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

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17
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10answers
4k views

Detect init system using the shell

This may have more to do with detecting operating systems, but I specifically need the init system currently in use on the system. Fedora 15 now uses systemd, Ubuntu uses Upstart, while others use ...
17
votes
1answer
20k views

Empty the contents of a file

I am aware of three methods to delete all entries from a file. They are >filename touch filename filename < /dev/null Of these three I abuse >filename the most as that requires the ...
17
votes
1answer
3k 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 ...
17
votes
2answers
296 views

Shell function definition: why is there a space after the opening brace?

You probably know of that fork bomb: :(){ :|:&};: #WARNING: harmful code I wonder why it is necessary, for it to parse, to include a space after the opening curly brace.
16
votes
4answers
26k views

File extensions for unix shell scripts

On wikipedia, the article for .sh says: For the .sh file extension type, see Bourne shell. How about other unix shells? I know that the shebang is used inside the file to indicate an ...
16
votes
3answers
1k views

Newlines in filenames

I understand and accept the premise that defensive1 shell scripting is both prudent and, in the longer term, more sustainable. Many of the answers to text processing questions here follow this ...
16
votes
7answers
3k views

How to efficiently generate large, uniformly distributed, random integers in bash?

I have been wondering what would be the best way to get good randomness in bash, i.e., what would be a procedure to get a random positive integer between MIN and MAX such that The range can be ...
16
votes
1answer
4k views

Can bash case statements cascade?

I'm trying to do something like this: case $level in 3) echo "Level Three" 2) echo "Level Two" 1) echo "Level one" ;; esac where if $level = 3, it ...
16
votes
3answers
26k views

Testing if a variable is empty in a shell script

I have seen the following technique used many times on many different shells, to test if a variable is empty: if [ "x$1" = "x" ]; then # Variable is empty fi Are there any advantages on using ...
15
votes
5answers
9k views

Bash script to get ASCII values for alphabet

How do I get the ASCII value of the alphabet? For example, 97 for a?
15
votes
8answers
2k views

What is the utility of the : command in shell scripting, given that it explicitly does nothing?

In the answer to this question about comments in shell scripting, it is indicated that the : is a null command that explicitly does nothing (but is not to be used for comments). What would be the ...
15
votes
6answers
2k views

Simultaneously calculate multiple digests (md5, sha256)?

Under the assumption that disk I/O and free RAM is a bottleneck (while CPU time is not the limitation), does a tool exist that can calculate multiple message digests at once? I am particularly ...
15
votes
2answers
33k views

How do I delete the first n lines of an ascii file using shell commands?

I have multiple files that contain ascii text information in the first 5-10 lines, followed by well-tabulated matrix information. In a shell script, I want to remove these first few lines of text so ...
15
votes
1answer
28k views

How can I execute local script on remote machine and include arguments?

I have written a script that runs fine when executed locally: ./sysMole -time Aug 18 18 The arguments "-time", "Aug", "18", and "18" are successfully passed on to the script. Now, this script is ...
15
votes
6answers
9k views

Best way to follow a log and execute a command when some text appears in the log

I have a server log that outputs a specific line of text into its log file when the server is up. I want to execute a command once the server is up, and hence do something like the following: tail -f ...
15
votes
5answers
6k views

How can I get bash to exit on backtick failure in a similar way to pipefail?

So I like to harden my bash scripts wherever I can (and when not able to delegate to a language like Python/Ruby) to ensure errors do not go uncaught. In that vein I have a strict.sh, which contains ...
15
votes
3answers
14k views

Variable as command; eval vs bash -c

I was reading a bash script someone made and I noticed that the author doesn't use eval to evaluate a variable as a command The author used bash -c "$1" instead of eval "$1" I assume using eval ...
15
votes
2answers
5k 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 ...
15
votes
2answers
1k 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, ...
15
votes
2answers
4k 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 ...
14
votes
6answers
18k views

How can I test if a variable is empty or contains only spaces?

The following bash syntax verifies if param isn't empty: [[ ! -z $param ]] For example: param="" [[ ! -z $param ]] && echo "I am not zero" No output and its fine. But when param ...
14
votes
5answers
2k views

Bash in Linux v.s Mac OS

I am buying an iMac. I've been working on Linux for a long time and I do a bunch of bash scripting. I was wondering if all the bash commands and features are available on Mac OS ? Anyone has any ...
14
votes
5answers
641 views

Stateful bash function

I’d like to implement a function in Bash which increases (and returns) a count with every call. Unfortunately this seems non-trivial since I’m invoking the function inside a subshell and it ...
14
votes
5answers
17k 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?
14
votes
2answers
5k views

Simplest way to comment/uncomment certain lines using command line

Is there a way to comment/uncomment a shell/config/ruby script using command line? for example: $ comment 14-18 bla.conf $ uncomment 14-18 bla.conf this would add or remove # sign on bla.conf on ...
14
votes
3answers
7k views

How can one run multiple programs in the background with single command?

How can one run multiple programs in the background with single command? I have tried the commands below, but they do not work. nohup ./script1.sh & && nohup ./script2.sh & -bash: ...
14
votes
4answers
19k views

How can I use bash's if test and find commands together?

I have a directory with crash logs, and I'd like to use a conditional statement in a bash script based on a find command. The log files are stored in this format: /var/log/crashes/app-2012-08-28.log ...
14
votes
9answers
483 views

Filter or pipe certain sections of a file

I have an input file with some sections the are demarcated with start and end tags, for example: line A line B @@inline-code-start line X line Y line Z @@inline-code-end line C line D I want to ...
14
votes
4answers
633 views

What is the difference between '$ . foo' and '$ ./foo'? [duplicate]

t is a simple script What is different between two case below : $ . t and $ ./t
14
votes
4answers
4k views

How to make bash abort the execution of a script on syntax error?

To be on safe side, I'd like bash abort the execution of a script if it encounters a syntax error. To my surprise, I can't achieve this. (set -e is not enough.) Example: #!/bin/bash # Do exit on ...
14
votes
5answers
23k views

Check if folder is a mounted remote filesystem

What is the best way (reliable, portable, etc.) to check if a given folder is on a mounted remote (nfs) filesystem within a shell script? I am looking for a command that would look like: ...
14
votes
2answers
4k views

Rule for invoking subshell in Bash?

I seem to misunderstand the Bash rule for creating a subshell. I thought parentheses always creates a subshell, which runs as its own process. However, this doesn't seem to be the case. In Code ...
14
votes
4answers
938 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 ...
13
votes
4answers
14k views

Why doesn't “sudo su” in a shell script run the rest of the script as root?

A sample script can be as below: #!/bin/bash sudo su ls /root When using ./test.sh as the normal user, instead run ls as super user and exit, it switches to root; and when I logout, it executes ls ...
13
votes
4answers
9k views

What is the meaning of $? in a shell script?

When going through one shell script, I saw the term "$?". What is the significance of this term?
13
votes
3answers
2k views

Determining if a file is a hard link or symbolic link?

I'm creating a shell script that would take a a filename/path to a file and determines if the file is a symbolic link or a hard link. The only thing is, I don't know how to see if they are a hard ...
13
votes
3answers
5k views

Fix or alternative for mktemp in OS X

I'm looking at a bash script someone else wrote that uses mktemp: TEMP=`mktemp --directory` However, this line does not work on my machine (OS X 10.6). How would I fix this line so that it is ...
13
votes
4answers
820 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
17k 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
3answers
16k views

Comparing two strings in Bash

I have the following if block in my bash script: if [ ${PACKAGENAME} -eq kakadu-v6_4-00902C ]; then echo "successfully entered if block!!" fi The script execution is not entering my if block even ...
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
6answers
3k views

Portable way to get script's absolute path?

What is a portable way for a (zsh) script to determine its absolute path? On Linux I use something like mypath=$(readlink -f $0) ...but this is not portable. (E.g., readlink on darwin does not ...
13
votes
4answers
48k views

Splitting string by the first occurrence of a delimiter

I have a string in the next format id;some text here with possible ; inside and want to split it to 2 strings by first occurrence of the ;. So, it should be: id and some text here with possible ; ...
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
511 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
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
2k views

GNU parallel vs & (I mean background) vs xargs -P

I'm confused about the difference or advantage (if any) of running a set of tasks in a .sh script using GNU parallel E.g. Ole Tange's answer: parallel ./pngout -s0 {} R{} ::: *.png rather than say ...
13
votes
1answer
12k views

How can I tell whether a package is installed via yum in a bash script?

I am trying to write a script that installs packages, but if it fails at any point later in the script rolls back whatever it installed. Of course if the user has already previously installed a ...
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 ...