I find a file whose name includes a given string, such as “abcde”?

Within a set of directories, how do I find a file whose name includes a given string, such as "abcde"?
4
votes
1answer
212 views

How do I come by this pty and what can I do with it?

It seems that a simple shell redirect from /dev/ptmx gets me a new pseudo-terminal. $ ls /dev/pts; ls /dev/pts </dev/ptmx 0 1 2 ptmx 0 1 2 3 ptmx It disappears as soon as the process that ...
2
votes
2answers
153 views

bash completion in Posix mode

I would like to use an interactive Posix bash shell but the autocomplete feature doesn't work. When I type the Tab character it simply prints a tab. The tab completion works only when I'm in a ...
80
votes
4answers
2k views

Can I somehow add a “&& prog2” to an already running prog1?

Most shells provide functions like && and ; to chain the execution of commands in certain ways. But what if a command is already running, can I still somehow add another command to be executed ...
21
votes
2answers
29k views

Difference between 2>&-, 2>/dev/null, |&, &>/dev/null and >/dev/null 2>&1

Just looking for the difference between 2>&- 2>/dev/null |& &>/dev/null >/dev/null 2>&1 and their portability with non-Bourne shells like tcsh, mksh, etc.
40
votes
6answers
62k views

How to monitor CPU/memory usage of a single process?

I would like to monitor one process's memory / cpu usage in real time. Similar to top but targeted at only one process, preferably with a history graph of some sort.
53
votes
2answers
47k views

How do I use pushd and popd commands?

What are the practical uses of both pushd and popd when there is an advantage of using these two commands over cd and cd -? EDIT: I'm looking for some practical examples of uses for both of these ...
32
votes
4answers
42k views

What does ` (backquote/backtick) mean in bash?

I came across the following command sudo chown `id -u` /somedir And I wonder what is the meaning of the ` symbol. I noticed for instance that while the command above works well the one below does ...
35
votes
7answers
2k views

Is test or [ or [[ more portable both between bash shells and between other shells?

I see I can do $ [ -w /home/durrantm ] && echo "writable" writable or $ test -w /home/durrantm && echo "writable" writable or $ [[ -w /home/durrantm ]] && echo ...
32
votes
3answers
14k views

Precedence of the shell logical operators &&, ||

I am trying to understand how the logical operator precedence works in bash. For example, I would have expected, that the following command does not echo anything. true || echo aaa && echo ...
26
votes
1answer
542 views

A shell tool to “tablify” input data

A long time ago I remember using a command that makes its input into a nicely formatted table. For example, for this input, apple 1 100 orange 20 19 pineapple 1000 87 avocado 4 30 The output will ...
30
votes
2answers
13k views

What does “3>&1 1>&2 2>&3” do in a script?

I saw this line in a script: DEVICE=`dialog --inputbox "Festplatten-Laufzeit auslesen. Gebe Sie das gewünschte Device an: " 0 70 "" 3>&1 1>&2 2>&3` What is 3>&1 ...
21
votes
6answers
10k views

How to check if bash can print colors

I want to know if there's any way to check if my program can output terminal output using colors or not. Running commands like less and looking at the output from a program that outputs using ...
38
votes
2answers
2k views

Why is '.' a hard link in Unix?

I've seen many explanations for why the link count for an empty directory in Unix based OSes is 2 instead of 1. They all say that it's because of the '.' directory, which every directory has pointing ...
30
votes
4answers
14k views

Clone ownership and permissions from another file?

Is there a command or flag to clone the user/group ownership and permissions on a file from another file? To make the perms and ownership exactly "like" that of another file?
28
votes
5answers
9k views

How to make bidirectional pipe between two programs?

Everyone knows how to make unidirectional pipe between two programs (bind stdout of first one and stdin of second one): first | second. But how to make bidirectional pipe, i.e. cross-bind stdin and ...
22
votes
2answers
932 views

Does ~ always equal $HOME

I know this has probably been asked before, but I couldn't find it with Google. Given Linux Kernel No configurations that change $HOME bash Will ~ == $HOME be true?
17
votes
2answers
4k views

recursive statistics on file types in directory?

I did a website scrape for a conversion project. I'd like to do some statistics on the types of files in there -- for instance, 400 .html files, 100 .gif, etc. What's an easy way to do this? It has to ...
31
votes
2answers
2k views

What does it mean to be “sh compatible”?

I've seen the phrase "sh compatible" used usually in reference to shells. I'm not sure if it also applies to the programs that might be run from within shells. What does it mean for a shell or other ...
14
votes
14answers
4k views

How to read an IP address backwards?

If we have this string (IP address): 192.168.1.1 How can I derive the (DNS reverse record form) from this string, so it will be shown like 1.1.168.192.in-addr.arpa using a shell script?
14
votes
3answers
2k views

How to delete part of a path in an interactive shell?

Is there a shortcut in bash and zsh to delete one component of a path? For example, if I type ls ~/local/color/, and the cursor is at the end of line, is there a shortcut to delete the color/ at the ...
13
votes
2answers
5k views

What is meant by a shell is in “vi” mode or “emacs” mode?

This question follows directly from the answer to one of my previous questions here. I am newbie to unix world and this site has helped a lot to understand some core concepts. In this case I am ...
20
votes
2answers
11k views

how can I use bash as my login shell when my sysadmin refuses to let me change it

The unix sysadmin where I'm working is reluctant to give me access to change my login shell from ksh to bash. He has given various excuses, the funniest being that since they write all their scripts ...
17
votes
4answers
4k views

What happens if you edit a script during execution?

I have a general question, which might be a result of misunderstanding of how processes are handled in Linux. For my purposes I am going to define a 'script' as a snippet of bash code saved to a text ...
17
votes
3answers
8k views

bulk rename (or correctly display) files with special characters

I have a bunch of directories and subdirectories that contain files with special characters, like this file: robbie@phil:~$ ls test�sktest.txt test?sktest.txt Find reveals an escape sequence: ...