A shell script is a script written for the shell, or command line interpreter, of an operating system.
25
votes
2answers
2k views
$VAR vs ${VAR} and to quote or not to quote
I can write
VAR=$VAR1
VAR=${VAR1}
VAR="$VAR1"
VAR="${VAR1}"
the end result to me all seems about the same. Why should I write one or the other? are any of these not portable/POSIX?
104
votes
9answers
8k 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 ...
40
votes
5answers
4k views
Why is it better to use “#!/usr/bin/env NAME” instead of “#!/path/to/NAME” as my shebang?
I notice that some scripts which I have acquired from others have the shebang #!/path/to/NAME while others (using the same tool, NAME) have the shebang #!/usr/bin/env NAME.
Both seem to work ...
6
votes
1answer
3k views
File descriptors & shell scripting
I am having a very hard time understanding how does one use file descriptors in shell scripts.
I know the basics such as
exec 5 > /tmp/foo
So fd 5 is attached to foo for writing.
exec 6 < ...
19
votes
8answers
8k views
Different ways to execute a shell script
There a several ways to execute a script, the ones I know are:
/path/to/script # using the path (absolute or relative)
. script # using the . (dot)
source script # using the `source` command
...
7
votes
4answers
2k views
How do I handle switches in a shell script?
Are there some built-in tools that will recognize -x and --xxxx as switches, and not arguments, or do you have to go through all the input variables, test for dashes, and then parse the arguments ...
13
votes
3answers
472 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 ...
6
votes
6answers
5k 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?
22
votes
6answers
7k views
Correct locking in shell scripts?
Sometimes you have to make sure that only one instance of a shell script is running at the same time.
For example a cron job which is executed via crond that does not provide
locking on its own (e.g. ...
16
votes
5answers
9k views
Split pages in pdf
I have a scanned pdf file which has scanned two pages on one virtual page (page in pdf file).
The resolution is with good quality. The problem is I have to zoom when reading and drag from left to the ...
7
votes
2answers
196 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 ...
5
votes
3answers
2k views
Storing output of command in shell variable
I have an operation using cut that I would like to assign result to a variable
var4=ztemp.xml |cut -f1 -d '.'
I get the error:
ztemp.xml is not a command
The value of var4 never gets ...
4
votes
4answers
1k views
Text between two tags
I wanna retrieve whatever is between these two tags – <tr> </tr> – from an html doc.
Now I don't have any specific html requirements that would warrant for an html parser. I just plain ...
14
votes
1answer
1k 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.
11
votes
4answers
601 views
keep duplicates out of $PATH on source
I have the following code that's source-d by my .shellrc
PATH="${PATH}:${HOME}/perl5/bin"
PATH="${PATH}:${HOME}/.bin"
export PATH
but if I make changes to other code and then source this file, my ...
