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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27
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2answers
1k 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?
33
votes
4answers
18k views

In bash, is it possible to use an integer variable in the loop control of a for loop?

I have the following bash script: #!/bin/bash upperlim=10 for i in {0..10} do echo $i done for i in {0..$upperlim} do echo $i done The first for loop (without the variable upperlim in the loop ...
18
votes
8answers
13k 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
4answers
1k views

keep duplicates out of $PATH on source [duplicate]

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 ...
10
votes
6answers
13k views

Shell script for moving oldest files?

How do I write a script for moving just the 20 oldest files from one folder to another? Is there a way to grab the oldest files in a folder?
8
votes
6answers
1k views

IO redirection and the head command

I was trying to quickly edit an .hgignore file from the Cygwin bash shell today, and I added a line that was a mistake. I'm not sure if this was the best way to do it, but I quickly thought of using ...
17
votes
9answers
14k views

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

I want to print lines from a file backwards without using tac command. Is there any other solution to do such thing with bash?
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 ...
5
votes
1answer
737 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 ...
9
votes
5answers
8k views

sh startup files over ssh

I have some important commands I need to execute before any sh shell starts. This is required for passing SSH commands in the SSH command (ssh host somecommand) and other programs that run commands. ...
4
votes
3answers
982 views

Why is brace expansion not supported?

Do I have to do any configuration to start brace expansion? When I run mkdir {1..10}, it just creates a dir naming {1..10}.
2
votes
2answers
2k views

Use of quotes in GNU grep regular expressions

I could see that certain patterns in the GNU Grep can be enclosed within brackets and certain others need not be. For example, matching the beginning of a word works only if it is enclosed within ...
1
vote
3answers
186 views

Why are non-environment variables passed to the subshell invoqued by command substitution?

Bash manual says: Command substitution, commands grouped with parentheses, and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment, except ...
0
votes
1answer
102 views

Modifying the search result of grep

Normally, we can search a keyword in a list of pdf files using the following command: for file in *pdf; do pdftotext "$file" - | grep keyword done Now, from the search result, If I want to find ...
0
votes
1answer
6k views

Script failing with “command not found: ^M”

When I try to run the following script in zsh, via the command /bin/zsh ~/.set_color_scheme.sh I get the following error: command not found: ^M The script has u+x permissions and it used to work on ...
67
votes
4answers
159k 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 ...
70
votes
8answers
77k 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 ...
32
votes
4answers
7k views

Process substitution and pipe

I was wondering how to understand the following: Piping the stdout of a command into the stdin of another is a powerful technique. But, what if you need to pipe the stdout of multiple ...
39
votes
11answers
22k views

How do I reuse the last output from the command line?

This is a noob question, but I'd like to know how to reuse the last output from the console, ie: pv-3:method Xavier$ python -c "from distutils.sysconfig import get_python_lib; print get_python_lib()" ...
22
votes
2answers
7k 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 ...
35
votes
3answers
9k 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 ...
31
votes
12answers
12k 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 ...
37
votes
5answers
7k views

difference between “function foo() {}” and “foo() {}”

I can define bash functions using or omitting the function keyword. Is there any difference? #!/bin/bash function foo() { echo "foo" } bar() { echo "bar" } foo bar Both calls to functions ...
28
votes
5answers
33k views

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?
26
votes
6answers
3k 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) ...
15
votes
4answers
13k views

Check if pipe is empty and run a command on the data if it isn't

I have piped a line in bash script and want to check if the pipe has data, before feeding it to a program. Searching I found about test -t 0 but it doesn't work here. Always returns false. So how to ...
9
votes
3answers
10k views

How to defer variable expansion

I was wanting to initialize some strings at the top of my script with variables that have no yet been set, such as: str1='I went to ${PLACE} and saw ${EVENT}' str2='If you do ${ACTION} you will ...
23
votes
1answer
8k 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 ...
14
votes
2answers
26k 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 ...
7
votes
2answers
858 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 ...
17
votes
9answers
18k views

How do I check if a variable exists in bash?

I need to check a variable's existence in an if statement. Something to the effect of: if [ -v $somevar ] then echo "Variable somevar exists!" else echo "Variable somevar does not exist!" ...
16
votes
6answers
13k views

Why are PATH variables different when running via sudo and su?

On my fedora VM, when running with my user account I have /usr/local/bin in my path: [justin@justin-fedora12 ~]$ env | grep PATH ...
13
votes
2answers
7k 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
3answers
2k views

Writing a character N times using the printf command

I found the following command to repeat a character in Linux: printf 'H%.0s' {1..5000} > H.txt I want, for example, H to repeat 5000 times. What does %.0s mean here?
8
votes
2answers
4k 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
3answers
351 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 ...
5
votes
3answers
18k views

executing a sh script from the cron

I have a test.sh script #!/bin/sh php /home/v/file.php sh /root/x/some.sh when I execute the file as root from command line it works. sh /home/v/test.sh when I set it to crontab -e (is the root ...
4
votes
2answers
3k 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. ...
4
votes
2answers
3k views

Is there a way to push shell config information when SSHing to a host?

I know how to set the GNOME-terminals (or xterms!) prompt to green/red regarding the last exit code: vi .bashrc export PROMPT_COMMAND='PS1="` if [[ \$? = "0" ]]; then echo "\\[\\033[0;32m\\]"; else ...
11
votes
2answers
11k 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 ...
6
votes
4answers
1k 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
383 views

how to sort by the day of the week?

I have two files. file1: Dave 734.838.9800 Bob 313.123.4567 Carol 248.344.5576 Mary 313.449.1390 Ted 248.496.2204 Alice 616.556.4458 file2: Bob Tuesday Carol Monday Ted Sunday ...
4
votes
2answers
386 views

Do redirection operators always open file descriptors in parallel?

1. Consider snippet#1: $ cat test.txt > test.txt cat: test.txt: input file is output file It seems that cat makes its input file descriptor point to test.txt and then when it tries to set its ...
3
votes
3answers
7k views

Scripting `rm` and `rm -i`

I want to create a script that whenever there are more than three files being removed, it invokes rm -i by default. I know it will go in initialization files but can't seem to get it working. So far: ...
2
votes
5answers
3k 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 ...
224
votes
4answers
20k views

How does a Segmentation Fault work under-the-hood?

I can't seem to find any information on this aside from "the CPU's MMU sends a signal" and "the kernel directs it to the offending program, terminating it". I assumed that it probably sends the ...
95
votes
5answers
92k 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 ...
91
votes
6answers
8k 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 ...
57
votes
5answers
72k 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 ...
59
votes
6answers
96k 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.