Difficulty with CAT redirection in script Post: 302990335

Sponsored Content

Top Forums Shell Programming and Scripting Difficulty with CAT redirection in script Post 302990335 by bakunin on Wednesday 25th of January 2017 04:05:15 AM

01-25-2017

Registered User

Quote:

Originally Posted by demmith

Oh my God, I am such an idiot! LOL!

Don't worry, it happens to the best of us.

As a general rule: do not change directories inside a script at all (for exactly this reason, to avoid this confusion). Whenever you work on files make sure you always use absolute pathes like this:

Code:

# cat wrong.sh

cd /some/where
command > ./output.file

# cat correct.sh

outdir="/some/where"
command > "${outdir}/output.file"

The same goes for all other filenames. This way your script will work regardless of where you started it. If you work on several files using a variable for the directory part ensures they all land in the same place. The absolute worst you can do, though, is to use relative pathes:

Code:

# cat worst-of-worst.sh

command1 > outfile1
cd ..
command2 > outfile2

If you ever find that in anyones code: have them promise never to write any shell script again. This is a surefire recipe for disaster because the script will (maybe) work if you call it from one directory and fail if you call it from another.

In general the script you call inherits the environment from its calling process - the command shell you used to call it. "Envrionent" means not only the values for variables (all that have been "export"ed before) like PATH, TZ (timezone), LANG, etc.. but also the current directory and similar things. It is good practice to make your script independent from this environment by setting it to a certain state except for the few variables where you explicitly want this effect to take place. This includes (but is not limited to) making it independent of the current path it was called from.

I hope this helps.

bakunin

This User Gave Thanks to bakunin For This Post:

bakunin

View Public Profile for bakunin

Find all posts by bakunin

9 More Discussions You Might Find Interesting

1. UNIX for Dummies Questions & Answers

redirection to tty** with cat

I tried to cat a file to another user that was logged in, but I received an error message that displayed something like: %cat funny > /dev/ttyp3 zsh: permission denied: /dev/ttyp3 Thank you all for your help

2. UNIX for Dummies Questions & Answers

Difference between cat , cat > , cat >> and touch !!!

Hi Can anybody tell the difference between Difference between cat , cat > , cat >> and touch command in UNIX? Thanks

3. UNIX for Dummies Questions & Answers

Regarding redirection using cat.

The behavior of the following 2 operations is unexpected. K1 and K2 are files here :- 1) cat < K1 K2 The above operation should actually display contents of the both files. But it gives the contents of K2 only. How is that ? 2) cat > K1 K2 Above operation takes the contents of...

4. Shell Programming and Scripting

cat in the command line doesn't match cat in the script

Hello, So I sorted my file as I was supposed to: sort -n -r -k 2 -k 1 file1 | uniq > file2 and when I wrote > cat file2 in the command line, I got what I was expecting, but in the script itself ... sort -n -r -k 2 -k 1 averages | uniq > temp cat file2 It wrote a whole...

5. Shell Programming and Scripting

Difficulty using "execute immediate" in shell - Sql script

Hello members, I get an unexpected "end of file" error while trying to execute the following piece of code in the bash / ksh shell. I'm assuming the problem is with using the "execute immediate statement" #! /bin/bash tname="table" for i in * do sqlstr="create table $tname$i as select...

6. Shell Programming and Scripting

How to check for Input Redirection in my script?

All, I have a requirement to write a script where I check for Input redirection when the script was executed, based on which I handle my logic. Below is the example: my.script #! /bin/ksh # Not sure how to frame the if condition below if ; then echo "Input Redirected from a file" ...

7. Shell Programming and Scripting

STDOUT and STDERR redirection within a script

Hello all, I have a for loop executing in a script that I want to redirect STDOUT to screen and to file, while directing STDERR to the bit bucket. Here is the general sentax of what I'm doing: for i in thingy do some_command ${i} done 1>&1 | tee ${LOGFILE} 2> /dev/null What I am...

8. Shell Programming and Scripting

Input redirection script

Hi, #!/bin/bash while ; do rm -f /tmp/pipe mkfifo /tmp/pipe ./yuv4mpeg_to_v4l2 < /tmp/pipe & mplayer tom_and_jerry.mp4 -vf scale=480:360 -vo yuv4mpeg:file=/tmp/pipe sleep 65; done When I run this - after mplayer finishes playing video it says - Exiting... (End of...

9. Shell Programming and Scripting

Input redirection within bash script

Hi, when I try to redirect input and the command is described as a string within an array redirection does not work. why? #!/bin/bash dir=("tail < ./hello.txt") tail < ./hello.txt #works ${dir} #does not work

LEARN ABOUT CENTOS

service

service(8)						      System Manager's Manual							service(8)

NAME

       service - run a System V init script

SYNOPSIS

       service SCRIPT COMMAND [OPTIONS]

       service --status-all

       service --help | -h | --version

DESCRIPTION

       service runs a System V init script in as predictable environment as possible, removing most environment variables and with current working
       directory set to /.

       The SCRIPT parameter specifies a System V init script, located in /etc/init.d/SCRIPT.  The  supported  values  of  COMMAND  depend  on  the
       invoked	script,  service  passes  COMMAND and OPTIONS it to the init script unmodified.  All scripts should support at least the start and
       stop commands.  As a special case, if COMMAND is --full-restart, the script is run twice, first with the stop command, then with the  start
       command.

       service --status-all runs all init scripts, in alphabetical order, with the status command.

       If the init script file does not exist, the script tries to use legacy actions.	If there is no suitable legacy action found and COMMAND is
       one of actions specified in LSB Core Specification, input is redirected to the systemctl.  Otherwise the command fails with return code 2.

FILES

       /etc/init.d
	      The directory containing System V init scripts.

ENVIRONMENT

       LANG, TERM
	      The only environment variables passed to the init scripts.

SEE ALSO

       chkconfig(8), ntsysv(8), systemd(1), systemctl(8), systemd.service(5)

								     Jan 2006								service(8)