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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14
votes
4answers
973 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 ...
13
votes
7answers
20k views

How to compare to floating point number in a shell script

I want to compare two floating point numbers in a shell script. The following code is not working: #!/bin/bash min=12.45 val=10.35 if (( $val < $min )) ; then min=$val fi echo $min
13
votes
8answers
8k 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 ...
16
votes
2answers
4k 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?
15
votes
6answers
2k 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 ...
12
votes
4answers
11k 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.
7
votes
3answers
9k 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: ...
16
votes
9answers
11k 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 ...
15
votes
2answers
10k 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 ...
13
votes
4answers
7k 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 ...
10
votes
3answers
6k views

What is the purpose of -e in sed command?

I can't find any documentation about the sed -e switch, for simple replace, do I need it? e.g. sed 's/foo/bar/' VS sed -e 's/foo/bar/'
8
votes
2answers
16k 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
5answers
6k 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" ...
3
votes
6answers
1k 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.
3
votes
1answer
3k 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
3k 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
1answer
266 views

Security Implications of using unsanitized data in Shell Arithmetic evaluation

In a comment to a recent question, Stéphane Chazelas mentions that there are security implications to double parentheses arithmetic such as: x=$((1-$x)) on most shells. My Google skills seem to be ...
9
votes
2answers
6k 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 ...
6
votes
3answers
608 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
654 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?
11
votes
6answers
8k 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
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 ...
5
votes
7answers
578 views

Turn list into single line with delimiter

I have to take a list (loads) of IP addresses in this format: 134.27.128.0 111.245.48.0 109.21.244.0 and turn them into this format with a pipe in-between (IPs made up) 134.27.128.0 | ...
5
votes
2answers
7k views

Trouble in script with spaces in filename

I've got a script that scp's a file from remote host back to local. Sometimes the file names contain spaces. scp does not like spaces in its file names. For some reason my attempts at handling the ...
4
votes
1answer
105 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?
10
votes
1answer
308 views

regarding portable sed -e… d b or ! b?

In this edit Stéphane Chazelas POSIXifies (again) my sed formatting by inserting an -expression break and another -expression statement. Now, I might just ask him why in the comments, I suppose, but ...
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
800 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 ...
5
votes
2answers
6k 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 ...
5
votes
4answers
229 views

is the behaviour of .* to include . and .. defined in LSB or POSIX or some other specification?

as title: is the behaviour of .* to include . and .. defined in LSB or POSIX or some other specification?
4
votes
4answers
405 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
9k 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
987 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
519 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 ...
101
votes
6answers
120k 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 ...
68
votes
5answers
176k 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 ...
78
votes
6answers
4k 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 ...
70
votes
11answers
74k views

How to add a newline to the end of a file?

Using version control systems I get annoyed at the noise when the diff says No newline at end of file. So I was wondering: How to add a newline at the end of a file to get rid off those messages?
36
votes
4answers
66k 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 ...
30
votes
6answers
3k 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
2k 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 ...
94
votes
3answers
10k 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 ...
20
votes
2answers
39k 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 ...
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, ...
60
votes
4answers
12k 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
39
votes
3answers
2k views

How do I open an incognito bash session?

Is it possible to open an incognito session in bash? For example, when we need to enter passwords in commands and don't want bash to add them to history.