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

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13
votes
4answers
832 views

How do I run a command and get its exit code in fewer characters?

I used to write my scripts like this: some_command while [ $? -ne 0 ] ; do sleep 1 some_command done It always irked me that I have to write some_command twice. Can I put it inside the loop ...
13
votes
7answers
18k views

How to compare to floating point number in a shell script

I want to compare two floating point numbers in a shell script. The following code is not working: #!/bin/bash min=12.45 val=10.35 if (( $val < $min )) ; then min=$val fi echo $min
13
votes
4answers
8k views

How to define and load your own shell function in zsh

I am having a hard time defining and running my own shell functions in zsh. I followed the instructions on the official documentation and tried with easy example first, but I failed to get it work. ...
13
votes
5answers
8k views

How do I write a retry logic in script to keep retrying to run it upto 5 times?

I want to write logic in shell script which will retry it to run again after 15 sec upto 5 times based on "status code=FAIL" if it fails due to some issue.
13
votes
4answers
18k 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?
13
votes
3answers
40k views

linux + add X days to date and get new virtual date

I have Linux ( RH 5.3) machine I need to add/calculate 10 days plus date so then I will get new date (expiration date)) for example # date Sun Sep 11 07:59:16 IST 2012 So I need to get ...
13
votes
3answers
3k views

Can't indent heredoc to match nesting's indent

If there's a "First World Problems" for scripting, this would be it. I have the following code in a script I'm updating: if [ $diffLines -eq 1 ]; then dateLastChanged=$(stat --format '%y' ...
13
votes
4answers
6k 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 ...
13
votes
2answers
518 views

When running a shell script, is it possible to pass specific positional parameters without having to enter them all in order?

For example, if I have script ./foo that takes 10 parameters, and I only want to pass the 8th parameter. The only way I know how to do this currently is: ./foo '' '' '' '' '' '' '' 'bar' Is there ...
13
votes
9answers
41k views

How to compare two dates in a shell

How to compare two dates in a shell script? For example something like todate=2013-07-18 cond=2013-07-15 if [ $todate -ge $cond ]; then break fi It doesn't work. ...
13
votes
7answers
4k views

Test if multiple variables are set

I'd like to make sure that at a certain point of a script, after sourceing a configuration file, several variables are set and, if they are not, to stop execution, telling the user about the missing ...
13
votes
1answer
1k views

How does a fork bomb work?

WARNING DO NOT ATTEMPT TO RUN THIS ON A PRODUCTION MACHINE In reading the Wikipedia page on the topic I generally follow what's going on with the following code: :(){ :|:& };: excerpt of ...
13
votes
4answers
23k views

printing colored text using echo

I know that for printing a colored text using echo, for example red color, the code is: echo -e "\e[1;31m This is red text \e[0m" and I know that in this example, 31 is code of red color and the ...
13
votes
3answers
497 views

When does `ls -s` print “0”

Of course, the standard way of testing if a file is empty is with test -s FILE, but one of our clients have received a script containing tests like this: RETVAL=`ls -s ./log/cr_trig.log | awk '{print ...
12
votes
4answers
4k views

Bash syntax error when “else” follows an empty “then” clause

Why would following script not execute, but give a syntax error of else: LOGS3_DIR=~/logs if [ -d "$LOGS3_DIR" ]; then cd cd "$LOGS3_DIR" echo "$LOGS3_DIR" for filename in `find "." -mtime 1 ...
12
votes
7answers
6k views

bash: test if $WORD is in set

I am looking for a construct in bash, to decide if a variable $WORD is one of defined words. I need something like this: if "$WORD" in dog cat horse ; then echo yes else echo no fi does ...
12
votes
9answers
35k views

Show sum of file sizes in directory listing

The Windows dir directory listing command has a line at the end showing the total amount of space taken up by the files listed. For example, dir *.exe shows all the .exe files in the current ...
12
votes
4answers
949 views

What is the meaning of IFS=$'\n' in bash scripting ?

at the beginning of a shell script program in the bash is the following line: IFS=$'\n' What is the meaning behind this collection of symbols ?
12
votes
3answers
11k 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 ...
12
votes
4answers
23k views

How to read from two input files using while loop

I wanted to know if there is any way of reading from two input files in a nested while loop one line at a time. For example, lets say I have two files FileA and FileB. FileA: [jaypal:~/Temp] cat ...
12
votes
8answers
8k 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 ...
12
votes
4answers
1k views

Recursively create directories for all letters

I want to create a directory in such a way that I need to label the directories from a to z. Inside each of these directories, I need to create sub-directories so that they are labelled as aa, ab etc. ...
12
votes
5answers
23k views

How to delete a file on remote machine via SSH by using a Shell Script?

I am writing a Shell Script where I have to delete a file on a remote machine via a Shell Script. Flow manually: ssh username@domain.com .. then at domain: cd ./some/where rm some_file.war How ...
12
votes
5answers
10k views

how do I trim each line in bash?

I would like to remove all leading and trailing spaces and tabs from each line in an output. Is there a simple tool like trim I could pipe my output into?
12
votes
2answers
852 views

When to use a semi-colon between environment variables and a command

Can anyone explain why the semi-colon is necessary in order for the LANG to be seen as updated by bash? Doesn't work: > LANG=Ja_JP bash -c "echo $LANG" en_US Works: > LANG=Ja_JP ; bash -c ...
12
votes
2answers
2k 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 ...
12
votes
3answers
595 views

How do I wait on a program started in another shell

I have a program that does a large a amount of work (takes about 4-5 hours) that gets started by cron when all the data it works with becomes available. Sometimes, when I am waiting for it to finish, ...
12
votes
3answers
509 views

When to use redirection to stderr in shell scripts

I know that well-behaved utilities like grep output "normal" messages to stdout, and error messages to stderr. $ grep '^foo' file1 file2 file1:foo grep: file2: No such file or directory When I'm ...
12
votes
0answers
356 views

Should I use $* or $@? [duplicate]

Which one of $* or $@ is better to use (in array syntax,command line parameters, etc.) to avoid any bugs/problems in code? Or it does not make any difference?
11
votes
2answers
2k views

find and remove duplicates in a directory

I have a directory with multiple img files and some of them are identical but they all have different names. I need to remove duplicates but with no external tools only with a bash script. I'm a ...
11
votes
2answers
6k views

Bash: run command2 if command1 fails

I want to do something like this: if cmd1 && cmd2 echo success else echo epic fail fi How should I do it?
11
votes
3answers
6k 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?
11
votes
5answers
597 views

Convert underscore to PascalCase, ie UpperCamelCase

If I have a string that looks like this: "this_is_the_string" Inside bash script, I would like to convert it to PascalCase, ie UpperCamelCase to look like this: "ThisIsTheString" I found that ...
11
votes
5answers
7k views

How can I use environment variables in my shebang?

I have a Python script that need to be run with a particular python installation. Is there a way to craft a shebang so that it runs with $FOO/bar/MyCustomPython?
11
votes
7answers
2k views

Shortest way to extract last 3 characters of base (minus suffix) filename

I'm trying to set a variable in an sh script to the last 3 characters of the base name of a file (by base name I mean without the path and without the suffix). I've succeeded in doing this but, purely ...
11
votes
6answers
49k views

How can I monitor all outgoing requests/connections from my machine?

My machine is a server so I want to ignore connections being made to my server (e.g. when someone visits my website). I want to see only connections/requests being made by my server to other places. ...
11
votes
1answer
943 views

How to generate a lot of (blank) images files?

For a test, I need to quickly create 1700 jpg-images (even blank) in a defined size 1000x706. I wonder if we could do this with a shell script or a program (like ImageMagick). I wish they called ...
11
votes
3answers
5k views

How can I receive top-like CPU statistics from the shell?

I'm trying to get an accurate read of my used CPU (in percent) from top. This is the command I'm running for testing: top -n1 | awk '/Cpu\(s\):/ {print $2}' This returns: 10.7%us, Which is the ...
11
votes
1answer
11k 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 < ...
11
votes
2answers
8k views

Regular expression in bash script

This is my first time bash scripting so I'm probably making an easy mistake. Basically, I'm trying to write a script that gets the groups of a user, and if they are in a certain group, it will log ...
11
votes
3answers
1k views

Are free shell accounts safe?

I am trying to identify how safe the free shell accounts provided by the likes of shellmix.com are. I have never used one myself but want to suggest it to someone interested in learning shell ...
11
votes
2answers
253 views

How to start tailing a file that has not been yet created

I use tail to monitor the progress of jobs that I know will write their progress to disk. Almost always, I know which file they will create before they start running (the jobs are dispatched by a ...
10
votes
3answers
6k views

How to sort the string which combined with string + numeric using bash script?

This is the data what I want to sort. But sort treats the numeric to string, the data it no sorted as I expected. /home/files/profile1 /home/files/profile10 /home/files/profile11 ...
10
votes
2answers
11k views

Fastest way to concatenate files

I've got 10k+ files totaling over 20GB that I need to concatenate into one file. Is there a faster way than cat input_file* >> out ? The preferred way would be a bash command, Python is ...
10
votes
1answer
807 views

How do you omit “./” when running scripts on current directory

On some tutorials over the internet they can run a script on the current directory without doing: ./script How do you omit this? I am using CentOS 5.6 x64. I want to run it this way script
10
votes
7answers
4k views

A bash function that takes argument like other languages?

I have a bash function to set the $PATH like this -- assign-path() { str=$1 # if the $PATH is empty, assign it directly. if [ -z $PATH ]; then PATH=$str; # if the $PATH does ...
10
votes
3answers
5k 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 ...
10
votes
4answers
30k views

Using the not equal operator for string comparison

if [ "$PHONE_TYPE" != "NORTEL" ] || [ "$PHONE_TYPE" != "NEC" ] || [ "$PHONE_TYPE" != "CISCO" ] then echo "Phone type must be nortel,cisco or nec" exit fi The above code did not work for me, so I ...
10
votes
6answers
1k views

How to extract a key value pair from ps command

I am trying to extract application name from ps command: ps -af -u sas |grep "java"| grep -v "grep" above command produces the following: sas 24431 1 0 Oct10 ? 00:51:08 ...
10
votes
4answers
13k 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 ...