Questions about shell scripts, executable files that are interpreted by a shell (bash, zsh, etc.).

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10
votes
1answer
12k views

show the year while listing files in the current directory

I am working on a red hat server I want to list the files in a way where the year when each was file created would appear in the Date how is that possible ?
10
votes
2answers
2k views

Piping for loop output prevents local variable modification

I'm trying to write a simple bash function that takes, as its arguments, a number of files and/or directories. It should: Fully qualify the filenames. Sort them. Remove duplicates. Print all that ...
10
votes
3answers
959 views

What does `1>>` and `2>>` mean in a bash script?

I have the following bash script, from what I understand >> is used to append the output of a command to an existing file instead of overwrite, but what is it doing in this case? This script is ...
10
votes
4answers
9k views

What's the easiest way to find an unused local port?

What's the easiest way to find an unused local port? Currently I'm using something similar to this: port=$RANDOM quit=0 while [ "$quit" -ne 1 ]; do netstat -a | grep $port >> /dev/null if ...
10
votes
2answers
10k views

How can I remove an element from an array completely?

unset array[0] removes the element but still if I do echo ${array[0]} I get a null value moreover there are other ways of doing this but if an element of an array contains spaces like below ...
10
votes
4answers
824 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 ...
10
votes
1answer
4k views

SSH causes while loop to stop

I have finally managed to boil down a problem I have been struggling with for a few weeks. I use SSH with "authorized keys" to run commands remotely. All is fine except when I do it in a while loop. ...
10
votes
1answer
2k views

How do I make a shell script that sends output to a process

I'm currently running a server console program in a screen because I need to both read it and occasionally send commands. I'd like to run the app as a deamon in the background (start/stop it with ...
10
votes
4answers
9k views

find -exec in bash script with variable expansion

I'm trying to run a command similar to the one below in a bash script. It should search through all subfolders of $sourcedir and copy all files of a certain type to the root level of $targetdir. ...
10
votes
1answer
1k views

Suppress output unless non-zero exit code

What's the best way to suppress output (stdout and stderr) unless the program exits with a non-zero code? I'm thinking: quiet_success() { file=$(mktemp) if ! ( "$@" > "$file" 2>&1 ); ...
9
votes
4answers
8k 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 ...
9
votes
3answers
18k views

What does $# mean in shell?

What does $# mean in shell? I have code such as if [ $# -eq 0 ] then I want to understand what $# means, but Google search is very bad for searching these kinds of things.
9
votes
3answers
39k views

How to rename multiple files in single command or script in Unix? [duplicate]

I have the below list of files aro_tty-mIF-45875564pmo_opt aro_tty-mIF-45875664pmo_opt aro_tty-mIF-45875964pmo_opt aro_tty-mIF-45875514pmo_opt aro_tty-mIF-45875524pmo_opt that I need to rename to ...
9
votes
5answers
2k views

Get total duration of video files in a directory

I have a list of .ts files: out1.ts ... out749.ts out8159.ts out8818.ts How can I get the total duration (running time) of all these files?
9
votes
5answers
5k views

generate consistent machine unique ID

Can we generate a unique id for each PC, something like uuuidgen, but it will never change unless there are hardware changes? I was thinking about merging CPUID and MACADDR and hash them to generate ...
9
votes
3answers
2k views

Substituting strings in a very large file

I have a very long series of urls with no separating character, in the same format as below: http://example.comhttp://example.nethttp://example.orghttp://etc... I want each URL to be on a new line. ...
9
votes
2answers
212 views

How does the syntax “${foo##*.}” work to get file extensions? [duplicate]

Why does this command work in successfully retrieving file extension name? file_ext=${filename##*.}
9
votes
3answers
4k views

Use & (ampersand) in single line bash loop

I have been using this command successfully, which changes a variable in a config file then executes a Python script within a loop: for((i=114;i<=255;i+=1)); do echo $i > numbers.txt;python ...
9
votes
2answers
9k views

Bash scripting - loop until return value is 0

I need to umount something in my script, but sometimes it unmount's before all of the data has finished being copied and causes the umount to fail. I looked for a way to do a "blocking" umount, but I ...
9
votes
2answers
825 views

date - years prior to 1901 are treated as invalid

I have date (GNU coreutils) 7.1 installed on my system. If I try to check dates prior to 14-Dec-1901, I get an "invalid date" error. For example, $ date -d 1901-12-13 date: invalid date ...
9
votes
6answers
10k 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?
9
votes
2answers
1k views

How can I delete all files in a folder that haven't been accessed in a certain amount of time?

I'd like to run a nightly cron job that deletes all the files in a folder that haven't been accessed in a week or more. What is the most efficient way to do this in bash?
9
votes
4answers
19k views

How do I append text to the beginning and end of multiple text files in Bash?

I have a directory full of text files. My goal is to append text to the beginning and end of all of them. The text that goes at the beginning and end is the same for each file. Based on code I got ...
9
votes
7answers
970 views

How to merge text of alphabetic lines with the numeric lines in shell?

I Have a file that has text like this: AAAA BBBB CCCC DDDD 1234 5678 9012 3456 EEEE 7890 etc... And i want to match up the Alphabetic lines with the Numeric lines so they are like this: ...
9
votes
2answers
3k 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 (( ...
9
votes
5answers
15k views

Binary to hexadecimal and decimal in a shell script

I have a context where I need to convert binary to hexadecimal and decimal and viceversa in a shell script. Can someone suggest me a tool for this?
9
votes
1answer
726 views

communication between multiple processes

I have a bash script, which runs manager() function as a separate process for x-times. How is it possible to forward messages to all manager() processes from within the script? I've read about ...
9
votes
2answers
26k views

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 background a 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 that ...
9
votes
4answers
333 views

Unexpected EOF and syntax error

I am currently writing my third ever shell script and I have run into a problem. This is my script so far: #!/bin/bash echo "choose one of the following options : \ 1) display all current users \ ...
9
votes
5answers
25k views

Add lines to the beginning and end of the huge file

I have the scenario where lines to be added on begining and end of the huge files. I have tried as shown below. for the first line: sed -i '1i\'"$FirstLine" $Filename for the last line: sed -i ...
9
votes
2answers
1k views

Are quotes needed for local variable assignment?

Can I safely omit quotes on the right side of a local assignment? function foo { local myvar=${bar} stuff() } I'm mainly interested in bash, but any info on corner cases in other shells are ...
9
votes
1answer
2k views

Are dash, ash and sh script 100% compatible?

I wanted to know whether scripts written for dash, ash, and sh are 100% compatible? Are there any added features to dash or ash, or syntax change? From what I heard, ash is a direct descendant of ...
9
votes
4answers
182 views

Script to extract selected entries from a bibtex file

I have a large bibtex file with many entries where each entry has the general structure @ARTICLE{AuthorYear, item = {...}, item = {...}, item = {...}, etc } (in some cases ARTICLE might be a ...
9
votes
2answers
405 views

Using a shell script as a virtual printer

I have a shell script that takes a PDF file as an argument and prints it with additional printer specific options. How can I add this script as a virtual printer that can be selected from the GUI? ...
9
votes
2answers
535 views

Cache all gpg subkey passwords at once? Prevent need for multiple gpg password entry?

Can I enter my gpg password just once and unlock all my sub keys (signing, decryption, authentication)? At the moment, I need to enter my gpg password three times (for signing, for decryption, for ...
8
votes
8answers
2k views

Is there a unix command that gives the maximum of two numbers?

I was looking for a command to limit numbers read in from stdin. I wrote a little script for that purpose (critique is welcome), but I was wondering if there was not a standard command for this, ...
8
votes
8answers
826 views

Grep for pattern at start or middle of a line

I'll start by saying that I think this problem is a little less innocent than it sounds. What I need to do: check for a folder within the PATH environment variable. It could be at the start or ...
8
votes
10answers
6k views

Decoding URL encoding (percent encoding)

I want to decode URL encoding, is there any built-in tool for doing this or could anyone provide me with a sed code that will do this? I did search a bit through unix.stackexchange.com and on the ...
8
votes
7answers
3k views

cron Vs. sleep - which is the better one in terms of efficient cpu/memory utilization?

The Case: I need to run some commands/script at certain intervals of time and for this I have two options - 1. set up a cron-job or, 2. implement a loop with sleep in the script itself. Question: ...
8
votes
4answers
541 views

How can measure how long it takes my script to run AND include that in an email it generates?

I have a simple bash script that runs a series of checks (ping, nslookup, etc) and then sends an email report with the output of that data. I'd like the email to include information on how long it ...
8
votes
5answers
3k views

Bash CD up until in certain folder

I do a lot of work in Magento, and so do a lot of the people I work with, and it gets annoying to have to type: cd ../../../../../../ To only find you're still a few directories from your root ...
8
votes
4answers
4k views

How to run parallel processes and combine outputs when both finished

I have a bash shell script in which I pipe some data through about 5 or 6 different programs then the final results into a tab delimited file. I then do the same again for a separate similar dataset ...
8
votes
4answers
3k views

How to prepend a license header recursively for all .h and .cpp files in a directory

I'm trying to add a license header to all header files and source files in a project directory using a for loop. This is not working, is there any other approach using sed?
8
votes
2answers
2k views

Do parentheses really put the command in a subshell?

From what I've read, putting a command in parentheses should run it in a subshell, similar to running a script. If this is true, how does it see the variable x if x isn't exported? x=1 Running ...
8
votes
2answers
11k views

Extracting a string, according to a pattern, in a bash script

In bash, suppose that I have a string strname: strname="ph7go04325r" I would like to extract the characters between the first "3" character and the last "r" character in strname, saving the result ...
8
votes
3answers
34k views

How can I concatenate a shell variable to other other parameters in my command lines?

How can I concatenate a shell variable to other other parameters in my command lines ? For example, #!/bin/sh WEBSITE="danydiop" /usr/bin/mysqldump --opt -u root --ppassword $WEBSITE > ...
8
votes
2answers
861 views

How to non-invasively test for write access to a file?

In a shell script, how do I easily and non-invasively test for write access to a file without actually attempting to modify the file? I could parse the output of stat, but that seems really complex, ...
8
votes
2answers
924 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 ...
8
votes
2answers
9k views

Remove end of line characters from stdout? Multiple lines into a single line

I have a script which outputs the following text. This is the output from a Netopia 2210-02 ADSL2 modem. ADSL Line State: Up ADSL Startup Attempts: 1 ADSL Modulation: DMT ADSL Data ...
8
votes
1answer
740 views

How to append multiple lines to a file with bash, with “--” in front of string

Question very similar to How to append multiple lines to a file with bash but I want to start the file with --, and also append to the file, if possible. printf ...