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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4
votes
2answers
915 views

Can I read and write to the same file in Linux without overwriting it? [duplicate]

Possible Duplicate: How can I make iconv replace the input file with the converted output? I frequently connect to amazon ec2 using their public DNS names ...
2
votes
2answers
5k views

bash: syntax error near unexpected token `}'

I tried using this: $ if [$a == 1] then { echo 'yes'; } fi; but I get an error: -bash: syntax error near unexpected token `}' What is the correct format? I tried several with no luck.
75
votes
4answers
1k 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 ...
33
votes
5answers
41k 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.
40
votes
7answers
34k views

What's the difference between /sbin/nologin and /bin/false

Technically, unless pam is set up to check your shell with pam_shells neither of these can actually prevent your login, if you're not on the shell. On my system they are even different sizes, so I ...
25
votes
1answer
483 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 ...
18
votes
4answers
26k 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 ...
21
votes
4answers
8k views

Quoting in ssh $host $FOO and ssh $host “sudo su user -c $FOO” type constructs

I often end up issuing complex commands over ssh; these commands involve piping to awk or perl one-lines, and as a result contain single quotes and $'s. I have neither been able to figure out a hard ...
35
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 ...
27
votes
2answers
9k 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 ...
18
votes
6answers
7k 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 ...
13
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 ...
20
votes
2answers
7k 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
3answers
6k 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: ...
14
votes
4answers
873 views

keep duplicates out of $PATH on source

I have the following code that's source-d by my .shellrc PATH="${PATH}:${HOME}/perl5/bin" PATH="${PATH}:${HOME}/.bin" export PATH but if I make changes to other code and then source this file, my ...
16
votes
2answers
3k views

How to apply recursively chmod directories without affecting files?

After I apply chmod -R to a directory, permissions are changed for everything within (files and directories). How can I add execute/search (x) permissions to directories without modifying the files?
11
votes
2answers
2k 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 ...
16
votes
9answers
10k views

Tool in UNIX to subtract dates

Is there any tool in Solaris UNIX (so no GNU tool available) to subtract dates? I know that in Linux we have gawk that can subtract one date from another. But in Solaris the maximum we have is nawk ...
14
votes
3answers
1k 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 ...
13
votes
6answers
872 views

How to achieve portability with sed -i (in-place editing)?

I'm writing shell scripts for my server, which is a shared hosting running FreeBSD. I also want to be able to test them locally, on my PC running Linux. Hence, I'm trying to write them in a portable ...
13
votes
1answer
8k views

bash multi line command with comments after the continuation character

Consider echo \ # this is a comment foo This gives: $ sh foo.sh # this is a comment foo.sh: line 2: foo: command not found After some searching on the web, I found a solution by DigitalRoss on ...
8
votes
2answers
9k views

How to cut (select) a field from text line counting from the end?

I know how to select a field from a line using the cut command. For instance, given the following data: a,b,c,d,e f,g,h,i,j k,l,m,n,o This command: cut -d, -f2 # returns the second field of the ...
8
votes
4answers
9k views

How do I reverse a for loop?

How do I properly do a for loop in reverse order? for f in /var/logs/foo*.log; do bar "$f" done I need a solution that doesn't break for funky characters in the file names.
6
votes
8answers
6k views

Can shell variable include - character?

I am not able to use - in variables in shell. Is there a way to be able to use it, because I have one script which depends on such named variables: $export a-b=c -bash: export: `a-b=c': not a valid ...
3
votes
6answers
733 views

When does a shell get executed during the linux startup process

I do not understand when does a shell, lets say bash, get executed, which program runs bash initially first.
19
votes
1answer
4k views

Difference between environment variables and exported environment variables in bash

Bash seems to differentiate between variables which have been exported and those which have not. example: $ FOO=BAR $ env | grep FOO $ set | grep FOO FOO=BAR set sees the variable but env does ...
10
votes
4answers
5k views

How to comment multi-line commands in shell scripts?

When invoking long, switch-heavy commands, it's a good practice to write them in shell scripts. Is there an easy way to comment lines in such scripts? I've tried the following methods, but neither ...
9
votes
2answers
749 views

Difference between executing multiple commands with && and ; [duplicate]

What's the difference between executing multiple commands with && and ;? Examples: echo "Hi\!" && echo "How are you?" and echo "Hi\!"; echo "How are you?"
9
votes
2answers
4k views

What is the significance of single and double quotes in environment variables?

I defined some environment variables in my .profile like this: MY_HOME="/home/my_user" but the variable does not seem to evaluate unless I strip off the quotes and re-source the file. I believe ...
7
votes
2answers
655 views

How can I use $var in a shell brace expansion of a sequence?

I want to use $var in a shell brace expansion with a range, in bash. Simply putting {$var1..$var2} doesn't work, so I went "lateral"... The following works, but it's a bit kludgey. # remove the ...
7
votes
3answers
7k views

How do I change folder timestamps recursively to the newest file?

I was wondering if anyone knows how to change the timestamps of folders recursively based on the latest timestamp found of the files in that folder. So for example: ...
5
votes
3answers
301 views

Correct textual name for <<

I've just come across the << command, used like so: cat > outfile.txt <<EOF Multi-line content that will be written to outfile.txt EOF Now, I've no idea what this is called, but I'd ...
4
votes
1answer
482 views

What's the conf file reading between login and non-login shell?

I found that those are .bash_profile, .bashrc, .bash_login, .profile. What's the reading sequence between them?
9
votes
1answer
314 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 ...
6
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 ...
6
votes
5answers
969 views

Shell programming, avoiding tempfiles

I often write KSH shell scripts that follow the same pattern: (1) retrieve output from one or more command (2) format it using grep|cut|awk|sed and print it to the screen or to a file In order to ...
4
votes
5answers
3k views

How do I perform an action on all files with a specific extension in subfolders in an elegant way?

My current best bet is: for i in $(find . -name *.jpg); do echo $i; done Problem: does not handle spaces in filenames. Note: I would also love a graphical way of doing this, such as the "tree" ...
2
votes
1answer
2k views

Why the behavior of while loop and for loop is different?

I am trying to read user and server details from file tempo.txt and then check the disk space usage of the file system on that unix account using another script server_disk_space.sh.But I am not able ...
19
votes
3answers
4k views

ls command: what does the first line mean?

When I do ls -l I get this: calico@A000505:~/Documentos$ ls -l total 2020 -rwxr-xr-x 1 calico calico 8559 2010-11-16 11:12 a.out -rwxrw-rw- 1 smt smt 2050138 2010-10-14 10:40 Java2.pdf ...
12
votes
0answers
356 views

Should I use $* or $@? [duplicate]

Which one of $* or $@ is better to use (in array syntax,command line parameters, etc.) to avoid any bugs/problems in code? Or it does not make any difference?
10
votes
6answers
6k views

How can I print lines from file backwards (without using “tac”)?

I have a simple question (I think). I want to print lines from a file backwards without using tac command. Is there any other solution to do such thing with bash?
5
votes
4answers
544 views

Shell, Concatenating 2 strings to reference a 3rd variable

I am trying to do the following in my script: #/bin/bash -x var1=www var2=www2 var3=www3 var4=www4 for i in 1 2 3 4; do echo \$var$i done Obviously, this doesn't work as expected and results in ...
4
votes
4answers
295 views

Grep of many keywords over many files: speeding it up

I am currently facing a "performance problem" while using grep. I am trying to locate the occurrences of many (10,000+) keywords in many (think Linux kernel repository size) files. The objective is to ...
2
votes
5answers
6k views

Carriage return issue

I have this in a bash script DAY2="20130605"<Cr> echo "This is yesterday date:"$DAY2"end" Why is the output the following? It seems as though there is a carriage return in DAY2 but where is ...
2
votes
2answers
3k views

What does the tilde (~) mean at the end of a filename? [duplicate]

What is the file with the ~ at the end of the filename for? $ ls # aliased to add flags -rwxrwxr-x 1 durrantm 2741 May 16 09:28 strip_out_rspec_prep_cmds.sh~* drwxrwxr-x 13 durrantm 4096 May 16 ...
1
vote
4answers
264 views

List all commands that a shell knows

What can I type at my shell (which happens to be bash) that will list all the commands that are recognized? Also, does this differ by shell? Or do all shells just have a "directory" of commands they ...
1
vote
2answers
1k 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"
1
vote
1answer
118 views

What is the “~/” directory?

I'm trying to locate where some files are stored and I can easily browse to them via ssh by going to "cd ~/foldername", however, I have no idea what directory "~/" actually is. When I browse around ...
75
votes
6answers
83k views

How to split the terminal into more than one “view”?

From vi, if you issue the command :sp, the screen splits into two "views", allowing you to edit more than one file from the same terminal. Along those same lines, is there a way to have multiple ...
71
votes
6answers
3k views

Run a command that is shadowed by an alias

Let's say i have the following alias in bash - alias ls='ls --color=auto' - and i want to call ordinary ls without options. Is the only way to do that is to unalias, do the command and then alias ...