Questions about bash only, as opposed to other Bourne/POSIX shells. For all Bourne/POSIX-style shells, use the /shell tag instead.

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236
votes
6answers
396k views

How to correctly add a path to PATH?

I'm wondering where a new path has to be added to PATH environment variable. I know this is accomplished editing .bash_rc (for example), but it's not clear how to do this. This way: export ...
203
votes
5answers
83k views

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 ...
198
votes
10answers
23k views

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 ...
193
votes
13answers
56k views

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 ...
140
votes
16answers
84k views

How to do integer & float calculations, in bash or other languages/frameworks?

Using echo "20+5" literally produces the text "20+5" What command can I use to get the numeric sum, e.g. 25 in this case. Also, what's the easiest way to do it just using bash for floating point, ...
115
votes
3answers
20k views

What does “rc” in .bashrc stand for?

Is it "resource configuration", by any chance?
109
votes
1answer
8k views

Bash: What does “>|” do?

I have just seen this written down; $ some-command >| /tmp/output.txt Vertical pipes are used in standard redirects "piping" the output of one command to another, is >| in fact completely ...
103
votes
15answers
143k views

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 ...
102
votes
14answers
140k views

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.
100
votes
2answers
17k views

When was the shellshock (CVE-2014-6271/7169) bug introduced, and what is the patch that fully fixes it?

Some context about the bug: CVE-2014-6271 Bash supports exporting not just shell variables, but also shell functions to other bash instances, via the process environment to (indirect) child ...
97
votes
4answers
22k views

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 ...
87
votes
19answers
75k views

How can I get my external IP address in bash?

I need to find my external IP address from a bash script. At the moment I use this function: myip () { lwp-request -o text checkip.dyndns.org | awk '{ print $NF }' } But it depends on ...
80
votes
4answers
2k views

Can I somehow add a “&& prog2” to an already running prog1?

Most shells provide functions like && and ; to chain the execution of commands in certain ways. But what if a command is already running, can I still somehow add another command to be executed ...
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 ...
71
votes
1answer
5k views

Have backticks (i.e. `cmd`) in *sh shells been deprecated?

I've seen this comment many times on Unix & Linux as well as on other sites that use the phrasing "backticks have been deprecated", with respect to shells such as Bash & Zsh. Is this ...
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?
68
votes
4answers
68k views

Why do I need a tty to run sudo if I can sudo without a password?

I have configured sudo to run without a password, but when I try to ssh 'sudo Foo", I still get sudo: sorry, you must have a tty to run sudo. Why does this happen and how can I work around it?
67
votes
4answers
28k views

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 ...
65
votes
12answers
27k views

How to grep a specific line _and_ the first line of a file?

Assuming a simple grep such as: $ psa aux | grep someApp 1000 11634 51.2 0.1 32824 9112 pts/1 SN+ 13:24 7:49 someApp This provides much information, but as the first line of the ps ...
63
votes
9answers
3k views

Remember a half-typed command while I check something

I often find myself in the following position: I've started typing a long command at the bash prompt, but half-way through I find out I need to check something with another command. This is a problem ...
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
60
votes
3answers
41k views

how can shellshock be exploited over SSH?

Apparently, the shellshock Bash exploit CVE-2014-6271 can be exploited over the network via SSH. I can imagine how the exploit would work via Apache/CGI, but I cannot imagine how that would work over ...
59
votes
6answers
42k views

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 ...
58
votes
2answers
9k views

Why does bash think: 016 + 1 = 15?

Can somebody explain to me why a number with a leading 0 gives this funny behaviour? #!/bin/bash NUM=016 SUM=$((NUM + 1)) echo "$NUM + 1 = $SUM" Will print: 016 + 1 = 15
54
votes
2answers
2k views

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

If you've been following unix.stackexchange.com 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 ...
53
votes
2answers
51k views

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 ...
49
votes
6answers
5k views

Is there a way to cd back multiple times in bash? [duplicate]

I often use cd - to go back to where I was. How can I do this multiple times in bash? Or would zsh or some other tool support this?
49
votes
5answers
2k views

cd by just typing the directory's name?

I either had this somewhere 20 years ago or I dreamed about it. Basically: If if type blobblob I get blobblob: command not found Fair enough. I would like it so that when my shell gets those ...
48
votes
4answers
6k views

Alternative to .bashrc

Is ~/.bashrc the only place where I can specify user specific ENV Variables, aliases, modifications to PATH variable, etc? I ask because it seems that ~/.bashrc would be BASH only...
47
votes
9answers
65k views

Looping through files with spaces in the names?

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 ...
47
votes
3answers
17k views

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 ...
44
votes
3answers
23k views

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

In the terminal, I can type CRTL+R to search for a matching command previously typed in BASH. E.g., if I type CTRL+R then grep is lists my last grep command, and I can hit enter to use it. This only ...
44
votes
2answers
24k views

Awesome symbols and characters in a bash prompt

I just ran across a screenshot of someone's terminal: Is there a list of all of the characters which can be used in a Bash prompt, or can someone get me the character for the star and the right ...
44
votes
6answers
5k views
+200

Can I configure my shell to print STDERR and STDOUT in different colors?

I want to set my terminal up so stderr is printed in a different color than stdout; maybe red. This would make it easier to tell the two apart. Is there a way to configure this in .bashrc? If not, is ...
43
votes
4answers
4k views

What features are in zsh and missing from bash, or vice versa?

As a Linux user, I've always just used bash because it was the default on every distro I used. People using other Unix systems such as BSD seem to use other shells far more frequently. In the ...
42
votes
3answers
26k views

Multiline shell script comments - how does this work?

Recently, I stumbled upon a multiline comment type I have never seen before - here is a script example: echo a # : aaa : ddd # echo b This seems to work, hell, even vim syntax-highlights it. What ...
42
votes
2answers
3k views

Why does rm *(1)* remove all files in a directory?

I expected that: $ rm *(1)* would remove all files containing (1) in the name. I was wrong. It removed all files in the directory. Why?
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.
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 ...
38
votes
4answers
5k views

Is there any way to keep a command from being added to your history?

Is there any way to keep a command from being added to your history? I have a command that I want to keep out of my history file, and I really don't care to have it there when I search the history ...
38
votes
1answer
10k views

What is $debian_chroot in .bashrc?

What is the debian_chroot variable in my bashrc file? and what is it doing here? PS1='${debian_chroot:+($debian_chroot)}\u@\h:\w\$ '
37
votes
4answers
49k views

How to cut by tab character

I want to use cut command to break input into fields and get the second field. cut -f2 -d' ' <--- How can I tell it to use tab instead of space
37
votes
7answers
59k views

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 ...
37
votes
4answers
106k views

How can i count the number of lines of file with common tools in a bash?

I am redirecting grepping-results to a file, and then use cat to show its content on the screen. I want to know how many lines of results I have in my results file and then += it to some counter. ...
37
votes
10answers
62k views

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, but I want to loop only through directories. ...
37
votes
4answers
12k views

What is the purpose of the hash command?

If you run hash it shows the path of all commands run since the hash was last reset (hash -r) [root@c04c ~]# hash hash: hash table empty [root@c04c ~]# whoami root [root@c04c ~]# hash hits ...
36
votes
7answers
5k views

why does ls -d also list files, and where is it documented?

when specifying ls --directory a* it should list only directories starting with a* BUT it lists files AND directories starting with a Questions: where might I find some documentation on this, ...
36
votes
4answers
101k views

What is the “eval” command in bash?

What can a man do with the "eval" command, why is it usefull? It's some kind of built-in function in bash? There is no "man" page for it..
36
votes
3answers
10k views

using single or double bracket - bash

I'm confused with using single or double bracket. look at code: dir="/home/mazimi/VirtualBox VMs" if [[ -d ${dir} ]]; then echo "yep" fi It works perfectly although the string contains space. ...
36
votes
4answers
4k views

unix, difference between path starting with '/' and '//'

In unix/linux, any number of consecutive forwardslashes in a path is generally equivalent to a single forwardslash. eg. $ cd /home/shum $ pwd /home/shum $ cd /home//shum $ pwd /home/shum $ cd ...