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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13
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Can't indent heredoc to match nesting's indent

If there's a "First World Problems" for scripting, this would be it. I have the following code in a script I'm updating: if [ $diffLines -eq 1 ]; then dateLastChanged=$(stat --format '%y' ...
8
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5answers
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Is there a difference between `;` and `&&` and `|`? [duplicate]

When you want to run multiple commands you can use ;, && and | Like this: killall Finder; killall SystemUIServer, cd ~/Desktop/ && rm Caches Or: man grep | man cat for example. But, ...
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2answers
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What does “${-#*i}” != “$-” mean?

In /etc/profile I see this: for i in /etc/profile.d/*.sh ; do if [ -r "$i" ]; then if [ "${-#*i}" != "$-" ]; then . "$i" else . "$i" >/dev/null 2>&1 ...
25
votes
3answers
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Multicolored Grep

I'm trying to get each grep command to highlight it's results in a different color. I can do it manually with a line like this: ls -l GREP_COLORS='mt=01;32' grep c | GREP_COLORS='mt=01;31' grep o | ...
19
votes
3answers
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How do I switch from an unknown shell to bash?

I was surprised that I didn't find this question already on the site. So, today $ came up after I logged in as a new user. This was unexpected because my main user's prompt starts with ...
17
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2answers
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What happens when we type a simple command on shell? [duplicate]

I have a simple doubt on execution of a simple command. As per my understanding, when we type a command such as ls on interactive shell, Shell interpretes the command. Shell creates a child process ...
16
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4answers
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Why do I have to re-set env vars in tmux when I re-attach?

I mainly work on a mac and ssh/tmux attach to a Linux machine to do my work. I have ssh-agent running on the Linux machine. I have set -g update-environment "SSH_AUTH_SOCK SSH_ASKPASS WINDOWID ...
15
votes
5answers
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find searching in parent directories instead of subdirectories

I'm nested deep in a file tree, and I'd like to find which parent directory contains a file. E.g. I'm in a set of nested Git repositories and want to find the .git directory controlling the files I'm ...
11
votes
3answers
2k views

What does “rm is hashed” mean?

I'm going through http://mywiki.wooledge.org/BashGuide/CommandsAndArguments and came across this: $ type rm rm is hashed (/bin/rm) $ type cd cd is a shell builtin Just a little earlier, the guide ...
11
votes
1answer
915 views

Can I restrict a user to use special programs?

Is it possible to add a user and restrict him to run special programs? For example after this user logs in, he can only open Firefox to use Internet and no other programs can be run by this user. For ...
6
votes
2answers
463 views

Expansion of a shell variable and effect of glob and split on it

This post actually contains two separate questions but I think grouping them together will give some more context. I have gone through this question on quotes around variables but I do not quite ...
19
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5answers
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Parenthesis in bash arithmetic: 3 * (2 + 1)

expr does not seem to like parenthesis (used in mathematics to explicit operator priority): expr 3 * (2 + 1) bash: syntax error near unexpected token `(' How to express operator priority in bash?
13
votes
2answers
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/dev/tcp listen instead of nc listen

With a netcat listener like: nc -l <port> < ~/.bashrc I can grab my .bashrc on a new machine (doesn't have nc or LDAP) with: cat < /dev/tcp/<ip>/<port> > ~/.bashrc My ...
9
votes
2answers
5k views

Can files be created with permissions set on the command line?

When creating directories, mkdir -m <mode> <dir> provides for creating one or more directories with the given mode/permissions set (atomically). Is there an equivalent for creating files, ...
7
votes
2answers
3k views

List elements with spaces in zsh

I've been studying zsh scripting for all of 2 hours at this point and I've hit a wall. I want to go through a list of files that may have spaces in them. I'm open to completely different approaches ...
6
votes
2answers
4k views

Manipulate file name piped from find command

I'm relatively new to Bash and am trying to do something that on the surface seemed pretty straightforward - run find over a directory hierarchy to get all of the *.wma files, pipe that output to a ...
5
votes
6answers
3k views

Portable way to find inode number

At first I used stat -c %i file (to help detect the presence of a jail), which seemed to work on any Linux distribution under the sun. On OS X' I had to use ls -i file | cut -d ' ' -f 1. Is there ...
23
votes
4answers
28k views

Usage of dash (-) in place of a filename

For a command, if using - as an argument in place of a file name will mean STDIN or STDOUT. But in this example, it creates a file with the name -: echo hello > - How can I make - in this ...
19
votes
5answers
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How can I create an alias for a git [action] command (which includes spaces)?

Most of my my aliases are of this form: alias p='pwd' I want to alias git commit so that it does git commit -v But trying to create an alias with a space gives an error: $ alias 'git commit'='git ...
17
votes
2answers
6k views

Are there naming conventions for variables in shell scripts?

Most languages have naming conventions for variables, the most common style I see in shell scripts is MY_VARIABLE=foo. Is this the convention or is it only for global variables? What about variables ...
16
votes
3answers
26k views

Why I can't escape spaces on a bash script? [duplicate]

I'm trying to escape the space char for a path in Bash, but neither using a backslash or quotes works. .sh script: ROOT="/home/hogar/Documents/files/" FILE=${ROOT}"bdd.encrypted" ...
12
votes
2answers
11k views

Solving “mv: Argument list too long”?

I have a folder with more than a million files that needs sorting, but I cant really do anything because mv outputs this message all the time -bash: /bin/mv: Argument list too long I'm using this ...
12
votes
3answers
16k views

Confusing use of && and || operators

I was skimming through an /etc/rc.d/init.d/sendmail file (I know this is hardly ever used, but I'm studying for an exam), and I've become a bit confused about the && and the || operators. ...
11
votes
3answers
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How to “send” variable to sub-shell?

I run the following script: VAR="Test" sh -c 'echo "Hello $VAR"' But I get : # ./test.sh Hello How can I "send" the variable VAR of my script to the shell created with sh -c?
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2answers
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Closing a file descriptor, >&- vs <&-

In the bash tutorial I am reading, it says that if you open a file descriptor for reading, i.e. exec 3< echolist Then you must close it like this, exec 3<&- However, if you open a file ...
9
votes
1answer
260 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
7k views

How to make exported shell variables permanent?

export LD_PRELOAD=/usr/lib/libtsocks.so It's ok that I can export in this way, but how can I make it permanent? I want LD_PRELOAD to still be changed after a reboot. I'm using Ubuntu and Fedora
9
votes
2answers
2k views

Use HERE file and redirect output of command

I have the following code in a batch script: mpirun -np 6 ./laplace <<END 100 100 100 0.01 100 3 2 1 END | tail -n 1 > output But it isn't working. What I want it to do is to use the HERE ...
8
votes
2answers
431 views

Is it possible to use shell parameter variables ($1, …, $@) directly in CLI?

Sometimes it is necessary to emulate and verify the above variables in small examples and then can be copied immediately to some script, etc. I tried to solve by using a simple example in the ...
7
votes
3answers
667 views

why for loop doesn't raise “argument too long” error?

I found that this would raise the "argument too long" error: ls *.* And this would not raise it: for file in *.* do echo $file done Why?
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2answers
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How to trigger error using Trap command

I am using Ubuntu 12.04.2. I am trying to use "trap" command to capture abnormal or error in my shell script but I am also trying to manually trigger "Error" exit. I have tried exit 1, but it won't ...
5
votes
2answers
231 views

How can I force bash to deallocate a brace expanded variable without name?

For benchmarking, I ran the command: for i in {1..100000000}; do echo "$i" line >> file done Bash expanded the braces and stored the list 1 2 3 4 5 6 ... 100000000 in memory. I thought ...
5
votes
5answers
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How to start a program in the background

The program Boblight does not run in background. There is no noticeable difference between executing sudo boblightd and sudo boblightd& How can I solve this problem that the console will ...
5
votes
4answers
2k views

Where do I change my shell?

If I wanted to use zsh, for instance, rather than bash, where would I make this change for the current user?
5
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4answers
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Is it possible in unix to search inside zip files

I have 100s of directories and within those I have a few zip files. Now there are images named abc.jpg in those zip files. The zip files may be in any folder or in any subfolder so its difficult to ...
3
votes
7answers
837 views

Batch renaming of files [duplicate]

I have a directory full of images: DSCN4749.JPG DSCN4750.JPG DSCN4892.JPG ... And I would like a one-liner to rename them to (say). dscn4749.jpg dscn4750.jpg dscn4892.jpg ... How do I do this? ...
15
votes
4answers
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Why do some commands 'hang' the terminal until they've finished?

Sometimes you run a program from the terminal, say, lxpanel†. The terminal won't drop you back to the prompt, it'll hang. You can press Ctrl+C to get back to the prompt, but that will kill ...
8
votes
5answers
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Convert file contents to lower case

I have temp file with some lower-case and upper-case contents. Input Contents of my temp file: hi Jigar GANDHI jiga I want to convert all upper to lower. Command I tried the following command: ...
8
votes
1answer
7k views

How to hide commands typed in a Linux shell?

Is there a way to hide what I'm typing in a shell, so the terminal doesn't echo my keystrokes, while still keeping the output of the command? For example, if I run ip addr show, the only thing on the ...
8
votes
5answers
3k views

`highlight` command

Is there a command which can be used to easily highlight (with colors, bold text or inverted back-/foreground) specific strings in stdin while passing the entire string to stdout? I'm thinking of ...
7
votes
2answers
1k views

What is word splitting? Why is it important in shell programming?

I'm getting confused about the role word splitting plays in zsh. I have not been exposed to this concept when programming in C, Python or MATLAB, and this has triggered my interest of why word ...
6
votes
1answer
471 views

Bash is automatically reloading (injecting) updates into a running script upon saving it: Why? Any practical use?

I was writing a bash script, and happened to updated the code (saved the script file to disk) while the script was waiting for some input in a while loop. After I returned to the terminal and ...
6
votes
5answers
4k views

Convert linux sysuptime to well format date

I want to display /proc/uptime in well format as: DD:HH:MM:SS /proc/uptime give me up time of system in seconds, is there a standard solution that convert seconds to this format?
6
votes
1answer
6k views

Changing the default shell without chsh or administrator priviledges

I saw the following snippet in this thread: How to change from csh to bash as default shell sleep 2 if (-x /usr/local/bin/bash) then exec /bin/sh -c '. ~/.profile; exec /usr/local/bin/bash' endif ...
6
votes
4answers
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Why is echo ignoring my quote characters?

The echo command isn't including the complete text that I give it. For example, if I do: $ echo ' echo PARAM=` grep $ARG /var/tmp/setfile | awk '{print $2}' ` ' It outputs: echo PARAM=` ...
6
votes
3answers
411 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 ...
6
votes
1answer
349 views

Can I have my shell history record how wildcards expanded?

If I run: $ ls * foo bar buzz $ history | tail -1 ls * You can see that in my shell history it remembers that I ran ls * rather than ls foo bar buzz. Ideally, I'd like to record both in separate ...
5
votes
3answers
217 views

Echoing stdin when running an ed(1) script

Given the following ed script, $ cat helloworld a hello world . ,n ,s,o,O,g ,n Q I would like to obtain somehow the interactive output $ ed a hello world . ,n 1 hello 2 world ,s,o,O,g ,n 1 ...
5
votes
2answers
610 views

Why doesn't my loop over the output of ls work?

I am trying to feed Directory names into a for loop. My code is as follows: td='/Test/TopDir/' cd "$td" for x in $(ls -d */ | cut -f1 -d'/'); do echo x=$x done The top directory I run this on ...
5
votes
4answers
165 views

Is it shell portable to run a command on the same line after variable assignment?

Is there any standard that covers the portability of running a command after variable assignment on the same line? APPLE="cider" echo hi How portable is something like that? Where will it work and ...