A shell script is a script written for the shell, or command line interpreter, of an operating system.
23
votes
2answers
1k 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?
87
votes
9answers
5k 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 ...
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 < ...
17
votes
8answers
7k 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
...
32
votes
4answers
3k 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 ...
13
votes
3answers
438 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
4k 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?
20
votes
6answers
6k 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. ...
6
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 ...
4
votes
4answers
769 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
5answers
8k 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 ...
14
votes
1answer
985 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.
10
votes
4answers
542 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 ...
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 ...
17
votes
2answers
513 views
Should I care about unnecessary cats?
A lot of command-line utilities can take their input either from a pipe or as a filename argument. For long shell scripts, I find starting the chain off with a cat makes it more readable, especially ...
9
votes
2answers
341 views
running script with “. ” and with “source ”
I was wondering if the following two ways of running a bash script
are equivalent?
. ./myScript.sh
source myScript.sh
Are they both running the content of the script instead of running the script, ...
7
votes
2answers
1k views
Parallelizing a for loop
I want to parallelize the for loops of the following code. How to do this?
#!/bin/bash
N=$1
n=$2
for (( i=1; i<=$N; i++ )); do
min=100000000000000 //set min to some garbage value
for (( ...
2
votes
4answers
1k views
Is there a way to sum up the size of files listed?
This is the command I am using to list some files:
find . -name \*.extract.sys -size +1000000c -exec ls -lrt {} \;
-rw-r--r-- 1 qa1wrk15 test 1265190 Sep 29 01:14 ...
13
votes
5answers
726 views
What purpose does the colon builtin serve?
I've hacked on a lot of shell scripts, and sometimes the simplest things baffle me. Today I ran across a script that made extensive use of the : (colon) bash builtin.
The documenation seems simple ...
7
votes
4answers
982 views
Executing user defined function in a find -exec call
I'm on Solaris 10 and I have tested the following with ksh (88), bash (3.00) and zsh (4.2.1).
The following code doesn't yield any result:
function foo {
echo "Hello World"
}
find somedir -exec ...
3
votes
2answers
988 views
Arg list too long error while using find
I have the find command as follows:
find /jobs/logs/* -prune -name '*.log' -type f -mtime +1 |
while read LOGFILE
do
echo $LOGFILE
done
This is failing due to the following ...
6
votes
2answers
381 views
Measuring time within a script
This thread shows how to measure the time it takes to run a script. In my case, I am interested in measuring time between two points within a script. Here is an example of how I would like to use ...
4
votes
4answers
381 views
Is there any book , Tutorial on very very advanced shell scripting
I am looking for some tutorial where a simple script is written using very advanced methods so that i can learn more from
7
votes
4answers
559 views
What is the easiest way to execute text from tail at the command line?
Sometimes I'm working on a new (ubuntu) box and I type git and am alerted:
The program 'git' is currently not installed. You can install it by typing:
apt-get install git-core
If that happens I ...
4
votes
3answers
2k views
Why is my variable being localized in one 'while read' loop, but not in another seemingly similar loop
Why do I get different values for $x from the snippets below?
#!/bin/bash
x=1
echo fred>junk ; while read var ; do x=55 ; done <junk
echo x=$x
# x=55 .. I'd expect this result
x=1
cat ...
2
votes
2answers
194 views
How do I prevent a script from terminating when the shell exits?
In particular, I created a script to start Firefox which I double-click and choose 'Run in terminal,' but when the shell exits Firefox is killed immediately.
How do I prevent this from happening?
7
votes
4answers
5k views
How to introduce timeout for shell scripting?
I want to run a shell script that got a loop in it and it can go for ever which I do not want to happen. So I need to introduce a timeout for the whole script.
How can I introduce a timeout for the ...
4
votes
8answers
1k views
Can shell variable include - character?
I am not able to use - in variables in shell. Is there a way to be able to use it, because I have one script which depends on such named variables:
$export a-b=c
-bash: export: `a-b=c': not a valid ...
6
votes
3answers
1k views
How to “send” variable to sub-shell?
I run the following script:
VAR="Test"
sh -c 'echo "Hello $VAR"'
But I get :
# ./test.sh
Hello
How can I "send" the variable VAR of my script to the shell created with sh -c?
5
votes
3answers
3k views
Find files which are created a certain time after or before a particular file was created
I need a shell script which finds files which are created 1 hour before or 1 hour after a particular file (test.txt) was created.
If I go with find -newer, that means I'd have to create a temporary ...
5
votes
5answers
757 views
Unix bash script check empty gz files
I'm trying to write a bash script that will go into a directory loop through the .gz files and delete them if they are empty (ie the uncompressed file contained within in empty.
I've got a couple of ...
2
votes
3answers
468 views
Switching to superuser while shell script is running
I have a shell script that contains the following lines:
CURRENT_USER=${USER}
echo $CURRENT_USER
su
echo switch to `who am i`
Once the 2nd line is executed, the shell prompts me to enter the root ...
1
vote
1answer
10k views
How do I execute a remote shell script over SSH and be prompted for passwords by commands that require it in that script?
I want to do something like this
ssh user@remote-domain.com ./remote_script.sh
Contents of remote_script.sh
#!/bin/bash
hg pull
This example is much simpler than what I am actually doing. I know ...
0
votes
1answer
960 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 ...
0
votes
1answer
120 views
How to automatically install packages?
I want to install packages automatically by running a script
I can write yum install php, but I want it to install another package once installation of the php package is completed.
Is there any ...
0
votes
2answers
173 views
Autentification on SSH connection in OneLine [duplicate]
Possible Duplicate:
Shell Script for logging into a ssh server
I need to connect via SSH to a remote server, however I must do this with out interactive mode and with out use Public keys ...
-4
votes
1answer
58 views
How to copy datasets from prior version directory to latest version directory [closed]
I've go a number of directories named like:
/data/db/OX/8_10
/data/db/OX/9_1
/data/db/OX/9_2
And need to copy some files (all the pt.* files) from the second latest one (above 9_1) to the latest ...
10
votes
2answers
827 views
How to safely pass variables to root-enabled scripts?
This question is totally general and not only applicable to my situation, but... I have a small busybox appliance where I want a non-root user to be able to execute a particular script with root ...
24
votes
3answers
1k 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 ...
6
votes
4answers
2k views
How can I detect if the shell is controlled from SSH?
I want to detect from a shell script (more specifically .zshrc) if it is controlled through SSH. I tried the HOST variable but it's always the name of the computer which is running the shell. Can I ...
10
votes
2answers
1k views
How do I test to see if an application exists in $PATH?
I'm trying to write all of my sh startup/env scripts to work with as much DRY and as much: "works on every *nix I clone it to", as possible. This means making sure that if I try to run code that's not ...
9
votes
2answers
928 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 ...
8
votes
4answers
3k views
Bash script testing if a command has run correctly
I am working on a bash script that I would like to work for several types of VCS.
I am thinking of testing if a directory is a repo for a system by running a typical info command and checking the ...
7
votes
1answer
2k views
find n most frequent words in a file
I want to find, say, 10 most common word in a text file. Firstly, solution should be optimized for keystrokes (in other words - my time). Secondly, for the performance. Here is what I have so far to ...
6
votes
4answers
6k views
How to inject keystrokes via a shell script?
I'm trying to inject keystrokes into the input daemon so as to simulate typing from a Bash script. Is this possible, and if so, how can I make it happen?
4
votes
1answer
365 views
Shell script printing the most visited directories
Is there a way to write a bash script with the following functionalities?
Be launched when I press some key or key combination. (not so important requirement)
Identify the 7 most visited directories ...
11
votes
3answers
945 views
What does it mean to have a $“dollarsign-prefixed string” in a script?
I just saw this in an init script:
echo $"Stopping Apache"
What is that dollar-sign for?
My research so far:
I found this in the bash manual:
extquote
If set, $'string' and ...
6
votes
2answers
103 views
When is double-quoting necessary?
The old advice used to be to double-quote any expression involving a $VARIABLE, at least if one wanted it to be interpreted by the shell as one single item, otherwise, any spaces in the content of ...
4
votes
3answers
2k views
Why the double quotes and backquotes in a shell script?
I'm looking at this script mysql backup script and I don't understand what is the reason for using both backticks and double quotes around command names?
379 WHICH="`which which`"
380 AWK="`${WHICH} ...
1
vote
2answers
603 views
How to move files with specific extensions to newly created directories
I want to mv files with specific extensions to newly created directories. This script moves files with all the extentions in all the newly created folders, which is not the intent. Instead, it should ...
