I am trying do execute a script, detached, with a delay and in the background.
I have the following script:
#!/bin/sh
echo "[$(date)][${HOST}] Immediate date" > ~/out.log
nohup bash -c "sleep 2 && echo '[$(date)][${HOST}] Delayed date'" >> ~/out.log 2>&1 &
I expect that out.log would contain exactly two lines like so:
[Tue Apr 4 08:55:56 CEST 2017][] Immediate date
[Tue Apr 4 08:55:58 CEST 2017][] Delayed date
Note the timestamps being 2 seconds apart. But instead bot dates read exactly the same time...
[Tue Apr 4 08:55:56 CEST 2017][] Immediate date
[Tue Apr 4 08:55:56 CEST 2017][] Delayed date
When I try it without the nohup and bash -c the output is as expected, but the calling script would have to keep running, otherwise the second command will be aborted.
What causes the date not being different and to fix it?
edit:
I had simplified the command for stackoverflow. But now when I expand it again I run into another variable expansion problem. When I add the HOST variable:
HOST=foo.bar
nohup bash -c 'sleep 2 && echo "[$(date)][${HOST}]" Delayed date' >> ~/out.log 2>&1 &
the output file dos not contain foo.bar:
[Tue Apr 4 12:52:11 CEST 2017][]
The $(date) is evaluated by the outside shell script. If you swap the " with the ', it works as you expect it to work. This is because $(..) in double quotes is resolved, even if you put it in single quotes inside the double quoted string.
#!/bin/sh
echo "[$(date)][${HOST}] Immediate date" > ~/out.log
nohup bash -c 'sleep 2 && echo "[$(date)]['"${HOST}"'] Delayed date"' >> ~/out.log 2>&1 &
Edit: Make the code work w.r.t. $HOST + $HOST containing whitespace
I believe that double quotes in the nohup will evaluate the expression in the nohup command immediately.
Changing your double quotes into single quotes (and using the double quotes for the echo may help, e.g
#!/bin/sh
echo "[$(date)][${HOST}] Immediate date" > ~/out.log
nohup bash -c 'sleep 2 && echo "[$(date)][${HOST}] Delayed date"' >> ~/out.log 2>&1 &
Related
I would like to store a command to use at a later time in a variable (not the output of the command, but the command itself).
I have a simple script as follows:
command="ls";
echo "Command: $command"; #Output is: Command: ls
b=`$command`;
echo $b; #Output is: public_html REV test... (command worked successfully)
However, when I try something a bit more complicated, it fails. For example, if I make
command="ls | grep -c '^'";
The output is:
Command: ls | grep -c '^'
ls: cannot access |: No such file or directory
ls: cannot access grep: No such file or directory
ls: cannot access '^': No such file or directory
How could I store such a command (with pipes/multiple commands) in a variable for later use?
Use eval:
x="ls | wc"
eval "$x"
y=$(eval "$x")
echo "$y"
Do not use eval! It has a major risk of introducing arbitrary code execution.
BashFAQ-50 - I'm trying to put a command in a variable, but the complex cases always fail.
Put it in an array and expand all the words with double-quotes "${arr[#]}" to not let the IFS split the words due to Word Splitting.
cmdArgs=()
cmdArgs=('date' '+%H:%M:%S')
and see the contents of the array inside. The declare -p allows you see the contents of the array inside with each command parameter in separate indices. If one such argument contains spaces, quoting inside while adding to the array will prevent it from getting split due to Word-Splitting.
declare -p cmdArgs
declare -a cmdArgs='([0]="date" [1]="+%H:%M:%S")'
and execute the commands as
"${cmdArgs[#]}"
23:15:18
(or) altogether use a bash function to run the command,
cmd() {
date '+%H:%M:%S'
}
and call the function as just
cmd
POSIX sh has no arrays, so the closest you can come is to build up a list of elements in the positional parameters. Here's a POSIX sh way to run a mail program
# POSIX sh
# Usage: sendto subject address [address ...]
sendto() {
subject=$1
shift
first=1
for addr; do
if [ "$first" = 1 ]; then set --; first=0; fi
set -- "$#" --recipient="$addr"
done
if [ "$first" = 1 ]; then
echo "usage: sendto subject address [address ...]"
return 1
fi
MailTool --subject="$subject" "$#"
}
Note that this approach can only handle simple commands with no redirections. It can't handle redirections, pipelines, for/while loops, if statements, etc
Another common use case is when running curl with multiple header fields and payload. You can always define args like below and invoke curl on the expanded array content
curlArgs=('-H' "keyheader: value" '-H' "2ndkeyheader: 2ndvalue")
curl "${curlArgs[#]}"
Another example,
payload='{}'
hostURL='http://google.com'
authToken='someToken'
authHeader='Authorization:Bearer "'"$authToken"'"'
now that variables are defined, use an array to store your command args
curlCMD=(-X POST "$hostURL" --data "$payload" -H "Content-Type:application/json" -H "$authHeader")
and now do a proper quoted expansion
curl "${curlCMD[#]}"
var=$(echo "asdf")
echo $var
# => asdf
Using this method, the command is immediately evaluated and its return value is stored.
stored_date=$(date)
echo $stored_date
# => Thu Jan 15 10:57:16 EST 2015
# (wait a few seconds)
echo $stored_date
# => Thu Jan 15 10:57:16 EST 2015
The same with backtick
stored_date=`date`
echo $stored_date
# => Thu Jan 15 11:02:19 EST 2015
# (wait a few seconds)
echo $stored_date
# => Thu Jan 15 11:02:19 EST 2015
Using eval in the $(...) will not make it evaluated later:
stored_date=$(eval "date")
echo $stored_date
# => Thu Jan 15 11:05:30 EST 2015
# (wait a few seconds)
echo $stored_date
# => Thu Jan 15 11:05:30 EST 2015
Using eval, it is evaluated when eval is used:
stored_date="date" # < storing the command itself
echo $(eval "$stored_date")
# => Thu Jan 15 11:07:05 EST 2015
# (wait a few seconds)
echo $(eval "$stored_date")
# => Thu Jan 15 11:07:16 EST 2015
# ^^ Time changed
In the above example, if you need to run a command with arguments, put them in the string you are storing:
stored_date="date -u"
# ...
For Bash scripts this is rarely relevant, but one last note. Be careful with eval. Eval only strings you control, never strings coming from an untrusted user or built from untrusted user input.
For bash, store your command like this:
command="ls | grep -c '^'"
Run your command like this:
echo $command | bash
Not sure why so many answers make it complicated!
use alias [command] 'string to execute'
example:
alias dir='ls -l'
./dir
[pretty list of files]
I tried various different methods:
printexec() {
printf -- "\033[1;37m$\033[0m"
printf -- " %q" "$#"
printf -- "\n"
eval -- "$#"
eval -- "$*"
"$#"
"$*"
}
Output:
$ printexec echo -e "foo\n" bar
$ echo -e foo\\n bar
foon bar
foon bar
foo
bar
bash: echo -e foo\n bar: command not found
As you can see, only the third one, "$#" gave the correct result.
I faced this problem with the following command:
awk '{printf "%s[%s]\n", $1, $3}' "input.txt"
I need to build this command dynamically:
The target file name input.txt is dynamic and may contain space.
The awk script inside {} braces printf "%s[%s]\n", $1, $3 is dynamic.
Challenge:
Avoid extensive quote escaping logic if there are many " inside the awk script.
Avoid parameter expansion for every $ field variable.
The solutions bellow with eval command and associative arrays do not work. Due to bash variable expansions and quoting.
Solution:
Build bash variable dynamically, avoid bash expansions, use printf template.
# dynamic variables, values change at runtime.
input="input file 1.txt"
awk_script='printf "%s[%s]\n" ,$1 ,$3'
# static command template, preventing double-quote escapes and avoid variable expansions.
awk_command=$(printf "awk '{%s}' \"%s\"\n" "$awk_script" "$input")
echo "awk_command=$awk_command"
awk_command=awk '{printf "%s[%s]\n" ,$1 ,$3}' "input file 1.txt"
Executing variable command:
bash -c "$awk_command"
Alternative that also works
bash << $awk_command
As you don't specify any scripting language, I would recommand tcl, the Tool Command Language for this kind of purpose.
Then in the first line, add the appropriate shebang:
#!/usr/local/bin/tclsh
with appropriate location you can retrieve with which tclsh.
In tcl scripts, you can call operating system commands with exec.
#!/bin/bash
#Note: this script works only when u use Bash. So, don't remove the first line.
TUNECOUNT=$(ifconfig |grep -c -o tune0) #Some command with "Grep".
echo $TUNECOUNT #This will return 0
#if you don't have tune0 interface.
#Or count of installed tune0 interfaces.
First of all, there are functions for this. But if you prefer variables then your task can be done like this:
$ cmd=ls
$ $cmd # works
file file2 test
$ cmd='ls | grep file'
$ $cmd # not works
ls: cannot access '|': No such file or directory
ls: cannot access 'grep': No such file or directory
file
$ bash -c $cmd # works
file file2 test
$ bash -c "$cmd" # also works
file
file2
$ bash <<< $cmd
file
file2
$ bash <<< "$cmd"
file
file2
Or via a temporary file
$ tmp=$(mktemp)
$ echo "$cmd" > "$tmp"
$ chmod +x "$tmp"
$ "$tmp"
file
file2
$ rm "$tmp"
Be careful registering an order with the: X=$(Command)
This one is still executed. Even before being called. To check and confirm this, you can do:
echo test;
X=$(for ((c=0; c<=5; c++)); do
sleep 2;
done);
echo note the 5 seconds elapsed
It is not necessary to store commands in variables even as you need to use it later. Just execute it as per normal. If you store in variables, you would need some kind of eval statement or invoke some unnecessary shell process to "execute your variable".
I would like to store a command to use at a later time in a variable (not the output of the command, but the command itself).
I have a simple script as follows:
command="ls";
echo "Command: $command"; #Output is: Command: ls
b=`$command`;
echo $b; #Output is: public_html REV test... (command worked successfully)
However, when I try something a bit more complicated, it fails. For example, if I make
command="ls | grep -c '^'";
The output is:
Command: ls | grep -c '^'
ls: cannot access |: No such file or directory
ls: cannot access grep: No such file or directory
ls: cannot access '^': No such file or directory
How could I store such a command (with pipes/multiple commands) in a variable for later use?
Use eval:
x="ls | wc"
eval "$x"
y=$(eval "$x")
echo "$y"
Do not use eval! It has a major risk of introducing arbitrary code execution.
BashFAQ-50 - I'm trying to put a command in a variable, but the complex cases always fail.
Put it in an array and expand all the words with double-quotes "${arr[#]}" to not let the IFS split the words due to Word Splitting.
cmdArgs=()
cmdArgs=('date' '+%H:%M:%S')
and see the contents of the array inside. The declare -p allows you see the contents of the array inside with each command parameter in separate indices. If one such argument contains spaces, quoting inside while adding to the array will prevent it from getting split due to Word-Splitting.
declare -p cmdArgs
declare -a cmdArgs='([0]="date" [1]="+%H:%M:%S")'
and execute the commands as
"${cmdArgs[#]}"
23:15:18
(or) altogether use a bash function to run the command,
cmd() {
date '+%H:%M:%S'
}
and call the function as just
cmd
POSIX sh has no arrays, so the closest you can come is to build up a list of elements in the positional parameters. Here's a POSIX sh way to run a mail program
# POSIX sh
# Usage: sendto subject address [address ...]
sendto() {
subject=$1
shift
first=1
for addr; do
if [ "$first" = 1 ]; then set --; first=0; fi
set -- "$#" --recipient="$addr"
done
if [ "$first" = 1 ]; then
echo "usage: sendto subject address [address ...]"
return 1
fi
MailTool --subject="$subject" "$#"
}
Note that this approach can only handle simple commands with no redirections. It can't handle redirections, pipelines, for/while loops, if statements, etc
Another common use case is when running curl with multiple header fields and payload. You can always define args like below and invoke curl on the expanded array content
curlArgs=('-H' "keyheader: value" '-H' "2ndkeyheader: 2ndvalue")
curl "${curlArgs[#]}"
Another example,
payload='{}'
hostURL='http://google.com'
authToken='someToken'
authHeader='Authorization:Bearer "'"$authToken"'"'
now that variables are defined, use an array to store your command args
curlCMD=(-X POST "$hostURL" --data "$payload" -H "Content-Type:application/json" -H "$authHeader")
and now do a proper quoted expansion
curl "${curlCMD[#]}"
var=$(echo "asdf")
echo $var
# => asdf
Using this method, the command is immediately evaluated and its return value is stored.
stored_date=$(date)
echo $stored_date
# => Thu Jan 15 10:57:16 EST 2015
# (wait a few seconds)
echo $stored_date
# => Thu Jan 15 10:57:16 EST 2015
The same with backtick
stored_date=`date`
echo $stored_date
# => Thu Jan 15 11:02:19 EST 2015
# (wait a few seconds)
echo $stored_date
# => Thu Jan 15 11:02:19 EST 2015
Using eval in the $(...) will not make it evaluated later:
stored_date=$(eval "date")
echo $stored_date
# => Thu Jan 15 11:05:30 EST 2015
# (wait a few seconds)
echo $stored_date
# => Thu Jan 15 11:05:30 EST 2015
Using eval, it is evaluated when eval is used:
stored_date="date" # < storing the command itself
echo $(eval "$stored_date")
# => Thu Jan 15 11:07:05 EST 2015
# (wait a few seconds)
echo $(eval "$stored_date")
# => Thu Jan 15 11:07:16 EST 2015
# ^^ Time changed
In the above example, if you need to run a command with arguments, put them in the string you are storing:
stored_date="date -u"
# ...
For Bash scripts this is rarely relevant, but one last note. Be careful with eval. Eval only strings you control, never strings coming from an untrusted user or built from untrusted user input.
For bash, store your command like this:
command="ls | grep -c '^'"
Run your command like this:
echo $command | bash
Not sure why so many answers make it complicated!
use alias [command] 'string to execute'
example:
alias dir='ls -l'
./dir
[pretty list of files]
I tried various different methods:
printexec() {
printf -- "\033[1;37m$\033[0m"
printf -- " %q" "$#"
printf -- "\n"
eval -- "$#"
eval -- "$*"
"$#"
"$*"
}
Output:
$ printexec echo -e "foo\n" bar
$ echo -e foo\\n bar
foon bar
foon bar
foo
bar
bash: echo -e foo\n bar: command not found
As you can see, only the third one, "$#" gave the correct result.
I faced this problem with the following command:
awk '{printf "%s[%s]\n", $1, $3}' "input.txt"
I need to build this command dynamically:
The target file name input.txt is dynamic and may contain space.
The awk script inside {} braces printf "%s[%s]\n", $1, $3 is dynamic.
Challenge:
Avoid extensive quote escaping logic if there are many " inside the awk script.
Avoid parameter expansion for every $ field variable.
The solutions bellow with eval command and associative arrays do not work. Due to bash variable expansions and quoting.
Solution:
Build bash variable dynamically, avoid bash expansions, use printf template.
# dynamic variables, values change at runtime.
input="input file 1.txt"
awk_script='printf "%s[%s]\n" ,$1 ,$3'
# static command template, preventing double-quote escapes and avoid variable expansions.
awk_command=$(printf "awk '{%s}' \"%s\"\n" "$awk_script" "$input")
echo "awk_command=$awk_command"
awk_command=awk '{printf "%s[%s]\n" ,$1 ,$3}' "input file 1.txt"
Executing variable command:
bash -c "$awk_command"
Alternative that also works
bash << $awk_command
As you don't specify any scripting language, I would recommand tcl, the Tool Command Language for this kind of purpose.
Then in the first line, add the appropriate shebang:
#!/usr/local/bin/tclsh
with appropriate location you can retrieve with which tclsh.
In tcl scripts, you can call operating system commands with exec.
#!/bin/bash
#Note: this script works only when u use Bash. So, don't remove the first line.
TUNECOUNT=$(ifconfig |grep -c -o tune0) #Some command with "Grep".
echo $TUNECOUNT #This will return 0
#if you don't have tune0 interface.
#Or count of installed tune0 interfaces.
First of all, there are functions for this. But if you prefer variables then your task can be done like this:
$ cmd=ls
$ $cmd # works
file file2 test
$ cmd='ls | grep file'
$ $cmd # not works
ls: cannot access '|': No such file or directory
ls: cannot access 'grep': No such file or directory
file
$ bash -c $cmd # works
file file2 test
$ bash -c "$cmd" # also works
file
file2
$ bash <<< $cmd
file
file2
$ bash <<< "$cmd"
file
file2
Or via a temporary file
$ tmp=$(mktemp)
$ echo "$cmd" > "$tmp"
$ chmod +x "$tmp"
$ "$tmp"
file
file2
$ rm "$tmp"
Be careful registering an order with the: X=$(Command)
This one is still executed. Even before being called. To check and confirm this, you can do:
echo test;
X=$(for ((c=0; c<=5; c++)); do
sleep 2;
done);
echo note the 5 seconds elapsed
It is not necessary to store commands in variables even as you need to use it later. Just execute it as per normal. If you store in variables, you would need some kind of eval statement or invoke some unnecessary shell process to "execute your variable".
I would like to store a command to use at a later time in a variable (not the output of the command, but the command itself).
I have a simple script as follows:
command="ls";
echo "Command: $command"; #Output is: Command: ls
b=`$command`;
echo $b; #Output is: public_html REV test... (command worked successfully)
However, when I try something a bit more complicated, it fails. For example, if I make
command="ls | grep -c '^'";
The output is:
Command: ls | grep -c '^'
ls: cannot access |: No such file or directory
ls: cannot access grep: No such file or directory
ls: cannot access '^': No such file or directory
How could I store such a command (with pipes/multiple commands) in a variable for later use?
Use eval:
x="ls | wc"
eval "$x"
y=$(eval "$x")
echo "$y"
Do not use eval! It has a major risk of introducing arbitrary code execution.
BashFAQ-50 - I'm trying to put a command in a variable, but the complex cases always fail.
Put it in an array and expand all the words with double-quotes "${arr[#]}" to not let the IFS split the words due to Word Splitting.
cmdArgs=()
cmdArgs=('date' '+%H:%M:%S')
and see the contents of the array inside. The declare -p allows you see the contents of the array inside with each command parameter in separate indices. If one such argument contains spaces, quoting inside while adding to the array will prevent it from getting split due to Word-Splitting.
declare -p cmdArgs
declare -a cmdArgs='([0]="date" [1]="+%H:%M:%S")'
and execute the commands as
"${cmdArgs[#]}"
23:15:18
(or) altogether use a bash function to run the command,
cmd() {
date '+%H:%M:%S'
}
and call the function as just
cmd
POSIX sh has no arrays, so the closest you can come is to build up a list of elements in the positional parameters. Here's a POSIX sh way to run a mail program
# POSIX sh
# Usage: sendto subject address [address ...]
sendto() {
subject=$1
shift
first=1
for addr; do
if [ "$first" = 1 ]; then set --; first=0; fi
set -- "$#" --recipient="$addr"
done
if [ "$first" = 1 ]; then
echo "usage: sendto subject address [address ...]"
return 1
fi
MailTool --subject="$subject" "$#"
}
Note that this approach can only handle simple commands with no redirections. It can't handle redirections, pipelines, for/while loops, if statements, etc
Another common use case is when running curl with multiple header fields and payload. You can always define args like below and invoke curl on the expanded array content
curlArgs=('-H' "keyheader: value" '-H' "2ndkeyheader: 2ndvalue")
curl "${curlArgs[#]}"
Another example,
payload='{}'
hostURL='http://google.com'
authToken='someToken'
authHeader='Authorization:Bearer "'"$authToken"'"'
now that variables are defined, use an array to store your command args
curlCMD=(-X POST "$hostURL" --data "$payload" -H "Content-Type:application/json" -H "$authHeader")
and now do a proper quoted expansion
curl "${curlCMD[#]}"
var=$(echo "asdf")
echo $var
# => asdf
Using this method, the command is immediately evaluated and its return value is stored.
stored_date=$(date)
echo $stored_date
# => Thu Jan 15 10:57:16 EST 2015
# (wait a few seconds)
echo $stored_date
# => Thu Jan 15 10:57:16 EST 2015
The same with backtick
stored_date=`date`
echo $stored_date
# => Thu Jan 15 11:02:19 EST 2015
# (wait a few seconds)
echo $stored_date
# => Thu Jan 15 11:02:19 EST 2015
Using eval in the $(...) will not make it evaluated later:
stored_date=$(eval "date")
echo $stored_date
# => Thu Jan 15 11:05:30 EST 2015
# (wait a few seconds)
echo $stored_date
# => Thu Jan 15 11:05:30 EST 2015
Using eval, it is evaluated when eval is used:
stored_date="date" # < storing the command itself
echo $(eval "$stored_date")
# => Thu Jan 15 11:07:05 EST 2015
# (wait a few seconds)
echo $(eval "$stored_date")
# => Thu Jan 15 11:07:16 EST 2015
# ^^ Time changed
In the above example, if you need to run a command with arguments, put them in the string you are storing:
stored_date="date -u"
# ...
For Bash scripts this is rarely relevant, but one last note. Be careful with eval. Eval only strings you control, never strings coming from an untrusted user or built from untrusted user input.
For bash, store your command like this:
command="ls | grep -c '^'"
Run your command like this:
echo $command | bash
Not sure why so many answers make it complicated!
use alias [command] 'string to execute'
example:
alias dir='ls -l'
./dir
[pretty list of files]
I tried various different methods:
printexec() {
printf -- "\033[1;37m$\033[0m"
printf -- " %q" "$#"
printf -- "\n"
eval -- "$#"
eval -- "$*"
"$#"
"$*"
}
Output:
$ printexec echo -e "foo\n" bar
$ echo -e foo\\n bar
foon bar
foon bar
foo
bar
bash: echo -e foo\n bar: command not found
As you can see, only the third one, "$#" gave the correct result.
I faced this problem with the following command:
awk '{printf "%s[%s]\n", $1, $3}' "input.txt"
I need to build this command dynamically:
The target file name input.txt is dynamic and may contain space.
The awk script inside {} braces printf "%s[%s]\n", $1, $3 is dynamic.
Challenge:
Avoid extensive quote escaping logic if there are many " inside the awk script.
Avoid parameter expansion for every $ field variable.
The solutions bellow with eval command and associative arrays do not work. Due to bash variable expansions and quoting.
Solution:
Build bash variable dynamically, avoid bash expansions, use printf template.
# dynamic variables, values change at runtime.
input="input file 1.txt"
awk_script='printf "%s[%s]\n" ,$1 ,$3'
# static command template, preventing double-quote escapes and avoid variable expansions.
awk_command=$(printf "awk '{%s}' \"%s\"\n" "$awk_script" "$input")
echo "awk_command=$awk_command"
awk_command=awk '{printf "%s[%s]\n" ,$1 ,$3}' "input file 1.txt"
Executing variable command:
bash -c "$awk_command"
Alternative that also works
bash << $awk_command
As you don't specify any scripting language, I would recommand tcl, the Tool Command Language for this kind of purpose.
Then in the first line, add the appropriate shebang:
#!/usr/local/bin/tclsh
with appropriate location you can retrieve with which tclsh.
In tcl scripts, you can call operating system commands with exec.
#!/bin/bash
#Note: this script works only when u use Bash. So, don't remove the first line.
TUNECOUNT=$(ifconfig |grep -c -o tune0) #Some command with "Grep".
echo $TUNECOUNT #This will return 0
#if you don't have tune0 interface.
#Or count of installed tune0 interfaces.
First of all, there are functions for this. But if you prefer variables then your task can be done like this:
$ cmd=ls
$ $cmd # works
file file2 test
$ cmd='ls | grep file'
$ $cmd # not works
ls: cannot access '|': No such file or directory
ls: cannot access 'grep': No such file or directory
file
$ bash -c $cmd # works
file file2 test
$ bash -c "$cmd" # also works
file
file2
$ bash <<< $cmd
file
file2
$ bash <<< "$cmd"
file
file2
Or via a temporary file
$ tmp=$(mktemp)
$ echo "$cmd" > "$tmp"
$ chmod +x "$tmp"
$ "$tmp"
file
file2
$ rm "$tmp"
Be careful registering an order with the: X=$(Command)
This one is still executed. Even before being called. To check and confirm this, you can do:
echo test;
X=$(for ((c=0; c<=5; c++)); do
sleep 2;
done);
echo note the 5 seconds elapsed
It is not necessary to store commands in variables even as you need to use it later. Just execute it as per normal. If you store in variables, you would need some kind of eval statement or invoke some unnecessary shell process to "execute your variable".
I have 2 commands which I am executing over shell command1 and command2. command1 takes long to complete (~2 minutes). So I can put it running in background using & but after that I want to execute command2 automatically. Can I do this on shell command line?
Try:
( command1; command2 ) &
Edit: Larger PoC:
demo.sh:
#!/bin/bash
echo Starting at $(date)
( echo Starting background process at $(date); sleep 5; echo Ending background process at $(date) ) &
echo Last command in script at $(date)
Running demo.sh:
$ ./demo.sh
Starting at Thu Mar 1 09:11:04 MST 2018
Starting background process at Thu Mar 1 09:11:04 MST 2018
Last command in script at Thu Mar 1 09:11:04 MST 2018
$ Ending background process at Thu Mar 1 09:11:09 MST 2018
Note that the script ended after "Last command in script", but the background process did its "Ending background process" echo 5 seconds later. All of the commands in the (...)& structure are run serially, but are all collectively forked to the background.
You can do so by putting both the commends in a shell script and do like below:-
command1 &
wait
command2 & #if you want to run second command also in background
It looks like set -v doesn't work when in a subshell? Why?
(set -x does work in a subshell, but the output is a bit busier).
$ cat foo.sh
set -v
date
set +v > /dev/null 2>&1
$ bash foo.sh
date
Fri Mar 3 14:52:34 PST 2017
set +v > /dev/null 2>&1
$ cat foo_subshell.sh
(
set -v
date
)
$ bash foo_subshell.sh
Fri Mar 3 14:52:42 PST 2017
The documentation says:
-v Print shell input lines as they are read.
When you write a subshell in parentheses, the entire subshell is read, then it's executed. So the set -v command isn't executed until after all the commands are read, so it's too late for it to print them.