Why was `cp` designed to silently overwrite existing files?

up vote
4
down vote

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I tested cp with the following commands:

$ ls
first.html second.html third.html

$ cat first.html
first

$ cat second.html
second

$ cat third.html
third

Then I copy first.html to second.html:

$ cp first.html second.html

$ cat second.html
first

The file second.html is silently overwritten without any errors. However, if I do it in a desktop GUI by dragging and dropping a file with the same name, it will be suffixed as first1.html automatically. This avoids accidentally overwriting an existing file.

Why doesn't cp follow this pattern instead of overwriting files silently?

edited 3 mins ago

isanae

13816

asked 4 hours ago

rider dragon

1234

New contributor

3

I imagine only the coreutils designers can truly answer the question, but it's just the way it works for now. Usually the apps are built assuming the user really means what they're doing and to minimize the extra prompting. If you want to change the behavior, alias 'cp' to 'cp -i' or 'cp -n'.
â€“Â kevlinux
4 hours ago

Is it "do one thing at one time"? @kevlinux
â€“Â rider dragon
3 hours ago

1

@kevlinux The coreutils devs are just implementing the POSIX standard.
â€“Â Kusalananda
1 hour ago

add a commentÂ |Â

up vote
4
down vote

favorite

I tested cp with the following commands:

$ ls
first.html second.html third.html

$ cat first.html
first

$ cat second.html
second

$ cat third.html
third

Then I copy first.html to second.html:

$ cp first.html second.html

$ cat second.html
first

Why doesn't cp follow this pattern instead of overwriting files silently?

edited 3 mins ago

isanae

13816

asked 4 hours ago

rider dragon

1234

New contributor

3

I imagine only the coreutils designers can truly answer the question, but it's just the way it works for now. Usually the apps are built assuming the user really means what they're doing and to minimize the extra prompting. If you want to change the behavior, alias 'cp' to 'cp -i' or 'cp -n'.
â€“Â kevlinux
4 hours ago

Is it "do one thing at one time"? @kevlinux
â€“Â rider dragon
3 hours ago

1

@kevlinux The coreutils devs are just implementing the POSIX standard.
â€“Â Kusalananda
1 hour ago

add a commentÂ |Â

up vote
4
down vote

favorite

I tested cp with the following commands:

$ ls
first.html second.html third.html

$ cat first.html
first

$ cat second.html
second

$ cat third.html
third

Then I copy first.html to second.html:

$ cp first.html second.html

$ cat second.html
first

Why doesn't cp follow this pattern instead of overwriting files silently?

edited 3 mins ago

isanae

13816

asked 4 hours ago

rider dragon

1234

New contributor

I tested cp with the following commands:

$ ls
first.html second.html third.html

$ cat first.html
first

$ cat second.html
second

$ cat third.html
third

Then I copy first.html to second.html:

$ cp first.html second.html

$ cat second.html
first

Why doesn't cp follow this pattern instead of overwriting files silently?

cp history

edited 3 mins ago

isanae

13816

asked 4 hours ago

rider dragon

1234

New contributor

edited 3 mins ago

isanae

13816

asked 4 hours ago

rider dragon

1234

New contributor

edited 3 mins ago

isanae

13816

edited 3 mins ago

isanae

13816

edited 3 mins ago

isanae

13816

asked 4 hours ago

rider dragon

1234

New contributor

asked 4 hours ago

rider dragon

1234

asked 4 hours ago

rider dragon

1234

New contributor

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

3

I imagine only the coreutils designers can truly answer the question, but it's just the way it works for now. Usually the apps are built assuming the user really means what they're doing and to minimize the extra prompting. If you want to change the behavior, alias 'cp' to 'cp -i' or 'cp -n'.
â€“Â kevlinux
4 hours ago

Is it "do one thing at one time"? @kevlinux
â€“Â rider dragon
3 hours ago

1

@kevlinux The coreutils devs are just implementing the POSIX standard.
â€“Â Kusalananda
1 hour ago

add a commentÂ |Â

3

I imagine only the coreutils designers can truly answer the question, but it's just the way it works for now. Usually the apps are built assuming the user really means what they're doing and to minimize the extra prompting. If you want to change the behavior, alias 'cp' to 'cp -i' or 'cp -n'.
â€“Â kevlinux
4 hours ago

Is it "do one thing at one time"? @kevlinux
â€“Â rider dragon
3 hours ago

1

@kevlinux The coreutils devs are just implementing the POSIX standard.
â€“Â Kusalananda
1 hour ago

I imagine only the coreutils designers can truly answer the question, but it's just the way it works for now. Usually the apps are built assuming the user really means what they're doing and to minimize the extra prompting. If you want to change the behavior, alias 'cp' to 'cp -i' or 'cp -n'.
â€“Â kevlinux
4 hours ago

Is it "do one thing at one time"? @kevlinux
â€“Â rider dragon
3 hours ago

@kevlinux The coreutils devs are just implementing the POSIX standard.
â€“Â Kusalananda
1 hour ago

add a commentÂ |Â

3 Answers
3

active

oldest

votes

up vote
11
down vote

accepted

The default overwrite behavior of cp is specified in POSIX.

If source_file is of type regular file, the following steps shall be taken:

3.a. The behavior is unspecified if dest_file exists and was written by a previous step. Otherwise, if dest_file exists, the following steps shall be taken:

3.a.i. If the -i option is in effect, the cp utility shall write a prompt to the standard error and read a line from the standard input. If the response is not affirmative, cp shall do nothing more with source_file and go on to any remaining files.

3.a.ii. A file descriptor for dest_file shall be obtained by performing actions equivalent to the open() function defined in the System Interfaces volume of POSIX.1-2017 called using dest_file as the path argument, and the bitwise-inclusive OR of O_WRONLY and O_TRUNC as the oflag argument.

3.a.iii. If the attempt to obtain a file descriptor fails and the -f option is in effect, cp shall attempt to remove the file by performing actions equivalent to the unlink() function defined in the System Interfaces volume of POSIX.1-2017 called using dest_file as the path argument. If this attempt succeeds, cp shall continue with step 3b.

When the POSIX specification was written, there already was a large number of scripts in existence, with a built-in assumption for the default overwrite behavior. Many of those scripts were designed to run without direct user presence, e.g. as cron jobs or other background tasks. Changing the behavior would have broken them. Reviewing and modifying them all to add an option to force overwriting wherever needed was probably considered a huge task with minimal benefits.

Also, the Unix command line was always designed to allow an experienced user to work efficiently, even at the expense of a hard learning curve for a beginner. When the user enters a command, the computer is to expect that the user really means it, without any second-guessing; it is the user's responsibility to be careful with potentially destructive commands.

answered 3 hours ago

telcoM

13.2k11441

add a commentÂ |Â

up vote
5
down vote

You may insert the "if check" yourself with

alias cp='cp -i'

The -i option to cp makes it ask interactively if the operation would overwrite the target and with the above alias, your interactive use of cp would enable this option by default.

The cp utility is often used in scripts. These scripts may not always be run manually, but may be part of system maintenance tasks run by via cron or some similar facility. In those cases, there are no humans to ask for confirmation.

To have a script avoid overwriting a file with cp one may insert an explicit check:

if [ -e "$target" ]; then
 printf '%s already exists, refusing to overwriten' "$target" >&2
 exit 1
fi
cp "$source" "$target"

I'm opting for exiting the script with an error message if $target exists in the above example. The existence of the target is obviously unexpected, and allowing the script to continue after detecting this may lead to other more severe issues (depending on what the script is doing and what its purpose is).

edited 1 hour ago

answered 1 hour ago

Kusalananda

111k15216342

I remember some distros set such an alias at least for the rm command in interactive shells. AFAIR, SuSE is one of them.
â€“Â rexkogitans
9 mins ago

add a commentÂ |Â

up vote
0
down vote

Because these commands are also meant to be used in scripts, possibly running without any kind of human supervision, and also because there are plenty of cases where you indeed want to overwrite the target (the philosophy of the Linux shells is that the human knows what s/he is doing)

There still a few safeguards:

GNU cp has a -n|--no-clobber option

if you copy several files to a single one cp will complain that the last one is not a directory.

edited 1 hour ago

Kusalananda

111k15216342

answered 1 hour ago

xenoid

1,9711620

add a commentÂ |Â

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3 Answers
3

active

oldest

votes

3 Answers
3

active

oldest

votes

up vote
11
down vote

accepted

The default overwrite behavior of cp is specified in POSIX.

If source_file is of type regular file, the following steps shall be taken:

3.a. The behavior is unspecified if dest_file exists and was written by a previous step. Otherwise, if dest_file exists, the following steps shall be taken:

3.a.i. If the -i option is in effect, the cp utility shall write a prompt to the standard error and read a line from the standard input. If the response is not affirmative, cp shall do nothing more with source_file and go on to any remaining files.

3.a.ii. A file descriptor for dest_file shall be obtained by performing actions equivalent to the open() function defined in the System Interfaces volume of POSIX.1-2017 called using dest_file as the path argument, and the bitwise-inclusive OR of O_WRONLY and O_TRUNC as the oflag argument.

3.a.iii. If the attempt to obtain a file descriptor fails and the -f option is in effect, cp shall attempt to remove the file by performing actions equivalent to the unlink() function defined in the System Interfaces volume of POSIX.1-2017 called using dest_file as the path argument. If this attempt succeeds, cp shall continue with step 3b.

answered 3 hours ago

telcoM

13.2k11441

add a commentÂ |Â

up vote
11
down vote

accepted

The default overwrite behavior of cp is specified in POSIX.

If source_file is of type regular file, the following steps shall be taken:

3.a. The behavior is unspecified if dest_file exists and was written by a previous step. Otherwise, if dest_file exists, the following steps shall be taken:

3.a.i. If the -i option is in effect, the cp utility shall write a prompt to the standard error and read a line from the standard input. If the response is not affirmative, cp shall do nothing more with source_file and go on to any remaining files.

3.a.ii. A file descriptor for dest_file shall be obtained by performing actions equivalent to the open() function defined in the System Interfaces volume of POSIX.1-2017 called using dest_file as the path argument, and the bitwise-inclusive OR of O_WRONLY and O_TRUNC as the oflag argument.

3.a.iii. If the attempt to obtain a file descriptor fails and the -f option is in effect, cp shall attempt to remove the file by performing actions equivalent to the unlink() function defined in the System Interfaces volume of POSIX.1-2017 called using dest_file as the path argument. If this attempt succeeds, cp shall continue with step 3b.

answered 3 hours ago

telcoM

13.2k11441

add a commentÂ |Â

up vote
11
down vote

accepted

The default overwrite behavior of cp is specified in POSIX.

If source_file is of type regular file, the following steps shall be taken:

3.a. The behavior is unspecified if dest_file exists and was written by a previous step. Otherwise, if dest_file exists, the following steps shall be taken:

3.a.i. If the -i option is in effect, the cp utility shall write a prompt to the standard error and read a line from the standard input. If the response is not affirmative, cp shall do nothing more with source_file and go on to any remaining files.

3.a.ii. A file descriptor for dest_file shall be obtained by performing actions equivalent to the open() function defined in the System Interfaces volume of POSIX.1-2017 called using dest_file as the path argument, and the bitwise-inclusive OR of O_WRONLY and O_TRUNC as the oflag argument.

3.a.iii. If the attempt to obtain a file descriptor fails and the -f option is in effect, cp shall attempt to remove the file by performing actions equivalent to the unlink() function defined in the System Interfaces volume of POSIX.1-2017 called using dest_file as the path argument. If this attempt succeeds, cp shall continue with step 3b.

answered 3 hours ago

telcoM

13.2k11441

The default overwrite behavior of cp is specified in POSIX.

If source_file is of type regular file, the following steps shall be taken:

3.a. The behavior is unspecified if dest_file exists and was written by a previous step. Otherwise, if dest_file exists, the following steps shall be taken:

3.a.i. If the -i option is in effect, the cp utility shall write a prompt to the standard error and read a line from the standard input. If the response is not affirmative, cp shall do nothing more with source_file and go on to any remaining files.

3.a.ii. A file descriptor for dest_file shall be obtained by performing actions equivalent to the open() function defined in the System Interfaces volume of POSIX.1-2017 called using dest_file as the path argument, and the bitwise-inclusive OR of O_WRONLY and O_TRUNC as the oflag argument.

3.a.iii. If the attempt to obtain a file descriptor fails and the -f option is in effect, cp shall attempt to remove the file by performing actions equivalent to the unlink() function defined in the System Interfaces volume of POSIX.1-2017 called using dest_file as the path argument. If this attempt succeeds, cp shall continue with step 3b.

answered 3 hours ago

telcoM

13.2k11441

answered 3 hours ago

telcoM

13.2k11441

answered 3 hours ago

telcoM

13.2k11441

answered 3 hours ago

telcoM

13.2k11441

add a commentÂ |Â

up vote
5
down vote

You may insert the "if check" yourself with

alias cp='cp -i'

The -i option to cp makes it ask interactively if the operation would overwrite the target and with the above alias, your interactive use of cp would enable this option by default.

To have a script avoid overwriting a file with cp one may insert an explicit check:

if [ -e "$target" ]; then
 printf '%s already exists, refusing to overwriten' "$target" >&2
 exit 1
fi
cp "$source" "$target"

edited 1 hour ago

answered 1 hour ago

Kusalananda

111k15216342

I remember some distros set such an alias at least for the rm command in interactive shells. AFAIR, SuSE is one of them.
â€“Â rexkogitans
9 mins ago

add a commentÂ |Â

up vote
5
down vote

You may insert the "if check" yourself with

alias cp='cp -i'

The -i option to cp makes it ask interactively if the operation would overwrite the target and with the above alias, your interactive use of cp would enable this option by default.

To have a script avoid overwriting a file with cp one may insert an explicit check:

if [ -e "$target" ]; then
 printf '%s already exists, refusing to overwriten' "$target" >&2
 exit 1
fi
cp "$source" "$target"

edited 1 hour ago

answered 1 hour ago

Kusalananda

111k15216342

I remember some distros set such an alias at least for the rm command in interactive shells. AFAIR, SuSE is one of them.
â€“Â rexkogitans
9 mins ago

add a commentÂ |Â

up vote
5
down vote

You may insert the "if check" yourself with

alias cp='cp -i'

The -i option to cp makes it ask interactively if the operation would overwrite the target and with the above alias, your interactive use of cp would enable this option by default.

To have a script avoid overwriting a file with cp one may insert an explicit check:

if [ -e "$target" ]; then
 printf '%s already exists, refusing to overwriten' "$target" >&2
 exit 1
fi
cp "$source" "$target"

edited 1 hour ago

answered 1 hour ago

Kusalananda

111k15216342

You may insert the "if check" yourself with

alias cp='cp -i'

The -i option to cp makes it ask interactively if the operation would overwrite the target and with the above alias, your interactive use of cp would enable this option by default.

To have a script avoid overwriting a file with cp one may insert an explicit check:

if [ -e "$target" ]; then
 printf '%s already exists, refusing to overwriten' "$target" >&2
 exit 1
fi
cp "$source" "$target"

edited 1 hour ago

answered 1 hour ago

Kusalananda

111k15216342

edited 1 hour ago

answered 1 hour ago

Kusalananda

111k15216342

answered 1 hour ago

Kusalananda

111k15216342

answered 1 hour ago

Kusalananda

111k15216342

I remember some distros set such an alias at least for the rm command in interactive shells. AFAIR, SuSE is one of them.
â€“Â rexkogitans
9 mins ago

add a commentÂ |Â

I remember some distros set such an alias at least for the rm command in interactive shells. AFAIR, SuSE is one of them.
â€“Â rexkogitans
9 mins ago

I remember some distros set such an alias at least for the rm command in interactive shells. AFAIR, SuSE is one of them.
â€“Â rexkogitans
9 mins ago

add a commentÂ |Â

up vote
0
down vote

There still a few safeguards:

GNU cp has a -n|--no-clobber option

if you copy several files to a single one cp will complain that the last one is not a directory.

edited 1 hour ago

Kusalananda

111k15216342

answered 1 hour ago

xenoid

1,9711620

add a commentÂ |Â

up vote
0
down vote

There still a few safeguards:

GNU cp has a -n|--no-clobber option

if you copy several files to a single one cp will complain that the last one is not a directory.

edited 1 hour ago

Kusalananda

111k15216342

answered 1 hour ago

xenoid

1,9711620

add a commentÂ |Â

up vote
0
down vote

There still a few safeguards:

GNU cp has a -n|--no-clobber option

if you copy several files to a single one cp will complain that the last one is not a directory.

edited 1 hour ago

Kusalananda

111k15216342

answered 1 hour ago

xenoid

1,9711620

There still a few safeguards:

GNU cp has a -n|--no-clobber option

if you copy several files to a single one cp will complain that the last one is not a directory.

edited 1 hour ago

Kusalananda

111k15216342

answered 1 hour ago

xenoid

1,9711620

edited 1 hour ago

Kusalananda

111k15216342

edited 1 hour ago

Kusalananda

111k15216342

edited 1 hour ago

Kusalananda

111k15216342

answered 1 hour ago

xenoid

1,9711620

answered 1 hour ago

xenoid

1,9711620

answered 1 hour ago

xenoid

1,9711620

add a commentÂ |Â

rider dragon is a new contributor. Be nice, and check out our Code of Conduct.

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