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.

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296
votes
6answers
249k 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 ...
213
votes
3answers
21k 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 ...
126
votes
1answer
47k views

When is double-quoting necessary?

The old advice used to be to double-quote any expression involving a $VARIABLE, at least if one wanted it to be interpreted by the shell as one single item, otherwise, any spaces in the content of $...
113
votes
4answers
39k views

Is there a “.bashrc” equivalent file read by all shells?

Is ~/.bashrc the only place to specify user specific environment variables, aliases, modifications to PATH variable, etc? I ask because it seems that ~/.bashrc seems to be bash-only, but other ...
17
votes
3answers
6k views

Brackets in if condition: why am I getting syntax errors without whitespace?

I am using the below script to move two days back when script runs at starting two days of the year and also check first and second days of every month and move two days back. if [$month="01"] &&...
49
votes
5answers
48k views

How can we run a command stored in a variable?

$ ls -l /tmp/test/my\ dir/ total 0 I was wondering why the following ways to run the above command fail or succeed? $ abc='ls -l "/tmp/test/my dir"' $ $abc ls: cannot access '"/tmp/test/my': No ...
370
votes
14answers
64k 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 ...
80
votes
2answers
283k views

How can I assign the output of a command to a shell variable?

I want to assign the result of an expression to a variable and concatenate it with a string, then echo it. Here's what I've got: #!/bin/bash cd ~/Desktop; thefile= ls -t -U | grep -m 1 "Screen Shot"; ...
988
votes
11answers
2.4m views

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 ...
71
votes
4answers
72k views

Understanding “IFS= read -r line”

I obviously understand that one can add value to internal field separator variable. For example: $ IFS=blah $ echo "$IFS" blah $ I also understand that read -r line will save data from stdin to ...
156
votes
14answers
166k views

Batch renaming files

I have a directory full of images: image0001.png image0002.png image0003.png ... And I would like a one-liner to rename them to (say). 0001.png 0002.png 0003.png ... How do I do this?
90
votes
4answers
37k views

Why does parameter expansion with spaces without quotes work inside double brackets “[[” but not inside single brackets “[”?

I'm confused with using single or double brackets. Look at this code: dir="/home/mazimi/VirtualBox VMs" if [[ -d ${dir} ]]; then echo "yep" fi It works perfectly although the string contains a ...
37
votes
4answers
14k views

How can I use $variable in a shell brace expansion of a sequence?

I want to use $var in a shell brace expansion with a range, in bash. Simply putting {$var1..$var2} doesn't work, so I went "lateral"... The following works, but it's a bit kludgey. # remove the ...
80
votes
4answers
18k views

What is the difference between sourcing ('.' or 'source') and executing a file in bash?

What's the difference between executing a script like this: ./test.sh and executing a script like this: . test.sh? I tried a simple, two-line script to see if I could find if there was a ...
64
votes
4answers
88k views

How can I make environment variables “exported” in a shell script stick around?

I have multiple Amazon EC2 accounts and want to quickly be able to switch variables, such as $EC2_HOME, using a script. I have have a shell script set up like this: #!/bin/sh export EC2_HOME=/home/...
559
votes
20answers
190k 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 ...
36
votes
1answer
11k views

How do ${0##*/} and ${0%/*} work?

I'm quite confused about the following regular expressions I found in a shell script: ${0##*/} ${0%/*} How do they work?
126
votes
2answers
22k 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 ...
158
votes
10answers
200k views

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 $...
17
votes
3answers
7k views

Which shell interpreter runs a script with no shebang?

Suppose the default shell for my account is zsh but I opened the terminal and fired up bash and executed a script named prac002.sh, which shell interpreter would be used to execute the script, zsh or ...
120
votes
36answers
60k views

Quick directory navigation in the bash shell

I would like to frequently switch between directories that are in totally unrelated paths, for example /Project/Warnest/docs/ and ~/Dropbox/Projects/ds/test/. But I don't want to type cd /[full-path]/...
54
votes
10answers
150k views

Script to change current directory (cd, pwd)

I want to run a script to simply change the current working directory: #!/bin/bash cd web/www/project But, after I run it, the current pwd remains unchanged! How can I do that?
67
votes
2answers
22k views

Why is my bash prompt getting bugged when I browse the history? [duplicate]

Often when I start looking at history of commands some of the characters from a command displayed aren't erased, for example: What's happening: prompt$ some_command prompt$ some_commanother_command ...
183
votes
8answers
101k views

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 ~/...
291
votes
6answers
113k views

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 ...
189
votes
12answers
317k views

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 ...
38
votes
3answers
6k views

Order of redirections

I don't quite understand how the computer reads this command. cat file1 file2 1> file.txt 2>&1 If I understand, 2>&1 simply redirect Standard Error to Standard Output. By that ...
79
votes
7answers
43k views

When would you use an additional file descriptor?

I know you can create a file descriptor and redirect output to it. e.g. exec 3<> /tmp/foo # open fd 3. echo a >&3 # write to it exec 3>&- # close fd 3. But you can do the same ...
23
votes
2answers
8k views

in bash, read after a pipe is not setting values

Edit : original title was "read fails in bash" With ksh I'm using read as a convenient way to separate values: $ echo 1 2 3 4 5 | read a b dump $ echo $b $a 2 1 $ But it fails in bash : $ echo 1 ...
127
votes
3answers
71k 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 ...
173
votes
9answers
117k views

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 ...
273
votes
4answers
74k 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 ...
165
votes
10answers
67k views

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.
41
votes
2answers
16k views

Execute bash scripts on entering a directory

What is the best way to execute a script when entering into a directory? When I move into a new directory I would like bash to execute the projectSettings.bash script much like RVM does.
8
votes
2answers
1k views

Difference between ' and " on command line (bash)? [duplicate]

I used to use '' and "" (single and double-quotes) interchangeably on the command line, but I recently noticed that '$HOME/some/dir' is not expanded, while "$HOME/some/dir" is. I searched around a ...
8
votes
2answers
2k views

Use a shell variable in awk

Here is my script (to find the files that contain a specified pattern): find . -type f \ -exec awk -v vawk="$1" '/'"$vawk"'/ {c++} c>0 { print ARGV[1]; exit 0 } END { if (! c) {exit 1}}' \{\} \...
72
votes
4answers
23k views

What is the difference between a builtin command and one that is not?

Is there any intrinsic difference between a builtin command and another command which can nominally do the same thing? eg. Do builtins get "special" treatement? ... is there less overhead running ...
6
votes
2answers
4k views

Can I read and write to the same file in Linux without overwriting it? [duplicate]

Possible Duplicate: How can I make iconv replace the input file with the converted output? I frequently connect to amazon ec2 using their public DNS names (ec2-12-34-56-78.compute-1.amazonaws.com)...
57
votes
4answers
16k views

unix, difference between path starting with '/' and '//' [duplicate]

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 /home///...
491
votes
3answers
349k views

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 ...
149
votes
8answers
238k views

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 ...
207
votes
7answers
532k views

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..
76
votes
8answers
220k views

#!/bin/bash - no such file or directory

I've created a bash script but when I try to execute it, I get #!/bin/bash no such file or directory I need to run the command: bash script.sh for it to work. How can I fix this?
31
votes
4answers
12k views

Quoting in ssh $host $FOO and ssh $host “sudo su user -c $FOO” type constructs

I often end up issuing complex commands over ssh; these commands involve piping to awk or perl one-lines, and as a result contain single quotes and $'s. I have neither been able to figure out a hard ...
36
votes
3answers
3k views

What is the difference between [[ $a == z* ]] and [ $a == z* ]?

Is there is any difference between these two. [[ $a == z* ]] and [ $a == z* ] Can I have an example where they would have different outputs? Furthermore, how does the working of [[ ]] differs ...
49
votes
2answers
33k views

Output from ls has newlines but displays on a single line. Why?

I think I may be overlooking a relatively fundamental point regarding shell. Output from the ls command by default separates output with newlines, but the shell displays the output on a single line. ...
27
votes
4answers
2k views

What is this shell/Bash syntax: someVariable=someValue someCommand [duplicate]

One of my coworkers has provided me with a Bash syntax that I am unfamiliar with. My Google foo has failed me on figuring out what it does and why/when I should use it. The command that he sent me ...
122
votes
2answers
20k 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 ...
100
votes
11answers
6k 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 ...
77
votes
15answers
69k 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 ...

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