Bash – Conflict between `pushd .` and `cd -`

bashcd-commandpushdshell

I am a happy user of the cd - command to go to the previous directory. At the same time I like pushd . and popd.

However, when I want to remember the current working directory by means of pushd ., I lose the possibility to go to the previous directory by cd -. (As pushd . also performs cd .).

How can I use pushd to still be able to use cd -

By the way: GNU bash, version 4.1.7(1)

Best Answer

You can use something like this:

push() { 
    if [ "$1" = . ]; then
        old=$OLDPWD
        current=$PWD
        builtin pushd .
        cd "$old"
        cd "$current"
    else
        builtin pushd "$1"
    fi
}

If you name it pushd, then it will have precedence over the built-in as functions are evaluated before built-ins.

You need variables old and current as overwriting OLDPWD will make it lose its special meaning.

bash's built-in version

% time

GNU time aka. /usr/bin/time

% \time

The built-in time command to bash can be read up on here:

% help time
time: time [-p] PIPELINE
    Execute PIPELINE and print a summary of the real time, user CPU time,
    and system CPU time spent executing PIPELINE when it terminates.
    The return status is the return status of PIPELINE.  The `-p' option
    prints the timing summary in a slightly different format.  This uses
    the value of the TIMEFORMAT variable as the output format.

The GNU time, /usr/bin/time, is usually more useful than the built-in.

As to your precision problem it's covered here in this github gist, specifically:

Why is bash time more precise then GNU time?

The builtin bash command time gives milisecond precision of execution, and GNU time (usually /usr/bin/time) gives centisecond precision. The times(2) syscall gives times in clocks, and 100 clocks = 1 second (usually), so the precision is like GNU time. What is bash time using so that it is more precise?

Bash time internally uses getrusage() and GNU time uses times(). getrusage() is far more precise because of microsecond resolution.

You can see the centiseconds with the following example (see 5th line of output):

% /usr/bin/time -v sleep .22222
    Command being timed: "sleep .22222"
    User time (seconds): 0.00
    System time (seconds): 0.00
    Percent of CPU this job got: 0%
    Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.22
    Average shared text size (kbytes): 0
    Average unshared data size (kbytes): 0
    Average stack size (kbytes): 0
    Average total size (kbytes): 0
    Maximum resident set size (kbytes): 1968
    Average resident set size (kbytes): 0
    Major (requiring I/O) page faults: 0
    Minor (reclaiming a frame) page faults: 153
    Voluntary context switches: 2
    Involuntary context switches: 1
    Swaps: 0
    File system inputs: 0
    File system outputs: 0
    Socket messages sent: 0
    Socket messages received: 0
    Signals delivered: 0
    Page size (bytes): 4096
    Exit status: 0

More resolution can be had using bash's time command like so & you can control the resolution:

# 3 places 
% TIMEFORMAT='%3R'; time ( sleep .22222 )
0.224

From the Bash manual on variables:

TIMEFORMAT
The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed with the time reserved word should be displayed. The ‘%’ character introduces an escape sequence that is expanded to a time value or other information. The escape sequences and their meanings are as follows; the braces denote optional portions.

%%
A literal ‘%’.

%[p][l]R
The elapsed time in seconds.

%[p][l]U
The number of CPU seconds spent in user mode.

%[p][l]S
The number of CPU seconds spent in system mode.

%P
The CPU percentage, computed as (%U + %S) / %R.

The optional p is a digit specifying the precision, the number of fractional digits after a decimal point. A value of 0 causes no decimal point or fraction to be output. At most three places after the decimal point may be specified; values of p greater than 3 are changed to 3. If p is not specified, the value 3 is used.

The optional l specifies a longer format, including minutes, of the form MMmSS.FFs. The value of p determines whether or not the fraction is included.

If this variable is not set, Bash acts as if it had the value

$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'
If the value is null, no timing information is displayed. A trailing newline is added when the format string is displayed.

Shell Commands – How to Use Pushd and Popd Commands

pushd, popd, and dirs are shell builtins which allow you manipulate the directory stack. This can be used to change directories but return to the directory from which you came.

For example

start up with the following directories:

$ pwd
/home/saml/somedir
$ ls
dir1  dir2  dir3

pushd to dir1

$ pushd dir1
~/somedir/dir1 ~/somedir
$ dirs
~/somedir/dir1 ~/somedir

dirs command confirms that we have 2 directories on the stack now. dir1 and the original dir, somedir. NOTE: Our "current" directory is ~/somedir/dir1.

pushd to ../dir3 (because we're inside `dir1` now)

$ pushd ../dir3
~/somedir/dir3 ~/somedir/dir1 ~/somedir
$ dirs
~/somedir/dir3 ~/somedir/dir1 ~/somedir
$ pwd
/home/saml/somedir/dir3

dirs shows we have 3 directories in the stack now. dir3, dir1, and somedir. Notice the direction. Every new directory is getting added to the left. When we start popping directories off, they'll come from the left as well.

manually change directories to `../dir2`

$ cd ../dir2
$ pwd
/home/saml/somedir/dir2
$ dirs
~/somedir/dir2 ~/somedir/dir1 ~/somedir

Now start popping directories

$ popd
~/somedir/dir1 ~/somedir
$ pwd
/home/saml/somedir/dir1

Notice we popped back to dir1.

Pop again...

$ popd
~/somedir    
$ pwd
/home/saml/somedir

And we're back where we started, somedir.

Might get a little confusing, but the head of the stack is the directory that you're currently in. Hence when we get back to somedir, even though dirs shows this:

$ dirs
~/somedir

Our stack is in fact empty.

$ popd
bash: popd: directory stack empty