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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10
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4answers
10k 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.
10
votes
4answers
6k 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 ...
8
votes
2answers
13k 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 ...
6
votes
5answers
16k 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 ...
3
votes
6answers
933 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.
2
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1answer
2k views

pass 1 environment variable using sudo

Is this a correct way for passing an environment variable in sudo ? sudo -u www-data -b env FOOBAR="foobar" /home/user/folder/daemon
10
votes
2answers
2k 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
5k 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
5answers
4k 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" ...
6
votes
3answers
554 views

Understanding environmental variables in different contexts

Trying to understand the behaviour of the environment in Linux (Ubuntu 13.04 concretely), I've find different situations where setting envirionment variables are used or defined for/in different ...
4
votes
1answer
598 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?
10
votes
2answers
651 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 ...
6
votes
4answers
1k 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 ...
3
votes
1answer
90 views

Fine-tune LS_COLORS for directory suffixes?

I’m fine customizing most settings in LS_COLORS. I don’t need help customizing the colors for different file suffixes. What I would like to do is configure special colors for certain directories, ...
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 ...
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?
11
votes
6answers
7k 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?
6
votes
7answers
2k views

Is it bad practice to set root's shell to something other than the default?

Once a friend of mine (who is an experienced Unix/Linux user) told me that setting root's shell to something other than sh (i.e bash or zsh) might create problems, because some script might assume ...
5
votes
4answers
718 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
2answers
4k 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 ...
4
votes
4answers
376 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
8k 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 ...
1
vote
4answers
910 views

renaming multiple files increment order

I have the following files SSt12.precip.374.sub.climatology.ctl SSt12.precip.2874.sub.climatology.ctl SSt12.precip.3764.sub.climatology.ctl SSt12.precip.6774.sub.climatology.ctl I want to rename ...
1
vote
4answers
1k views

Why does the command shuf file > file leave an empty file?

I know this is sort of a duplicate of another question (Why this sort command gives me an empty file?) but I wanted to expand on the question in response to the answers given. The command shuf ...
1
vote
4answers
335 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
1answer
121 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 ...
90
votes
6answers
107k 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 ...
61
votes
4answers
149k views

How do I grep for multiple patterns?

I want to find all lines in several files that match one of two patterns. I tried to find the patterns I'm looking for by typing grep (foo|bar) *.txt but the shell interprets the | as a pipe and ...
75
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 ...
30
votes
6answers
2k views

When do you use brace expansion?

I understand what brace expansion is, but I don't know how best to use it. When do you use it? Please teach me some convenient and remarkable examples if you have your own tip.
31
votes
12answers
1k views

How to quickly store and access often used commands?

I have a lot of commands I routinely need to execute, often with the slightest variation. Right now I'm storing them all in .bash_history and use CTRL-R to access them, but I wonder if there's a ...
92
votes
3answers
9k views

Why is “echo” so much faster than “touch”?

I'm trying to update the timestamp to the current time on all of the xml files in my directory (recursively). I'm using Mac OSX 10.8.5. On about 300,000 files, the following echo command takes 10 ...
32
votes
4answers
55k views

How to suspend and bring a background process to foreground

I have a process originally running in the foreground. I suspended by Ctrl+Z, and then resume its running in the background by bg <jobid>. I wonder how to suspend a process running in the ...
29
votes
1answer
1k views

Avoiding “BASH-isms” in shell scripts

Does there exist a tool similar to Perl::Critic that will inspect your shell scripts and point out flaws, portability issues, uses of non-standard programs without fallbacks, depreciated program uses, ...
58
votes
4answers
9k views

What is the difference between “&&” and “;” when chaining commands

What is the difference between the following methods of chaining commands? cmd1; cmd2 cmd1 && cmd2
16
votes
1answer
32k views

How can I execute local script on remote machine and include arguments?

I have written a script that runs fine when executed locally: ./sysMole -time Aug 18 18 The arguments "-time", "Aug", "18", and "18" are successfully passed on to the script. Now, this script is ...
40
votes
5answers
11k views

How to go to the previous working directory in terminal?

In terminal, how can I define a key to go to the previous directory which I was in when changing directory with the cd command? For example, I'm in /opt/soft/bin and I cd into /etc/squid3 and I want ...
32
votes
5answers
7k views

Does /usr/sbin/nologin as a login shell serve a security purpose?

In my /etc/passwd file, I can see that the www-data user used by Apache, as well as all sorts of system users, have either /usr/sbin/nologin or /bin/false as their login shell. For example, here is a ...
20
votes
3answers
958 views

Which are the standard commands available in every Linux based distribution?

I would like to know which are the standard commands available in every Linux system. For example if you get a debian/ubuntu/redhat/suse/arch/slackware etc, you will always find there commands like: ...
30
votes
4answers
75k views

How to add repository from shell in Debian?

In Ubuntu one can add a repository to apt-get via the following command - sudo add-apt-repository ppa:yannubuntu/boot-repair As Ubuntu is based on Debian code base, I was expecting that the same ...
7
votes
3answers
603 views

How to scroll large datafile while keeping header

Suppose I have some large datafile, which overflow the screen in both vertical and horizontal direction. How can I browse this file, while the header-lines stay on the screen? For the moment, I am ...
4
votes
3answers
2k views

Best practice to use $? in bash?

When I read this answer about $? another question comes to mind. Is there any best practice for how to use $? in bash? Let's have a example: We have a linear script and I we would like to know ...
24
votes
6answers
10k views

determining path to sourced shell script

Is there a way for a sourced shell script to find out the path to itself? I'm mainly concerned with bash, though I have some coworkers who use tcsh. I'm guessing I may not have a ton of luck here, ...
22
votes
6answers
57k views

Is there a way to see details of all the threads that a process has in Linux?

For Windows, I think Process Explorer shows you all the threads under a process. Is there a similar command line utility for Linux that can show me details about all the threads a particular process ...
20
votes
2answers
5k views

How can I test for POSIX compliance for shell scripts?

Probably the main thing that this website has taught me so far is the importance of writing portable shell scripts. Considering that POSIX is the closest thing to a common standard between all ...
19
votes
6answers
817 views

How could I remember how to use redirection?

I know what program > /dev/null 2>&1 does. It redirects the output to /dev/null and 2>&1 means to redirect the error output in the same place where the output is sent. My ...
17
votes
2answers
4k views

Is there an easy way to programmatically extract IP address?

Is there an easy way to programmatically extract IP address, without tedious parsing of ifconfig? I would not mind simple command output processing using sed to do it but not processing multiline ...
13
votes
4answers
1k views

How to do a continous 'wc -l' with gnu texttools?

I know of course that cat logfile.txt | wc -l 120 will tell me the number of lines in a file. Whereas tail -f logfile.txt will show me the new lines that another program writes to logfile.txt. ...
9
votes
5answers
5k views

Shell Script mktemp, what's the best method to create temporary named pipe?

I'm aware its best to create temporary files with mktemp, but what about named pipes? I have an idea, I'm sure it would work, but I would really like an expert opinion. I prefer things to be as ...
39
votes
4answers
1k views

What exactly is an environment variable?

I know that VARIABLE=value creates an environment variable, and export VARIABLE=value makes it available to processes created by the current shell. env shows the current environment variables, but ...