Questions tagged [bash]

Questions specific to GNU’s Bourne Again SHell, as opposed to other Bourne/POSIX shells. For questions about Unix shells in general, use the /shell tag instead. For shell scripts with errors/syntax errors, please check them with the shellcheck program (or in the web shellcheck server at before posting here.

Filter by
Sorted by
Tagged with
1319 votes
12 answers

How to correctly add a path to PATH?

I'm wondering where a new path has to be added to the PATH environment variable. I know this can be accomplished by editing .bashrc (for example), but it's not clear how to do this. This way: export ...
  • 16.7k
765 votes
4 answers

How to cycle through reverse-i-search in BASH?

In the terminal, I can type Ctrl + R to search for a matching command previously typed in BASH. E.g., if I type Ctrl + R then grep, it lists my last grep command, and I can hit enter to use it. This ...
  • 4,705
695 votes
24 answers

Preserve bash history in multiple terminal windows

I consistently have more than one terminal open. Anywhere from two to ten, doing various bits and bobs. Now let's say I restart and open up another set of terminals. Some remember certain things, some ...
  • 15.6k
661 votes
19 answers

How to get execution time of a script effectively?

I would like to display the completion time of a script. What I currently do is - #!/bin/bash date ## echo the date at start # the script contents date ## echo the date at end This just show's ...
  • 26.1k
641 votes
3 answers

Using "${a:-b}" for variable assignment in scripts

I have been looking at a few scripts other people wrote (specifically Red Hat), and a lot of their variables are assigned using the following notation VARIABLE1="${VARIABLE1:-some_val}" or some expand ...
567 votes
27 answers

How can I resolve a hostname to an IP address in a Bash script?

What's the most concise way to resolve a hostname to an IP address in a Bash script? I'm using Arch Linux.
525 votes
7 answers

What is the difference between the Bash operators [[ vs [ vs ( vs ((?

I am a little bit confused on what do these operators do differently when used in bash (brackets, double brackets, parenthesis and double parenthesis). [[ , [ , ( , (( I have seen people use them on ...
  • 5,399
523 votes
10 answers

How to conditionally do something if a command succeeded or failed

How can I do something like this in bash? if "`command` returns any error"; then echo "Returned an error" else echo "Proceed..." fi
user avatar
501 votes
4 answers

In a bash script, using the conditional "or" in an "if" statement

This question is a sequel of sorts to my earlier question. The users on this site kindly helped me determine how to write a bash for loop that iterates over string values. For example, suppose that ...
  • 15.8k
486 votes
14 answers

How do I loop through only directories in bash?

I have a folder with some directories and some files (some are hidden, beginning with dot). for d in *; do echo $d done will loop through all files and directories, but I want to loop only through ...
  • 26.9k
426 votes
16 answers

In Bash, when to alias, when to script, and when to write a function?

It's taken me almost 10 years of Linux usage to ask this question. It was all trial and error and random late-night internet surfing. But people shouldn't need 10 years for this. If I were just ...
  • 13.3k
395 votes
17 answers

How do I change the extension of multiple files?

I would like to change a file extension from *.txt to *.text. I tried using the basename command, but I'm having trouble on changing more than one file. Here's my code: files=`ls -1 *.txt` for x in ...
  • 4,279
395 votes
13 answers

How can I get the size of a file in a bash script?

How can I get the size of a file in a bash script? How do I assign this to a bash variable so I can use it later?
  • 4,111
376 votes
12 answers

How to view the output of a running process in another bash session?

I have left a script running on a remote machine from when I was locally working at it. I can connect over SSH to the machine as the same user and see the script running in ps. $ ps aux | grep ...
  • 7,212
370 votes
6 answers

Why does my shell script choke on whitespace or other special characters?

… or an introductory guide to robust filename handling and other string passing in shell scripts. I wrote a shell script which works well most of the time. But it chokes on some inputs (e.g. on some ...
362 votes
13 answers

How to remove a single line from history?

I'm working in Mac OSX, so I guess I'm using bash...? Sometimes I enter something that I don't want to be remembered in the history. How do I remove it?
  • 7,519
352 votes
4 answers

How do I clear Bash's cache of paths to executables?

When I execute a program without specifying the full path to the executable, and Bash must search the directories in $PATH to find the binary, it seems that Bash remembers the path in some sort of ...
348 votes
6 answers

How to define 'tab' delimiter with 'cut' in Bash?

Here is an example of using cut to break input into fields using a space delimiter, and obtaining the second field: cut -f2 -d' ' How can the delimiter be defined as a tab, instead of a space?
343 votes
4 answers

What does "rc" in .bashrc stand for?

Is it "resource configuration", by any chance?
  • 34.1k
334 votes
4 answers

How to get the pid of the last executed command in shell script?

I want to have a shell script like this: my-app & echo $my-app-pid But I do not know how the get the pid of the just executed command. I know I can just use the jobs -p my-app command to grep ...
  • 5,469
328 votes
13 answers

How to colorize output of git?

Is there a way to color output for git (or any command)? Consider: baller@Laptop:~/rails/spunky-monkey$ git status # On branch new-message-types # Changes not staged for commit: # (use "git add &...
  • 7,519
315 votes
3 answers

How can I make "Press any key to continue" [duplicate]

I'm making a script to install my theme, after it finished installing it will appear the changelog and there will be "Press any key to continue" so that after users read the changelog then press any ...
314 votes
12 answers

How can I test if a variable is empty or contains only spaces?

The following bash syntax verifies if param isn't empty: [[ ! -z $param ]] For example: param="" [[ ! -z $param ]] && echo "I am not zero" No output and its fine. But when param is ...
  • 6,883
313 votes
5 answers

Why doesn't my Bash script recognize aliases?

In my ~/.bashrc file reside two definitions: commandA, which is an alias to a longer path commandB, which is an alias to a Bash script I want to process the same file with these two commands, so I ...
  • 10.2k
312 votes
5 answers

How can I get the current working directory? [duplicate]

I want to have a script that takes the current working directory to a variable. The section that needs the directory is like this dir = pwd. It just prints pwd how do I get the current working ...
user avatar
308 votes
25 answers

Colorizing your terminal and shell environment?

I spend most of my time working in Unix environments and using terminal emulators. I try to use color on the command line, because color makes the output more useful and intuitive. What options exist ...
303 votes
10 answers

What is the "eval" command in bash?

What can you do with the eval command? Why is it useful? Is it some kind of a built-in function in bash? There is no man page for it..
  • 38.4k
286 votes
4 answers

What is the meaning of IFS=$'\n' in bash scripting?

At the beginning of a bash shell script is the following line: IFS=$'\n' What is the meaning behind this collection of symbols?
267 votes
19 answers

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 of those messages?
  • 14.4k
267 votes
3 answers

What is the purpose of .bashrc and how does it work?

I found the .bashrc file and I want to know the purpose/function of it. Also how and when is it used?
  • 23.4k
266 votes
4 answers

Security implications of forgetting to quote a variable in bash/POSIX shells

If you've been following for a while, you should hopefully know by now that leaving a variable unquoted in list context (as in echo $var) in Bourne/POSIX shells (zsh being the ...
264 votes
12 answers

Execute a command once per line of piped input?

I want to run a java command once for every match of ls | grep pattern -. In this case, I think I could do find pattern -exec java MyProg '{}' \; but I'm curious about the general case - is there an ...
  • 3,553
263 votes
9 answers

How to determine where an environment variable came from?

I have a Linux instance that I set up some time ago. When I fire it up and log in as root there are some environment variables that I set up but I can't remember or find where they came from. I've ...
  • 2,733
254 votes
14 answers

Passing named arguments to shell scripts

Is there any easy way to pass (receive) named parameters to a shell script? For example, my_script -p_out '/some/path' -arg_1 '5' And inside receive them as: # I believe this ...
252 votes
12 answers

Is there a one-liner that allows me to create a directory and move into it at the same time?

I find myself repeating a lot of: mkdir longtitleproject cd longtitleproject Is there a way of doing it in one line without repeating the directory name? I'm on bash here.
249 votes
5 answers

What does env x='() { :;}; command' bash do and why is it insecure?

There is apparently a vulnerability (CVE-2014-6271) in bash: Bash specially crafted environment variables code injection attack I am trying to figure out what is happening, but I'm not entirely sure ...
  • 13.5k
247 votes
6 answers

How can I delete a word backward at the command line (bash and zsh)?

How can I delete a word backward at the command line? I'm truly used to some editors deleting the last 'word' using Ctrl+Backspace, and I'd like that functionality at the command line too. I am ...
  • 2,719
239 votes
10 answers

Terminal prompt not wrapping correctly

I have an issue where if I type in very long commands in bash the terminal will not render what I'm typing correctly. I'd expect that if I had a command like the following: username@someserver ~/...
  • 2,500
237 votes
4 answers

Combined `mkdir` and `cd`? [duplicate]

is there any way (what is the easiest way in bash) to combine the following: mkdir foo cd foo The manpage for mkdir does not describe anything like that, maybe there is a fancy version of mkdir? I ...
  • 3,498
236 votes
2 answers

There are stopped jobs (on bash exit)

I get the message There are stopped jobs. when I try to exit a bash shell sometimes. Here is a reproducible scenario in python 2.x: ctrl+c is handled by the interpreter as an exception. ctrl+z 'stops'...
  • 4,144
218 votes
9 answers

Looping through files with spaces in the names? [duplicate]

I wrote the following script to diff the outputs of two directores with all the same files in them as such: #!/bin/bash for file in `find . -name "*.csv"` do echo "file = $file"; diff $...
  • 6,953
218 votes
3 answers

How do I remove a directory and all its contents?

In bash all I know is that rmdir directoryname will remove the directory but only if it's empty. Is there a way to force remove subdirectories?
  • 2,475
216 votes
4 answers

What does <<< mean? [duplicate]

What does <<< mean? Here is an example: $ sed 's/a/b/g' <<< "aaa" bbb Is it something general that works with more Linux commands? It looks like it's feeding the sed program with ...
215 votes
8 answers

How to debug a bash script?

I'm having some problems with some scripts in bash, about errors and unexpected behaviors. I would like to investigate the causes of the problems so I can apply fixes. Is there a way I can turn some ...
  • 35k
213 votes
3 answers

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 ...
  • 5,011
212 votes
6 answers

remove particular characters from a variable using bash

I want to parse a variable (in my case it's development kit version) to make it dot(.) free. If version='2.3.3', desired output is 233. I tried as below, but it requires . to be replaced with another ...
  • 4,678
209 votes
6 answers

Can I redirect output to a log file and background a process at the same time?

Can I redirect output to a log file and a background process at the same time? In other words, can I do something like this? nohup java -jar myProgram.jar 2>&1 > output.log & Or, is ...
  • 3,830

2 3 4 5