“Variabilize” the ampersand (background a process)

I want to know if there's a way to put the ampersand in a variable and still use it to send a process to the background.

This works:

BCKGRND=yes

if [ "$BCKGRND" = "yes" ]; then

    sleep 5 &

else

    sleep 5

fi

But wouldn't it be cool to accomplish those five lines with only one? Like so:

BCKGRND='&'

sleep 5 ${BCKGRND}

But that doesn't work. If BCKGRND isn't set it works - but when it is set it's interpreted it as a literal '&' and errors out.

edited 2 days ago

Stephen Kitt

166k24367447

asked 2 days ago

BrowncoatOkie

464

New contributor

add a comment |

I want to know if there's a way to put the ampersand in a variable and still use it to send a process to the background.

This works:

BCKGRND=yes

if [ "$BCKGRND" = "yes" ]; then

    sleep 5 &

else

    sleep 5

fi

But wouldn't it be cool to accomplish those five lines with only one? Like so:

BCKGRND='&'

sleep 5 ${BCKGRND}

But that doesn't work. If BCKGRND isn't set it works - but when it is set it's interpreted it as a literal '&' and errors out.

edited 2 days ago

Stephen Kitt

166k24367447

asked 2 days ago

BrowncoatOkie

464

New contributor

add a comment |

I want to know if there's a way to put the ampersand in a variable and still use it to send a process to the background.

This works:

BCKGRND=yes

if [ "$BCKGRND" = "yes" ]; then

    sleep 5 &

else

    sleep 5

fi

But wouldn't it be cool to accomplish those five lines with only one? Like so:

BCKGRND='&'

sleep 5 ${BCKGRND}

But that doesn't work. If BCKGRND isn't set it works - but when it is set it's interpreted it as a literal '&' and errors out.

edited 2 days ago

Stephen Kitt

166k24367447

asked 2 days ago

BrowncoatOkie

464

New contributor

I want to know if there's a way to put the ampersand in a variable and still use it to send a process to the background.

This works:

BCKGRND=yes

if [ "$BCKGRND" = "yes" ]; then

    sleep 5 &

else

    sleep 5

fi

But wouldn't it be cool to accomplish those five lines with only one? Like so:

BCKGRND='&'

sleep 5 ${BCKGRND}

But that doesn't work. If BCKGRND isn't set it works - but when it is set it's interpreted it as a literal '&' and errors out.

bash shell-script variable background-process

edited 2 days ago

Stephen Kitt

166k24367447

asked 2 days ago

BrowncoatOkie

464

New contributor

edited 2 days ago

Stephen Kitt

166k24367447

asked 2 days ago

BrowncoatOkie

464

New contributor

edited 2 days ago

Stephen Kitt

166k24367447

edited 2 days ago

Stephen Kitt

166k24367447

edited 2 days ago

Stephen Kitt

166k24367447

asked 2 days ago

BrowncoatOkie

464

New contributor

asked 2 days ago

BrowncoatOkie

464

asked 2 days ago

BrowncoatOkie

464

New contributor

BrowncoatOkie is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment |

3 Answers
3

active

oldest

votes

It's not possible to use a variable to background the call because variable expansion happens after the command-line is parsed for control operators (such as && and &).

Yet another option would be to wrap the calls in a function:

mayberunbg() {

  if [ "$BCKGRND" = "yes" ]; then

    "$@" &

  else

    "$@"

  fi

}

... and then set the variable as needed:

$ BCKGRND=yes mayberunbg sleep 3

[1] 14137

$

[1]+  Done                    "$@"

# or

$ BCKGRND=yes

$ mayberunbg sleep 3

[1] 14203

$

[1]+  Done                    "$@"

$ BCKGRND=no mayberunbg sleep 3

# 3 seconds later

$

answered 2 days ago

Jeff Schaller

39.3k1054125

What, no ed? +1 anyway, this is the cleanest solution.

– Stephen Kitt
2 days ago

LOL @StephenKitt; now the gears are turning

– Jeff Schaller
2 days ago

I liked the eval answer for its simplicity but my actual real-world command I wanted to background was far too complicated with too many variables in it to be comfortable using eval. @jeff-schaller gave the answer that pointed me in the direction I went. Instead of a function, though, I put the entire command in a variable then used his if statement style to execute the command with or without the &.

– BrowncoatOkie
2 days ago

add a comment |

You can flip things and variabilise “foregrounding”:

FOREGROUND=fg

sleep 5 & ${FOREGROUND}

Set FOREGROUND to true or empty to run the process in the background. (Setting FOREGROUND to true to run in the background is admittedly confusing! Appropriate variable names are left as an exercise for the reader.)

answered 2 days ago

Stephen Kitt

166k24367447

4

that's nice but it won't work in a shell without job control (ie any script unless set -m was used).

– mosvy
2 days ago

add a comment |

You would probably have to use eval:

eval "sleep 5" "$BCKGRND"

eval causes the shell to re-evaluate the arguments given. A literal & would therefore be interpreted as & at the end of a command and not as an argument to the command, putting the command in the background.

edited 2 days ago

answered 2 days ago

Kusalananda

123k16232381

1

A answer containing eval should contain a warning, that this should be handles with care. See e.g. this answer.

– Ralf
2 days ago

I don't get what the problem is with "$BCKGRND" evaluating to an empty argument.

– mosvy
2 days ago

2

@Ralf absolutely irrelevant in this case. There's nothing special about eval -- you can execute commands via arithmetic expansions, for instance. Maybe there should be such a warning against using bash (or any similar shell) at all ;-)

– mosvy
2 days ago

1

@Kusalananda eval will join its arguments with spaces before doing the actual eval. Just try it: eval printf "'{%s}n'" foo "" "" "". eval foo "" "" "" "" is completely similar to eval foo, no matter what IFS or other thing is.

– mosvy
2 days ago

1

The command being eval'ed should be in double quotes if it contains any special characters, e.g. eval 'sleep $TIMEOUT' "$BACKGROUND". Otherwise you could get double expansions if the variable expands to another variable or contains special characters. Also, nested quoting can get tricky.

– Barmar
2 days ago

|
show 4 more comments

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3 Answers
3

active

oldest

votes

3 Answers
3

active

oldest

votes

It's not possible to use a variable to background the call because variable expansion happens after the command-line is parsed for control operators (such as && and &).

Yet another option would be to wrap the calls in a function:

mayberunbg() {

  if [ "$BCKGRND" = "yes" ]; then

    "$@" &

  else

    "$@"

  fi

}

... and then set the variable as needed:

$ BCKGRND=yes mayberunbg sleep 3

[1] 14137

$

[1]+  Done                    "$@"

# or

$ BCKGRND=yes

$ mayberunbg sleep 3

[1] 14203

$

[1]+  Done                    "$@"

$ BCKGRND=no mayberunbg sleep 3

# 3 seconds later

$

answered 2 days ago

Jeff Schaller

39.3k1054125

What, no ed? +1 anyway, this is the cleanest solution.

– Stephen Kitt
2 days ago

LOL @StephenKitt; now the gears are turning

– Jeff Schaller
2 days ago

I liked the eval answer for its simplicity but my actual real-world command I wanted to background was far too complicated with too many variables in it to be comfortable using eval. @jeff-schaller gave the answer that pointed me in the direction I went. Instead of a function, though, I put the entire command in a variable then used his if statement style to execute the command with or without the &.

– BrowncoatOkie
2 days ago

add a comment |

It's not possible to use a variable to background the call because variable expansion happens after the command-line is parsed for control operators (such as && and &).

Yet another option would be to wrap the calls in a function:

mayberunbg() {

  if [ "$BCKGRND" = "yes" ]; then

    "$@" &

  else

    "$@"

  fi

}

... and then set the variable as needed:

$ BCKGRND=yes mayberunbg sleep 3

[1] 14137

$

[1]+  Done                    "$@"

# or

$ BCKGRND=yes

$ mayberunbg sleep 3

[1] 14203

$

[1]+  Done                    "$@"

$ BCKGRND=no mayberunbg sleep 3

# 3 seconds later

$

answered 2 days ago

Jeff Schaller

39.3k1054125

What, no ed? +1 anyway, this is the cleanest solution.

– Stephen Kitt
2 days ago

LOL @StephenKitt; now the gears are turning

– Jeff Schaller
2 days ago

I liked the eval answer for its simplicity but my actual real-world command I wanted to background was far too complicated with too many variables in it to be comfortable using eval. @jeff-schaller gave the answer that pointed me in the direction I went. Instead of a function, though, I put the entire command in a variable then used his if statement style to execute the command with or without the &.

– BrowncoatOkie
2 days ago

add a comment |

It's not possible to use a variable to background the call because variable expansion happens after the command-line is parsed for control operators (such as && and &).

Yet another option would be to wrap the calls in a function:

mayberunbg() {

  if [ "$BCKGRND" = "yes" ]; then

    "$@" &

  else

    "$@"

  fi

}

... and then set the variable as needed:

$ BCKGRND=yes mayberunbg sleep 3

[1] 14137

$

[1]+  Done                    "$@"

# or

$ BCKGRND=yes

$ mayberunbg sleep 3

[1] 14203

$

[1]+  Done                    "$@"

$ BCKGRND=no mayberunbg sleep 3

# 3 seconds later

$

answered 2 days ago

Jeff Schaller

39.3k1054125

It's not possible to use a variable to background the call because variable expansion happens after the command-line is parsed for control operators (such as && and &).

Yet another option would be to wrap the calls in a function:

mayberunbg() {

  if [ "$BCKGRND" = "yes" ]; then

    "$@" &

  else

    "$@"

  fi

}

... and then set the variable as needed:

$ BCKGRND=yes mayberunbg sleep 3

[1] 14137

$

[1]+  Done                    "$@"

# or

$ BCKGRND=yes

$ mayberunbg sleep 3

[1] 14203

$

[1]+  Done                    "$@"

$ BCKGRND=no mayberunbg sleep 3

# 3 seconds later

$

answered 2 days ago

Jeff Schaller

39.3k1054125

answered 2 days ago

Jeff Schaller

39.3k1054125

answered 2 days ago

Jeff Schaller

39.3k1054125

answered 2 days ago

Jeff Schaller

39.3k1054125

What, no ed? +1 anyway, this is the cleanest solution.

– Stephen Kitt
2 days ago

LOL @StephenKitt; now the gears are turning

– Jeff Schaller
2 days ago

I liked the eval answer for its simplicity but my actual real-world command I wanted to background was far too complicated with too many variables in it to be comfortable using eval. @jeff-schaller gave the answer that pointed me in the direction I went. Instead of a function, though, I put the entire command in a variable then used his if statement style to execute the command with or without the &.

– BrowncoatOkie
2 days ago

add a comment |

What, no ed? +1 anyway, this is the cleanest solution.

– Stephen Kitt
2 days ago

LOL @StephenKitt; now the gears are turning

– Jeff Schaller
2 days ago

I liked the eval answer for its simplicity but my actual real-world command I wanted to background was far too complicated with too many variables in it to be comfortable using eval. @jeff-schaller gave the answer that pointed me in the direction I went. Instead of a function, though, I put the entire command in a variable then used his if statement style to execute the command with or without the &.

– BrowncoatOkie
2 days ago

What, no ed? +1 anyway, this is the cleanest solution.

– Stephen Kitt
2 days ago

LOL @StephenKitt; now the gears are turning

– Jeff Schaller
2 days ago

I liked the eval answer for its simplicity but my actual real-world command I wanted to background was far too complicated with too many variables in it to be comfortable using eval. @jeff-schaller gave the answer that pointed me in the direction I went. Instead of a function, though, I put the entire command in a variable then used his if statement style to execute the command with or without the &.

– BrowncoatOkie
2 days ago

add a comment |

You can flip things and variabilise “foregrounding”:

FOREGROUND=fg

sleep 5 & ${FOREGROUND}

answered 2 days ago

Stephen Kitt

166k24367447

4

that's nice but it won't work in a shell without job control (ie any script unless set -m was used).

– mosvy
2 days ago

add a comment |

You can flip things and variabilise “foregrounding”:

FOREGROUND=fg

sleep 5 & ${FOREGROUND}

answered 2 days ago

Stephen Kitt

166k24367447

4

that's nice but it won't work in a shell without job control (ie any script unless set -m was used).

– mosvy
2 days ago

add a comment |

You can flip things and variabilise “foregrounding”:

FOREGROUND=fg

sleep 5 & ${FOREGROUND}

answered 2 days ago

Stephen Kitt

166k24367447

You can flip things and variabilise “foregrounding”:

FOREGROUND=fg

sleep 5 & ${FOREGROUND}

answered 2 days ago

Stephen Kitt

166k24367447

answered 2 days ago

Stephen Kitt

166k24367447

answered 2 days ago

Stephen Kitt

166k24367447

answered 2 days ago

Stephen Kitt

166k24367447

4

that's nice but it won't work in a shell without job control (ie any script unless set -m was used).

– mosvy
2 days ago

add a comment |

4

that's nice but it won't work in a shell without job control (ie any script unless set -m was used).

– mosvy
2 days ago

that's nice but it won't work in a shell without job control (ie any script unless set -m was used).

– mosvy
2 days ago

add a comment |

You would probably have to use eval:

eval "sleep 5" "$BCKGRND"

edited 2 days ago

answered 2 days ago

Kusalananda

123k16232381

1

A answer containing eval should contain a warning, that this should be handles with care. See e.g. this answer.

– Ralf
2 days ago

I don't get what the problem is with "$BCKGRND" evaluating to an empty argument.

– mosvy
2 days ago

2

@Ralf absolutely irrelevant in this case. There's nothing special about eval -- you can execute commands via arithmetic expansions, for instance. Maybe there should be such a warning against using bash (or any similar shell) at all ;-)

– mosvy
2 days ago

1

@Kusalananda eval will join its arguments with spaces before doing the actual eval. Just try it: eval printf "'{%s}n'" foo "" "" "". eval foo "" "" "" "" is completely similar to eval foo, no matter what IFS or other thing is.

– mosvy
2 days ago

1

The command being eval'ed should be in double quotes if it contains any special characters, e.g. eval 'sleep $TIMEOUT' "$BACKGROUND". Otherwise you could get double expansions if the variable expands to another variable or contains special characters. Also, nested quoting can get tricky.

– Barmar
2 days ago

|
show 4 more comments

You would probably have to use eval:

eval "sleep 5" "$BCKGRND"

edited 2 days ago

answered 2 days ago

Kusalananda

123k16232381

1

A answer containing eval should contain a warning, that this should be handles with care. See e.g. this answer.

– Ralf
2 days ago

I don't get what the problem is with "$BCKGRND" evaluating to an empty argument.

– mosvy
2 days ago

2

@Ralf absolutely irrelevant in this case. There's nothing special about eval -- you can execute commands via arithmetic expansions, for instance. Maybe there should be such a warning against using bash (or any similar shell) at all ;-)

– mosvy
2 days ago

1

@Kusalananda eval will join its arguments with spaces before doing the actual eval. Just try it: eval printf "'{%s}n'" foo "" "" "". eval foo "" "" "" "" is completely similar to eval foo, no matter what IFS or other thing is.

– mosvy
2 days ago

1

The command being eval'ed should be in double quotes if it contains any special characters, e.g. eval 'sleep $TIMEOUT' "$BACKGROUND". Otherwise you could get double expansions if the variable expands to another variable or contains special characters. Also, nested quoting can get tricky.

– Barmar
2 days ago

|
show 4 more comments

You would probably have to use eval:

eval "sleep 5" "$BCKGRND"

edited 2 days ago

answered 2 days ago

Kusalananda

123k16232381

You would probably have to use eval:

eval "sleep 5" "$BCKGRND"

edited 2 days ago

answered 2 days ago

Kusalananda

123k16232381

edited 2 days ago

answered 2 days ago

Kusalananda

123k16232381

answered 2 days ago

Kusalananda

123k16232381

answered 2 days ago

Kusalananda

123k16232381

1

A answer containing eval should contain a warning, that this should be handles with care. See e.g. this answer.

– Ralf
2 days ago

I don't get what the problem is with "$BCKGRND" evaluating to an empty argument.

– mosvy
2 days ago

2

@Ralf absolutely irrelevant in this case. There's nothing special about eval -- you can execute commands via arithmetic expansions, for instance. Maybe there should be such a warning against using bash (or any similar shell) at all ;-)

– mosvy
2 days ago

1

@Kusalananda eval will join its arguments with spaces before doing the actual eval. Just try it: eval printf "'{%s}n'" foo "" "" "". eval foo "" "" "" "" is completely similar to eval foo, no matter what IFS or other thing is.

– mosvy
2 days ago

1

The command being eval'ed should be in double quotes if it contains any special characters, e.g. eval 'sleep $TIMEOUT' "$BACKGROUND". Otherwise you could get double expansions if the variable expands to another variable or contains special characters. Also, nested quoting can get tricky.

– Barmar
2 days ago

|
show 4 more comments

1

A answer containing eval should contain a warning, that this should be handles with care. See e.g. this answer.

– Ralf
2 days ago

I don't get what the problem is with "$BCKGRND" evaluating to an empty argument.

– mosvy
2 days ago

2

@Ralf absolutely irrelevant in this case. There's nothing special about eval -- you can execute commands via arithmetic expansions, for instance. Maybe there should be such a warning against using bash (or any similar shell) at all ;-)

– mosvy
2 days ago

1

@Kusalananda eval will join its arguments with spaces before doing the actual eval. Just try it: eval printf "'{%s}n'" foo "" "" "". eval foo "" "" "" "" is completely similar to eval foo, no matter what IFS or other thing is.

– mosvy
2 days ago

1

The command being eval'ed should be in double quotes if it contains any special characters, e.g. eval 'sleep $TIMEOUT' "$BACKGROUND". Otherwise you could get double expansions if the variable expands to another variable or contains special characters. Also, nested quoting can get tricky.

– Barmar
2 days ago

A answer containing eval should contain a warning, that this should be handles with care. See e.g. this answer.

– Ralf
2 days ago

I don't get what the problem is with "$BCKGRND" evaluating to an empty argument.

– mosvy
2 days ago

@Ralf absolutely irrelevant in this case. There's nothing special about eval -- you can execute commands via arithmetic expansions, for instance. Maybe there should be such a warning against using bash (or any similar shell) at all ;-)

– mosvy
2 days ago

@Kusalananda eval will join its arguments with spaces before doing the actual eval. Just try it: eval printf "'{%s}n'" foo "" "" "". eval foo "" "" "" "" is completely similar to eval foo, no matter what IFS or other thing is.

– mosvy
2 days ago

The command being eval'ed should be in double quotes if it contains any special characters, e.g. eval 'sleep $TIMEOUT' "$BACKGROUND". Otherwise you could get double expansions if the variable expands to another variable or contains special characters. Also, nested quoting can get tricky.

– Barmar
2 days ago

|
show 4 more comments

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