Visit Our UNIX and Linux User Community

Variable assignments specified with eval shell built-in

Thread Tools Search this Thread
Top Forums UNIX for Advanced & Expert Users Variable assignments specified with eval shell built-in
# 1  
Old 01-08-2011
Variable assignments specified with eval shell built-in

According to the POSIX specifications eval is a special shell built-in, which should imply that variable assignments specified together with it should remain in effect after the built-in completes. Thus one would expect IFS to be changed after this:
var=$'a\nb c'
$ IFS=$'\n' eval '
for i in $var; do
  echo "$i"
b c

But it isn't:
$ set | grep ^IFS
IFS=$' \t\n'

Last edited by fpmurphy; 01-08-2011 at 11:43 AM.. Reason: fixed link
# 2  
Old 01-08-2011

For this script:
#!/bin/bash --posix
#!/usr/bin/env dash
#!/usr/bin/env bash

# @(#) s1	Demonstrate IFS setting.

# Section 1, setup, pre-solution.
# Infrastructure details, environment, commands for forum posts. 
# Uncomment export command to test script as external user.
# export PATH="/usr/local/bin:/usr/bin:/bin"
set +o nounset
pe() { for i;do printf "%s" "$i";done; printf "\n"; }
pl() { pe;pe "-----" ;pe "$*"; }
# C=$HOME/bin/context && [ -f $C ] && . $C eval
version 2>&1 >/dev/null && version =o eval
set -o nounset
pe " posix setting: \"$(set -o | grep -i posix)\""

# Section 2, solution.
pl " Results:"
pe " original: IFS is $(set | grep ^IFS | od -bc)"
var=$'a\nb c'
IFS=$'\n' eval '
pe " before loop: IFS is $(set | grep ^IFS | od -bc)"
for i in $var; do
  echo "$i"
pe " after loop:  IFS is $(set | grep ^IFS | od -bc)"

exit 0

The setting stays the same:
% ./s1
OS, ker|rel, machine: Linux, 2.6.26-2-amd64, x86_64
Distribution        : Debian GNU/Linux 5.0 (lenny) 
eval - is a shell builtin [bash]
 posix setting: "posix          	on"

 original: IFS is 0000000 111 106 123 075 047 040 011 012
          I   F   S   =   '      \t  \n
 before loop: IFS is 0000000 111 106 123 075 047 012
          I   F   S   =   '  \n
b c
 after loop:  IFS is 0000000 111 106 123 075 047 012
          I   F   S   =   '  \n

I think this is what you expected.

The dash and plain bash did not keep the before-loop setting ... cheers, drl
This User Gave Thanks to drl For This Post:
# 3  
Old 01-08-2011
and for ksh93
 original: IFS is 0000000  111 106 123 075 044 047 040 134 164 134 156 047 012
           I   F   S   =   $   '       \   t   \   n   '
 before loop: IFS is 0000000  111 106 123 075 044 047 134 156 047 012
           I   F   S   =   $   '   \   n   '
b c
 after loop:  IFS is 0000000  111 106 123 075 044 047 134 156 047 012
           I   F   S   =   $   '   \   n   '

This User Gave Thanks to fpmurphy For This Post:
# 4  
Old 01-08-2011
Thanks guys.. Mystery solved! I ran it through ksh and dash before but I must have been looking at the wrong thing .... It is a bash-only thing then. I never realized default bash is so much different from POSIX and that you really need to use --posix for it to be compliant (at least the way it is compiled in my distribution)

That'll be item #23 on this list...
Bash POSIX Mode - Bash Reference Manual
# 5  
Old 01-08-2011
Note that Bash even in POSIX mode is not fully POSIX compliant. It is mostly compliant. There are weasel words used to avoid claiming full compliance.
will cause Bash to conform more closely to the POSIX standard
Chet R has no interest in full conformance.

Previous Thread | Next Thread
Test Your Knowledge in Computers #130
Difficulty: Easy
The original Unix code was developed by AT&T.
True or False?

10 More Discussions You Might Find Interesting

1. Shell Programming and Scripting

Safe way to eval variable declarations?

Is there a safe way to evaluate variable declarations within a script whether they come from a .conf file, user input, or stdin? Example .conf file: port=21 user="$USER" # Hopefully allow this type of substitution domain="$DOMAIN" server="$(malicious... (4 Replies)
Discussion started by: Michael Stora
4 Replies

2. Shell Programming and Scripting

'eval' used in variable assignment

pattern1=book { x=1 eval echo \$pattern$x } book (this is the output) But when I assign a variable to the output of the eval it doesn't work unless I prefix 2 times backslash before $ as shown below. { a=`eval echo \\$pattern$x` echo $a } book Why here twice "\" has to be... (3 Replies)
Discussion started by: ravisingh
3 Replies

3. Shell Programming and Scripting

Newline in my script-built shell variable

Hi all, I'm not exactly a shell script guru, so this is probably a very silly question and I'm not seeing the point, but you know, sometimes it happens... I have this script which adds entries to local arp cache using it to find the corresponding IP address. # export MAC=00:25:90:34:3d:f2... (16 Replies)
Discussion started by: micantox
16 Replies

4. Shell Programming and Scripting

awk built-in variable for input file

Hi guys, Does awk have a built-in variable which I can use to display the input file it's currently reading? I'm currently concatenating multiple files using awk and later on do some parsing. But for now, I want to add an extra column in the main output data file - basically putting in the... (3 Replies)
Discussion started by: Det7
3 Replies

5. Shell Programming and Scripting

assignment to variable from eval command

Hi Gurus, I am having 2 parameters as below parm1=value1 parm2=parm1 I want to evaluate parm1 value using eval echo \$$parm2 and later i want to assign this value to other variable which i will be using in if statement like : if ]; then do this....... fi could you please suggest... (5 Replies)
Discussion started by: k_vikash
5 Replies

6. Shell Programming and Scripting

eval and variable assignment

Hi, i have an issue with eval and variable assignment. 1) i have a date value in a variable and that date is part of a filename, var1=20100331 file1=${var1} file2=${var2} file3=${var3} i am passing the above variables to a script via... (11 Replies)
Discussion started by: mohanpadamata
11 Replies

7. Shell Programming and Scripting

Passing eval value to a variable

Hello, I have a script that does an scp to a server and then gets the number of process running on that server, the o/P should be stored in a variable for further processing eval `echo "ssh -q $Infa_user@$host 'csh -c $CMD '"` where CMD="ps -ef | grep -i ${INFA_REPO} | grep -v grep | wc... (2 Replies)
Discussion started by: amit1_x
2 Replies

8. UNIX for Dummies Questions & Answers

built in variable to hold the name of therunning script...

Hi, I have multiple functions that can be called by any shell script. These functions have inbuilt echo statements to logs their activity into a log file. When I run multiple shell scripts that call these functions, they all log into the same log file and I am not able to differentiate which... (2 Replies)
Discussion started by: new_learner
2 Replies

9. Shell Programming and Scripting

How to assign eval value as Variable..

Im facing problem in assigning value of eval array variable as normal variable.. x=0 eval DATA${x}="FJSVcpcu" x=`expr $x + 1` eval DATA${x}="FJSVcsr" if x=0, type -> eval echo \$DATA$x , its give me FJSVcpcu i want assign this value into an variable as variable=`eval echo... (3 Replies)
Discussion started by: neruppu
3 Replies

10. Shell Programming and Scripting

eval a variable that has a .

Hi, Is there any way that I can eval the following - eval abc.csv=def.csv I am getting the - bash: command not found error. thanks. (3 Replies)
Discussion started by: ttshell
3 Replies

Featured Tech Videos