Suppose there's a script called 'test.sh':
#!/bin/bash
while read line; do
APP=/apps echo "$line"
done < ./lines
And the 'lines':
cd $APP && pwd
If I bash test.sh, it prints out 'cd $APP && pwd'.
But when I type APP=/apps echo "cd $APP && pwd" in the terminal, it prints out 'cd /apps && pwd'.
Is it possible using echo to extract variables which are reading from a regular file?
Depending on the contents of the file, you may want to use eval:
#!/bin/bash
APP=/apps
while read line; do
eval "echo \"$line\"" # WARNING: dangerous
done < ./lines
However, eval is extremely dangerous. Although the quoting here will work for simple cases, it is quite easy to execute arbitrary commands by manipulating the input.
You should use eval to evaluate string line read from file
If you know the variable(s) you want to substitute, just substitute them.
sed 's%\$APP\>%/apps%g' ./lines
Related
If I run the script with ./test.sh 100 I do not get the output 100 because I am using a thread. What do I have to do to get the expected output? (I must not change test.sh though.)
test.sh
#!/bin/bash
FILE="number.txt"
echo "0" > $FILE
for (( x=1; x<=$1; x++)); do
exec "./increment.sh" $FILE &
done
wait
cat $FILE
increment.sh
#!/bin/bash
value=(< "$1")
let value++
echo $value > "$1"
EDIT
Well I tried this:
#!/bin/bash
flock $1 --shared 2>/dev/null
value=(< "$1")
let value++
echo $value > "$1"
Now i get something like 98 99 all the time if I use ./test.sh 100
I is not working very well and I do not know how to fix it.
If test.sh really cannot be improved, then each instance of increment.sh must serialize it's own access to $FILE.
Filesystem locking is the obvious solution for this under UNIX. However, there is no shell builtin to accomplish this. Instead, you must rely on an external utility program like flock, setlock, or chpst -l|-L. For example:
#!/bin/bash
(
flock 100 # Lock *exclusively* (not shared)
value=(< "$1")
let value++
echo $value > "$1"
) 100>>"$1" # A note of caution
A note of caution: using the file you'll be modifying as a lockfile gets tricky quickly — it's easy to truncate in shell when you didn't mean to, and the mixing of access modes above might offend some people — but the above avoids gross mistakes.
I made this script that should receive one or more parameter, and those parameter are all directories, and it has to browse those directories (one by one) and do some operations.
The operations work fine if the parameter is 1 (only one directory),
How should I modify my script to make it works if more than 1 parameter is passed
Example if I want it to do the same operations in 2 or 3 directories at the same time?
Thanks
#!/bin/sh
cd $1
for file in ./* # */
do
if [[ -d $file ]]
then
ext=dir
else
ext="${file##*.}"
fi
mv "${file}" "${file}.$ext"
done
First, if you are using bash use bash shebang (#! /bin/bash).
Then use
#! /bin/bash
for d in "$#"
do
echo "Do something with $d"
done
to iterate over the command line arguments (dirs in your case)
#!/bin/sh
for dir in "$#"; do
for file in "$dir"/*; do
echo "Doing something with '$file'"
done
done
Is it possible to save last entered value of a variable by the user in the bash script itself so that I reuse value the next time while executing again?.
Eg:
#!/bin/bash
if [ -d "/opt/test" ]; then
echo "Enter path:"
read path
p=$path
else
.....
........
fi
The above script is just a sample example I wanted to give(which may be wrong), is it possible if I want to save the value of p permanently in the script itself to so that I use it somewhere later in the script even when the script is re-executed?.
EDIT:
I am already using sed to overwrite the lines in the script while executing, this method works but this is not at all good practice as said. Replacing the lines in the same file as said in the below answer is much better than what I am using like the one below:
...
....
PATH=""; #This is line no 7
DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )";
name="$(basename "$(test -L "$0" && readlink "$0" || echo "$0")")";
...
if [ condition ]
fi
path=$path
sed -i '7s|.*|PATH='$path';|' $DIR/$name;
Someting like this should do the asked stuff :
#!/bin/bash
ENTERED_PATH=""
if [ "$ENTERED_PATH" = "" ]; then
echo "Enter path"
read path
ENTERED_PATH=$path
sed -i 's/ENTERED_PATH=""/ENTERED_PATH='$path'/g' $0
fi
This script will ask user a path only if not previously ENTERED_PATH were defined, and store it directly into the current file with the sed line.
Maybe a safer way to do this, would be to write a config file somewhere with the data you want to save and source it . data.saved at the begining of your script.
In the script itself? Yes with sed but it's not advisable.
#!/bin/bash
test='0'
echo "test currently is: $test";
test=`expr $test + 1`
echo "changing test to: $test"
sed -i "s/test='[0-9]*'/test='$test'/" $0
Preferable method:
Try saving the value in a seperate file you can easily do a
myvar=`cat varfile.txt`
And whatever was in the file is not in your variable.
I would suggest using the /tmp/ dir to store the file in.
Another option would be to save the value as an extended attribute attached to the script file. This has many of the same problems as editing the script's contents (permissions issues, weird for multiple users, etc) plus a few of its own (not supported on all filesystems...), but IMHO it's not quite as ugly as rewriting the script itself (a config file really is a better option).
I don't use Linux, but I think the relevant commands would be something like this:
path="$(getfattr --only-values -n "user.saved_path" "${BASH_SOURCE[0]}")"
if [[ -z "$path" ]]; then
read -p "Enter path:" path
setfattr -n "user.saved_path" -v "$path" "${BASH_SOURCE[0]}"
fi
#! /bin/bash
if [ $1 ]; then
redirect="<"$1
else
redirect="<&0"
fi
mysql --host=a.b --port=3306 --user=me --password='!' -D DCVBase2 $redirect
wanna redirect from either file or stdin. May be use some quatation around $redirect?
Something like this will work:
#!/bin/bash
if [ "$1" ]; then
redirect="$1"
else
redirect="/dev/tty"
fi
while read LINE
do
echo ${LINE}
done < ${redirect}
You can't put the "<..." in a variable, but you can put the parameter to it in a variable
Also, redirecting stdin from /dev/fd/0 is a no-op, because /dev/fd/0 is bash's stdin, which is what mysql would inherit by default.
so you can make this work by falling back to taking stdin from /dev/fd/0, which looks similar to James_R_Ferguson's answer, except that it uses /dev/fd/0 because using /dev/tty makes an assumption that bash's stdin is an actual terminal.
#! /bin/bash
if [ -n "$1" ]; then # note, I changed this to test for non-empty
redirect="$1"
else
redirect="/dev/fd/0"
fi3
mysql --host=a.b --port=3306 --user=me --password='!' -D DCVBase2 < "$redirect"
Not possible in standard bash because the redirects are processed before variable expansion (though I can't seem to find a reference for that, so you'll have to believe my empirical findings).
You can work around that using the eval command:
$ redirect=">foo"
$ ls $redirect
ls: cannot access >foo: No such file or directory
$ eval ls $redirect
Beware that this opens a can of worms regarding substitution, quoting and so, so you have to be careful to escape everything you do not want interpreted BEFORE the eval (the ! in your command is likely going to be a problem, for example).
$ foo=\$bar
$ bar=baz
$ echo $foo
$bar
$ eval echo $foo
baz
The following is iptable save file, which I modified by setting some variables like you see below.
-A OUTPUT -o $EXTIF -s $UNIVERSE -d $INTNET -j REJECT
I also have a bash script which is defining this variables and should call iptables-restore with the save file above.
#!/bin/sh
EXTIF="eth0"
INTIF="eth1"
INTIP="192.168.0.1/32"
EXTIP=$(/sbin/ip addr show dev "$EXTIF" | perl -lne 'if(/inet (\S+)/){print$1;last}');
UNIVERSE="0.0.0.0/0"
INTNET="192.168.0.1/24"
Now I need to use
/sbin/iptables-restore <the content of iptables save file>
in bash script and somehow insert the text file on top to this script, so the variables will be initialized. Is there any way to do that?
UPDATE: even tried this
/sbin/iptables-restore -v <<-EOF;
$(</etc/test.txt)
EOF
Something like this:
while read line; do eval "echo ${line}"; done < iptables.save.file | /sbin/iptables-restore -v
or more nicely formatted:
while read line
do eval "echo ${line}"
done < iptables.save.file | /sbin/iptables-restore -v
The eval of a string forces the variable expansion stuff.
Use . (dot) char to include one shell script to another:
#!/bin/sh
. /path/to/another/script
In your shell script:
. /path/to/variable-definitions
/sbin/iptables-restore < $(eval echo "$(</path/to/template-file)")
or possibly
/sbin/iptables-restore < <(eval echo "$(</path/to/template-file)")